\section{Built-in Types \label{types}} The following sections describe the standard types that are built into the interpreter. These are the numeric types, sequence types, and several others, including types themselves. There is no explicit Boolean type; use integers instead. \indexii{built-in}{types} \indexii{Boolean}{type} Some operations are supported by several object types; in particular, all objects can be compared, tested for truth value, and converted to a string (with the \code{`\textrm{\ldots}`} notation). The latter conversion is implicitly used when an object is written by the \keyword{print}\stindex{print} statement. (Information on \ulink{\keyword{print} statement}{../ref/print.html} and other language statements can be found in the \citetitle[../ref/ref.html]{Python Reference Manual} and the \citetitle[../tut/tut.html]{Python Tutorial}.) \subsection{Truth Value Testing \label{truth}} Any object can be tested for truth value, for use in an \keyword{if} or \keyword{while} condition or as operand of the Boolean operations below. The following values are considered false: \stindex{if} \stindex{while} \indexii{truth}{value} \indexii{Boolean}{operations} \index{false} \begin{itemize} \item \code{None} \withsubitem{(Built-in object)}{\ttindex{None}} \item \code{False} \withsubitem{(Built-in object)}{\ttindex{False}} \item zero of any numeric type, for example, \code{0}, \code{0L}, \code{0.0}, \code{0j}. \item any empty sequence, for example, \code{''}, \code{()}, \code{[]}. \item any empty mapping, for example, \code{\{\}}. \item instances of user-defined classes, if the class defines a \method{__nonzero__()} or \method{__len__()} method, when that method returns the integer zero or \class{bool} value \code{False}.\footnote{Additional information on these special methods may be found in the \citetitle[../ref/ref.html]{Python Reference Manual}.} \end{itemize} All other values are considered true --- so objects of many types are always true. \index{true} Operations and built-in functions that have a Boolean result always return \code{0} for false and \code{1} for true, unless otherwise stated. (Important exception: the Boolean operations \samp{or}\opindex{or} and \samp{and}\opindex{and} always return one of their operands.) \subsection{Boolean Operations \label{boolean}} These are the Boolean operations, ordered by ascending priority: \indexii{Boolean}{operations} \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} \lineiii{\var{x} or \var{y}} {if \var{x} is false, then \var{y}, else \var{x}}{(1)} \lineiii{\var{x} and \var{y}} {if \var{x} is false, then \var{x}, else \var{y}}{(1)} \hline \lineiii{not \var{x}} {if \var{x} is false, then \code{1}, else \code{0}}{(2)} \end{tableiii} \opindex{and} \opindex{or} \opindex{not} \noindent Notes: \begin{description} \item[(1)] These only evaluate their second argument if needed for their outcome. \item[(2)] \samp{not} has a lower priority than non-Boolean operators, so \code{not \var{a} == \var{b}} is interpreted as \code{not (\var{a} == \var{b})}, and \code{\var{a} == not \var{b}} is a syntax error. \end{description} \subsection{Comparisons \label{comparisons}} Comparison operations are supported by all objects. They all have the same priority (which is higher than that of the Boolean operations). Comparisons can be chained arbitrarily; for example, \code{\var{x} < \var{y} <= \var{z}} is equivalent to \code{\var{x} < \var{y} and \var{y} <= \var{z}}, except that \var{y} is evaluated only once (but in both cases \var{z} is not evaluated at all when \code{\var{x} < \var{y}} is found to be false). \indexii{chaining}{comparisons} This table summarizes the comparison operations: \begin{tableiii}{c|l|c}{code}{Operation}{Meaning}{Notes} \lineiii{<}{strictly less than}{} \lineiii{<=}{less than or equal}{} \lineiii{>}{strictly greater than}{} \lineiii{>=}{greater than or equal}{} \lineiii{==}{equal}{} \lineiii{!=}{not equal}{(1)} \lineiii{<>}{not equal}{(1)} \lineiii{is}{object identity}{} \lineiii{is not}{negated object identity}{} \end{tableiii} \indexii{operator}{comparison} \opindex{==} % XXX *All* others have funny characters < ! > \opindex{is} \opindex{is not} \noindent Notes: \begin{description} \item[(1)] \code{<>} and \code{!=} are alternate spellings for the same operator. (I couldn't choose between \ABC{} and C! :-) \index{ABC language@\ABC{} language} \index{language!ABC@\ABC} \indexii{C}{language} \code{!=} is the preferred spelling; \code{<>} is obsolescent. \end{description} Objects of different types, except different numeric types, never compare equal; such objects are ordered consistently but arbitrarily (so that sorting a heterogeneous array yields a consistent result). Furthermore, some types (for example, file objects) support only a degenerate notion of comparison where any two objects of that type are unequal. Again, such objects are ordered arbitrarily but consistently. \indexii{object}{numeric} \indexii{objects}{comparing} Instances of a class normally compare as non-equal unless the class \withsubitem{(instance method)}{\ttindex{__cmp__()}} defines the \method{__cmp__()} method. Refer to the \citetitle[../ref/customization.html]{Python Reference Manual} for information on the use of this method to effect object comparisons. \strong{Implementation note:} Objects of different types except numbers are ordered by their type names; objects of the same types that don't support proper comparison are ordered by their address. Two more operations with the same syntactic priority, \samp{in}\opindex{in} and \samp{not in}\opindex{not in}, are supported only by sequence types (below). \subsection{Numeric Types \label{typesnumeric}} There are four numeric types: \dfn{plain integers}, \dfn{long integers}, \dfn{floating point numbers}, and \dfn{complex numbers}. Plain integers (also just called \dfn{integers}) are implemented using \ctype{long} in C, which gives them at least 32 bits of precision. Long integers have unlimited precision. Floating point numbers are implemented using \ctype{double} in C. All bets on their precision are off unless you happen to know the machine you are working with. \obindex{numeric} \obindex{integer} \obindex{long integer} \obindex{floating point} \obindex{complex number} \indexii{C}{language} Complex numbers have a real and imaginary part, which are both implemented using \ctype{double} in C. To extract these parts from a complex number \var{z}, use \code{\var{z}.real} and \code{\var{z}.imag}. Numbers are created by numeric literals or as the result of built-in functions and operators. Unadorned integer literals (including hex and octal numbers) yield plain integers. Integer literals with an \character{L} or \character{l} suffix yield long integers (\character{L} is preferred because \samp{1l} looks too much like eleven!). Numeric literals containing a decimal point or an exponent sign yield floating point numbers. Appending \character{j} or \character{J} to a numeric literal yields a complex number. \indexii{numeric}{literals} \indexii{integer}{literals} \indexiii{long}{integer}{literals} \indexii{floating point}{literals} \indexii{complex number}{literals} \indexii{hexadecimal}{literals} \indexii{octal}{literals} Python fully supports mixed arithmetic: when a binary arithmetic operator has operands of different numeric types, the operand with the ``smaller'' type is converted to that of the other, where plain integer is smaller than long integer is smaller than floating point is smaller than complex. Comparisons between numbers of mixed type use the same rule.\footnote{ As a consequence, the list \code{[1, 2]} is considered equal to \code{[1.0, 2.0]}, and similar for tuples. } The functions \function{int()}, \function{long()}, \function{float()}, and \function{complex()} can be used to coerce numbers to a specific type. \index{arithmetic} \bifuncindex{int} \bifuncindex{long} \bifuncindex{float} \bifuncindex{complex} All numeric types (except complex) support the following operations, sorted by ascending priority (operations in the same box have the same priority; all numeric operations have a higher priority than comparison operations): \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} \lineiii{\var{x} + \var{y}}{sum of \var{x} and \var{y}}{} \lineiii{\var{x} - \var{y}}{difference of \var{x} and \var{y}}{} \hline \lineiii{\var{x} * \var{y}}{product of \var{x} and \var{y}}{} \lineiii{\var{x} / \var{y}}{quotient of \var{x} and \var{y}}{(1)} \lineiii{\var{x} \%{} \var{y}}{remainder of \code{\var{x} / \var{y}}}{(4)} \hline \lineiii{-\var{x}}{\var{x} negated}{} \lineiii{+\var{x}}{\var{x} unchanged}{} \hline \lineiii{abs(\var{x})}{absolute value or magnitude of \var{x}}{} \lineiii{int(\var{x})}{\var{x} converted to integer}{(2)} \lineiii{long(\var{x})}{\var{x} converted to long integer}{(2)} \lineiii{float(\var{x})}{\var{x} converted to floating point}{} \lineiii{complex(\var{re},\var{im})}{a complex number with real part \var{re}, imaginary part \var{im}. \var{im} defaults to zero.}{} \lineiii{\var{c}.conjugate()}{conjugate of the complex number \var{c}}{} \lineiii{divmod(\var{x}, \var{y})}{the pair \code{(\var{x} / \var{y}, \var{x} \%{} \var{y})}}{(3)(4)} \lineiii{pow(\var{x}, \var{y})}{\var{x} to the power \var{y}}{} \lineiii{\var{x} ** \var{y}}{\var{x} to the power \var{y}}{} \end{tableiii} \indexiii{operations on}{numeric}{types} \withsubitem{(complex number method)}{\ttindex{conjugate()}} \noindent Notes: \begin{description} \item[(1)] For (plain or long) integer division, the result is an integer. The result is always rounded towards minus infinity: 1/2 is 0, (-1)/2 is -1, 1/(-2) is -1, and (-1)/(-2) is 0. Note that the result is a long integer if either operand is a long integer, regardless of the numeric value. \indexii{integer}{division} \indexiii{long}{integer}{division} \item[(2)] Conversion from floating point to (long or plain) integer may round or truncate as in C; see functions \function{floor()} and \function{ceil()} in the \refmodule{math}\refbimodindex{math} module for well-defined conversions. \withsubitem{(in module math)}{\ttindex{floor()}\ttindex{ceil()}} \indexii{numeric}{conversions} \indexii{C}{language} \item[(3)] See section \ref{built-in-funcs}, ``Built-in Functions,'' for a full description. \item[(4)] Complex floor division operator, modulo operator, and \function{divmod()}. \deprecated{2.3}{Instead convert to float using \function{abs()} if appropriate.} \end{description} % XXXJH exceptions: overflow (when? what operations?) zerodivision \subsubsection{Bit-string Operations on Integer Types \label{bitstring-ops}} \nodename{Bit-string Operations} Plain and long integer types support additional operations that make sense only for bit-strings. Negative numbers are treated as their 2's complement value (for long integers, this assumes a sufficiently large number of bits that no overflow occurs during the operation). The priorities of the binary bit-wise operations are all lower than the numeric operations and higher than the comparisons; the unary operation \samp{\~} has the same priority as the other unary numeric operations (\samp{+} and \samp{-}). This table lists the bit-string operations sorted in ascending priority (operations in the same box have the same priority): \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} \lineiii{\var{x} | \var{y}}{bitwise \dfn{or} of \var{x} and \var{y}}{} \lineiii{\var{x} \^{} \var{y}}{bitwise \dfn{exclusive or} of \var{x} and \var{y}}{} \lineiii{\var{x} \&{} \var{y}}{bitwise \dfn{and} of \var{x} and \var{y}}{} \lineiii{\var{x} << \var{n}}{\var{x} shifted left by \var{n} bits}{(1), (2)} \lineiii{\var{x} >> \var{n}}{\var{x} shifted right by \var{n} bits}{(1), (3)} \hline \lineiii{\~\var{x}}{the bits of \var{x} inverted}{} \end{tableiii} \indexiii{operations on}{integer}{types} \indexii{bit-string}{operations} \indexii{shifting}{operations} \indexii{masking}{operations} \noindent Notes: \begin{description} \item[(1)] Negative shift counts are illegal and cause a \exception{ValueError} to be raised. \item[(2)] A left shift by \var{n} bits is equivalent to multiplication by \code{pow(2, \var{n})} without overflow check. \item[(3)] A right shift by \var{n} bits is equivalent to division by \code{pow(2, \var{n})} without overflow check. \end{description} \subsection{Iterator Types \label{typeiter}} \versionadded{2.2} \index{iterator protocol} \index{protocol!iterator} \index{sequence!iteration} \index{container!iteration over} Python supports a concept of iteration over containers. This is implemented using two distinct methods; these are used to allow user-defined classes to support iteration. Sequences, described below in more detail, always support the iteration methods. One method needs to be defined for container objects to provide iteration support: \begin{methoddesc}[container]{__iter__}{} Return an iterator object. The object is required to support the iterator protocol described below. If a container supports different types of iteration, additional methods can be provided to specifically request iterators for those iteration types. (An example of an object supporting multiple forms of iteration would be a tree structure which supports both breadth-first and depth-first traversal.) This method corresponds to the \member{tp_iter} slot of the type structure for Python objects in the Python/C API. \end{methoddesc} The iterator objects themselves are required to support the following two methods, which together form the \dfn{iterator protocol}: \begin{methoddesc}[iterator]{__iter__}{} Return the iterator object itself. This is required to allow both containers and iterators to be used with the \keyword{for} and \keyword{in} statements. This method corresponds to the \member{tp_iter} slot of the type structure for Python objects in the Python/C API. \end{methoddesc} \begin{methoddesc}[iterator]{next}{} Return the next item from the container. If there are no further items, raise the \exception{StopIteration} exception. This method corresponds to the \member{tp_iternext} slot of the type structure for Python objects in the Python/C API. \end{methoddesc} Python defines several iterator objects to support iteration over general and specific sequence types, dictionaries, and other more specialized forms. The specific types are not important beyond their implementation of the iterator protocol. The intention of the protocol is that once an iterator's \method{next()} method raises \exception{StopIteration}, it will continue to do so on subsequent calls. Implementations that do not obey this property are deemed broken. (This constraint was added in Python 2.3; in Python 2.2, various iterators are broken according to this rule.) \subsection{Sequence Types \label{typesseq}} There are six sequence types: strings, Unicode strings, lists, tuples, buffers, and xrange objects. Strings literals are written in single or double quotes: \code{'xyzzy'}, \code{"frobozz"}. See chapter 2 of the \citetitle[../ref/strings.html]{Python Reference Manual} for more about string literals. Unicode strings are much like strings, but are specified in the syntax using a preceeding \character{u} character: \code{u'abc'}, \code{u"def"}. Lists are constructed with square brackets, separating items with commas: \code{[a, b, c]}. Tuples are constructed by the comma operator (not within square brackets), with or without enclosing parentheses, but an empty tuple must have the enclosing parentheses, e.g., \code{a, b, c} or \code{()}. A single item tuple must have a trailing comma, e.g., \code{(d,)}. \obindex{sequence} \obindex{string} \obindex{Unicode} \obindex{tuple} \obindex{list} Buffer objects are not directly supported by Python syntax, but can be created by calling the builtin function \function{buffer()}.\bifuncindex{buffer} They support concatenation and repetition, but the result is a new string object rather than a new buffer object. \obindex{buffer} Xrange objects are similar to buffers in that there is no specific syntax to create them, but they are created using the \function{xrange()} function.\bifuncindex{xrange} They don't support slicing or concatenation, but do support repetition, and using \code{in}, \code{not in}, \function{min()} or \function{max()} on them is inefficient. \obindex{xrange} Most sequence types support the following operations. The \samp{in} and \samp{not in} operations have the same priorities as the comparison operations. The \samp{+} and \samp{*} operations have the same priority as the corresponding numeric operations.\footnote{They must have since the parser can't tell the type of the operands.} This table lists the sequence operations sorted in ascending priority (operations in the same box have the same priority). In the table, \var{s} and \var{t} are sequences of the same type; \var{n}, \var{i} and \var{j} are integers: \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} \lineiii{\var{x} in \var{s}}{\code{1} if an item of \var{s} is equal to \var{x}, else \code{0}}{} \lineiii{\var{x} not in \var{s}}{\code{0} if an item of \var{s} is equal to \var{x}, else \code{1}}{} \hline \lineiii{\var{s} + \var{t}}{the concatenation of \var{s} and \var{t}}{} \lineiii{\var{s} * \var{n}\textrm{,} \var{n} * \var{s}}{\var{n} shallow copies of \var{s} concatenated}{(1)} \hline \lineiii{\var{s}[\var{i}]}{\var{i}'th item of \var{s}, origin 0}{(2)} \lineiii{\var{s}[\var{i}:\var{j}]}{slice of \var{s} from \var{i} to \var{j}}{(2), (3)} \hline \lineiii{len(\var{s})}{length of \var{s}}{} \lineiii{min(\var{s})}{smallest item of \var{s}}{} \lineiii{max(\var{s})}{largest item of \var{s}}{} \end{tableiii} \indexiii{operations on}{sequence}{types} \bifuncindex{len} \bifuncindex{min} \bifuncindex{max} \indexii{concatenation}{operation} \indexii{repetition}{operation} \indexii{subscript}{operation} \indexii{slice}{operation} \opindex{in} \opindex{not in} \noindent Notes: \begin{description} \item[(1)] Values of \var{n} less than \code{0} are treated as \code{0} (which yields an empty sequence of the same type as \var{s}). Note also that the copies are shallow; nested structures are not copied. This often haunts new Python programmers; consider: \begin{verbatim} >>> lists = [[]] * 3 >>> lists [[], [], []] >>> lists[0].append(3) >>> lists [[3], [3], [3]] \end{verbatim} What has happened is that \code{lists} is a list containing three copies of the list \code{[[]]} (a one-element list containing an empty list), but the contained list is shared by each copy. You can create a list of different lists this way: \begin{verbatim} >>> lists = [[] for i in range(3)] >>> lists[0].append(3) >>> lists[1].append(5) >>> lists[2].append(7) >>> lists [[3], [5], [7]] \end{verbatim} \item[(2)] If \var{i} or \var{j} is negative, the index is relative to the end of the string: \code{len(\var{s}) + \var{i}} or \code{len(\var{s}) + \var{j}} is substituted. But note that \code{-0} is still \code{0}. \item[(3)] The slice of \var{s} from \var{i} to \var{j} is defined as the sequence of items with index \var{k} such that \code{\var{i} <= \var{k} < \var{j}}. If \var{i} or \var{j} is greater than \code{len(\var{s})}, use \code{len(\var{s})}. If \var{i} is omitted, use \code{0}. If \var{j} is omitted, use \code{len(\var{s})}. If \var{i} is greater than or equal to \var{j}, the slice is empty. \end{description} \subsubsection{String Methods \label{string-methods}} These are the string methods which both 8-bit strings and Unicode objects support: \begin{methoddesc}[string]{capitalize}{} Return a copy of the string with only its first character capitalized. \end{methoddesc} \begin{methoddesc}[string]{center}{width} Return centered in a string of length \var{width}. Padding is done using spaces. \end{methoddesc} \begin{methoddesc}[string]{count}{sub\optional{, start\optional{, end}}} Return the number of occurrences of substring \var{sub} in string S\code{[\var{start}:\var{end}]}. Optional arguments \var{start} and \var{end} are interpreted as in slice notation. \end{methoddesc} \begin{methoddesc}[string]{decode}{\optional{encoding\optional{, errors}}} Decodes the string using the codec registered for \var{encoding}. \var{encoding} defaults to the default string encoding. \var{errors} may be given to set a different error handling scheme. The default is \code{'strict'}, meaning that encoding errors raise \exception{ValueError}. Other possible values are \code{'ignore'} and \code{replace'}. \versionadded{2.2} \end{methoddesc} \begin{methoddesc}[string]{encode}{\optional{encoding\optional{,errors}}} Return an encoded version of the string. Default encoding is the current default string encoding. \var{errors} may be given to set a different error handling scheme. The default for \var{errors} is \code{'strict'}, meaning that encoding errors raise a \exception{ValueError}. Other possible values are \code{'ignore'} and \code{'replace'}. \versionadded{2.0} \end{methoddesc} \begin{methoddesc}[string]{endswith}{suffix\optional{, start\optional{, end}}} Return true if the string ends with the specified \var{suffix}, otherwise return false. With optional \var{start}, test beginning at that position. With optional \var{end}, stop comparing at that position. \end{methoddesc} \begin{methoddesc}[string]{expandtabs}{\optional{tabsize}} Return a copy of the string where all tab characters are expanded using spaces. If \var{tabsize} is not given, a tab size of \code{8} characters is assumed. \end{methoddesc} \begin{methoddesc}[string]{find}{sub\optional{, start\optional{, end}}} Return the lowest index in the string where substring \var{sub} is found, such that \var{sub} is contained in the range [\var{start}, \var{end}). Optional arguments \var{start} and \var{end} are interpreted as in slice notation. Return \code{-1} if \var{sub} is not found. \end{methoddesc} \begin{methoddesc}[string]{index}{sub\optional{, start\optional{, end}}} Like \method{find()}, but raise \exception{ValueError} when the substring is not found. \end{methoddesc} \begin{methoddesc}[string]{isalnum}{} Return true if all characters in the string are alphanumeric and there is at least one character, false otherwise. \end{methoddesc} \begin{methoddesc}[string]{isalpha}{} Return true if all characters in the string are alphabetic and there is at least one character, false otherwise. \end{methoddesc} \begin{methoddesc}[string]{isdigit}{} Return true if there are only digit characters, false otherwise. \end{methoddesc} \begin{methoddesc}[string]{islower}{} Return true if all cased characters in the string are lowercase and there is at least one cased character, false otherwise. \end{methoddesc} \begin{methoddesc}[string]{isspace}{} Return true if there are only whitespace characters in the string and the string is not empty, false otherwise. \end{methoddesc} \begin{methoddesc}[string]{istitle}{} Return true if the string is a titlecased string: uppercase characters may only follow uncased characters and lowercase characters only cased ones. Return false otherwise. \end{methoddesc} \begin{methoddesc}[string]{isupper}{} Return true if all cased characters in the string are uppercase and there is at least one cased character, false otherwise. \end{methoddesc} \begin{methoddesc}[string]{join}{seq} Return a string which is the concatenation of the strings in the sequence \var{seq}. The separator between elements is the string providing this method. \end{methoddesc} \begin{methoddesc}[string]{ljust}{width} Return the string left justified in a string of length \var{width}. Padding is done using spaces. The original string is returned if \var{width} is less than \code{len(\var{s})}. \end{methoddesc} \begin{methoddesc}[string]{lower}{} Return a copy of the string converted to lowercase. \end{methoddesc} \begin{methoddesc}[string]{lstrip}{\optional{chars}} Return a copy of the string with leading characters removed. If \var{chars} is omitted or \code{None}, whitespace characters are removed. If given and not \code{None}, \var{chars} must be a string; the characters in the string will be stripped from the beginning of the string this method is called on. \versionchanged[Support for the \var{chars} argument]{2.2.2} \end{methoddesc} \begin{methoddesc}[string]{replace}{old, new\optional{, maxsplit}} Return a copy of the string with all occurrences of substring \var{old} replaced by \var{new}. If the optional argument \var{maxsplit} is given, only the first \var{maxsplit} occurrences are replaced. \end{methoddesc} \begin{methoddesc}[string]{rfind}{sub \optional{,start \optional{,end}}} Return the highest index in the string where substring \var{sub} is found, such that \var{sub} is contained within s[start,end]. Optional arguments \var{start} and \var{end} are interpreted as in slice notation. Return \code{-1} on failure. \end{methoddesc} \begin{methoddesc}[string]{rindex}{sub\optional{, start\optional{, end}}} Like \method{rfind()} but raises \exception{ValueError} when the substring \var{sub} is not found. \end{methoddesc} \begin{methoddesc}[string]{rjust}{width} Return the string right justified in a string of length \var{width}. Padding is done using spaces. The original string is returned if \var{width} is less than \code{len(\var{s})}. \end{methoddesc} \begin{methoddesc}[string]{rstrip}{\optional{chars}} Return a copy of the string with trailing characters removed. If \var{chars} is omitted or \code{None}, whitespace characters are removed. If given and not \code{None}, \var{chars} must be a string; the characters in the string will be stripped from the end of the string this method is called on. \versionchanged[Support for the \var{chars} argument]{2.2.2} \end{methoddesc} \begin{methoddesc}[string]{split}{\optional{sep \optional{,maxsplit}}} Return a list of the words in the string, using \var{sep} as the delimiter string. If \var{maxsplit} is given, at most \var{maxsplit} splits are done. If \var{sep} is not specified or \code{None}, any whitespace string is a separator. \end{methoddesc} \begin{methoddesc}[string]{splitlines}{\optional{keepends}} Return a list of the lines in the string, breaking at line boundaries. Line breaks are not included in the resulting list unless \var{keepends} is given and true. \end{methoddesc} \begin{methoddesc}[string]{startswith}{prefix\optional{, start\optional{, end}}} Return true if string starts with the \var{prefix}, otherwise return false. With optional \var{start}, test string beginning at that position. With optional \var{end}, stop comparing string at that position. \end{methoddesc} \begin{methoddesc}[string]{strip}{\optional{chars}} Return a copy of the string with leading and trailing characters removed. If \var{chars} is omitted or \code{None}, whitespace characters are removed. If given and not \code{None}, \var{chars} must be a string; the characters in the string will be stripped from the both ends of the string this method is called on. \versionchanged[Support for the \var{chars} argument]{2.2.2} \end{methoddesc} \begin{methoddesc}[string]{swapcase}{} Return a copy of the string with uppercase characters converted to lowercase and vice versa. \end{methoddesc} \begin{methoddesc}[string]{title}{} Return a titlecased version of the string: words start with uppercase characters, all remaining cased characters are lowercase. \end{methoddesc} \begin{methoddesc}[string]{translate}{table\optional{, deletechars}} Return a copy of the string where all characters occurring in the optional argument \var{deletechars} are removed, and the remaining characters have been mapped through the given translation table, which must be a string of length 256. \end{methoddesc} \begin{methoddesc}[string]{upper}{} Return a copy of the string converted to uppercase. \end{methoddesc} \begin{methoddesc}[string]{zfill}{width} Return the numeric string left filled with zeros in a string of length \var{width}. The original string is returned if \var{width} is less than \code{len(\var{s})}. \versionadded{2.2.2} \end{methoddesc} \subsubsection{String Formatting Operations \label{typesseq-strings}} \index{formatting, string (\%{})} \index{interpolation, string (\%{})} \index{string!formatting} \index{string!interpolation} \index{printf-style formatting} \index{sprintf-style formatting} \index{\protect\%{} formatting} \index{\protect\%{} interpolation} String and Unicode objects have one unique built-in operation: the \code{\%} operator (modulo). This is also known as the string \emph{formatting} or \emph{interpolation} operator. Given \code{\var{format} \% \var{values}} (where \var{format} is a string or Unicode object), \code{\%} conversion specifications in \var{format} are replaced with zero or more elements of \var{values}. The effect is similar to the using \cfunction{sprintf()} in the C language. If \var{format} is a Unicode object, or if any of the objects being converted using the \code{\%s} conversion are Unicode objects, the result will be a Unicode object as well. If \var{format} requires a single argument, \var{values} may be a single non-tuple object. \footnote{A tuple object in this case should be a singleton.} Otherwise, \var{values} must be a tuple with exactly the number of items specified by the format string, or a single mapping object (for example, a dictionary). A conversion specifier contains two or more characters and has the following components, which must occur in this order: \begin{enumerate} \item The \character{\%} character, which marks the start of the specifier. \item Mapping key value (optional), consisting of an identifier in parentheses (for example, \code{(somename)}). \item Conversion flags (optional), which affect the result of some conversion types. \item Minimum field width (optional). If specified as an \character{*} (asterisk), the actual width is read from the next element of the tuple in \var{values}, and the object to convert comes after the minimum field width and optional precision. \item Precision (optional), given as a \character{.} (dot) followed by the precision. If specified as \character{*} (an asterisk), the actual width is read from the next element of the tuple in \var{values}, and the value to convert comes after the precision. \item Length modifier (optional). \item Conversion type. \end{enumerate} If the right argument is a dictionary (or any kind of mapping), then the formats in the string \emph{must} have a parenthesized key into that dictionary inserted immediately after the \character{\%} character, and each format formats the corresponding entry from the mapping. For example: \begin{verbatim} >>> count = 2 >>> language = 'Python' >>> print '%(language)s has %(count)03d quote types.' % vars() Python has 002 quote types. \end{verbatim} In this case no \code{*} specifiers may occur in a format (since they require a sequential parameter list). The conversion flag characters are: \begin{tableii}{c|l}{character}{Flag}{Meaning} \lineii{\#}{The value conversion will use the ``alternate form'' (where defined below).} \lineii{0}{The conversion will be zero padded for numeric values.} \lineii{-}{The converted value is left adjusted (overrides the \character{0} conversion if both are given).} \lineii{{~}}{(a space) A blank should be left before a positive number (or empty string) produced by a signed conversion.} \lineii{+}{A sign character (\character{+} or \character{-}) will precede the conversion (overrides a "space" flag).} \end{tableii} The length modifier may be \code{h}, \code{l}, and \code{L} may be present, but are ignored as they are not necessary for Python. The conversion types are: \begin{tableiii}{c|l|c}{character}{Conversion}{Meaning}{Notes} \lineiii{d}{Signed integer decimal.}{} \lineiii{i}{Signed integer decimal.}{} \lineiii{o}{Unsigned octal.}{(1)} \lineiii{u}{Unsigned decimal.}{} \lineiii{x}{Unsigned hexidecimal (lowercase).}{(2)} \lineiii{X}{Unsigned hexidecimal (uppercase).}{(2)} \lineiii{e}{Floating point exponential format (lowercase).}{} \lineiii{E}{Floating point exponential format (uppercase).}{} \lineiii{f}{Floating point decimal format.}{} \lineiii{F}{Floating point decimal format.}{} \lineiii{g}{Same as \character{e} if exponent is greater than -4 or less than precision, \character{f} otherwise.}{} \lineiii{G}{Same as \character{E} if exponent is greater than -4 or less than precision, \character{F} otherwise.}{} \lineiii{c}{Single character (accepts integer or single character string).}{} \lineiii{r}{String (converts any python object using \function{repr()}).}{(3)} \lineiii{s}{String (converts any python object using \function{str()}).}{(4)} \lineiii{\%}{No argument is converted, results in a \character{\%} character in the result.}{} \end{tableiii} \noindent Notes: \begin{description} \item[(1)] The alternate form causes a leading zero (\character{0}) to be inserted between left-hand padding and the formatting of the number if the leading character of the result is not already a zero. \item[(2)] The alternate form causes a leading \code{'0x'} or \code{'0X'} (depending on whether the \character{x} or \character{X} format was used) to be inserted between left-hand padding and the formatting of the number if the leading character of the result is not already a zero. \item[(3)] The \code{\%r} conversion was added in Python 2.0. \item[(4)] If the object or format provided is a \class{unicode} string, the resulting string will also be \class{unicode}. \end{description} % XXX Examples? Since Python strings have an explicit length, \code{\%s} conversions do not assume that \code{'\e0'} is the end of the string. For safety reasons, floating point precisions are clipped to 50; \code{\%f} conversions for numbers whose absolute value is over 1e25 are replaced by \code{\%g} conversions.\footnote{ These numbers are fairly arbitrary. They are intended to avoid printing endless strings of meaningless digits without hampering correct use and without having to know the exact precision of floating point values on a particular machine. } All other errors raise exceptions. Additional string operations are defined in standard modules \refmodule{string}\refstmodindex{string} and \refmodule{re}.\refstmodindex{re} \subsubsection{XRange Type \label{typesseq-xrange}} The xrange\obindex{xrange} type is an immutable sequence which is commonly used for looping. The advantage of the xrange type is that an xrange object will always take the same amount of memory, no matter the size of the range it represents. There are no consistent performance advantages. XRange objects have very little behavior: they only support indexing and the \function{len()} function. \subsubsection{Mutable Sequence Types \label{typesseq-mutable}} List objects support additional operations that allow in-place modification of the object. These operations would be supported by other mutable sequence types (when added to the language) as well. Strings and tuples are immutable sequence types and such objects cannot be modified once created. The following operations are defined on mutable sequence types (where \var{x} is an arbitrary object): \indexiii{mutable}{sequence}{types} \obindex{list} \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} \lineiii{\var{s}[\var{i}] = \var{x}} {item \var{i} of \var{s} is replaced by \var{x}}{} \lineiii{\var{s}[\var{i}:\var{j}] = \var{t}} {slice of \var{s} from \var{i} to \var{j} is replaced by \var{t}}{} \lineiii{del \var{s}[\var{i}:\var{j}]} {same as \code{\var{s}[\var{i}:\var{j}] = []}}{} \lineiii{\var{s}.append(\var{x})} {same as \code{\var{s}[len(\var{s}):len(\var{s})] = [\var{x}]}}{(1)} \lineiii{\var{s}.extend(\var{x})} {same as \code{\var{s}[len(\var{s}):len(\var{s})] = \var{x}}}{(2)} \lineiii{\var{s}.count(\var{x})} {return number of \var{i}'s for which \code{\var{s}[\var{i}] == \var{x}}}{} \lineiii{\var{s}.index(\var{x})} {return smallest \var{i} such that \code{\var{s}[\var{i}] == \var{x}}}{(3)} \lineiii{\var{s}.insert(\var{i}, \var{x})} {same as \code{\var{s}[\var{i}:\var{i}] = [\var{x}]} if \code{\var{i} >= 0}}{(4)} \lineiii{\var{s}.pop(\optional{\var{i}})} {same as \code{\var{x} = \var{s}[\var{i}]; del \var{s}[\var{i}]; return \var{x}}}{(5)} \lineiii{\var{s}.remove(\var{x})} {same as \code{del \var{s}[\var{s}.index(\var{x})]}}{(3)} \lineiii{\var{s}.reverse()} {reverses the items of \var{s} in place}{(6)} \lineiii{\var{s}.sort(\optional{\var{cmpfunc}})} {sort the items of \var{s} in place}{(6), (7), (8), (9)} \end{tableiii} \indexiv{operations on}{mutable}{sequence}{types} \indexiii{operations on}{sequence}{types} \indexiii{operations on}{list}{type} \indexii{subscript}{assignment} \indexii{slice}{assignment} \stindex{del} \withsubitem{(list method)}{ \ttindex{append()}\ttindex{extend()}\ttindex{count()}\ttindex{index()} \ttindex{insert()}\ttindex{pop()}\ttindex{remove()}\ttindex{reverse()} \ttindex{sort()}} \noindent Notes: \begin{description} \item[(1)] The C implementation of Python has historically accepted multiple parameters and implicitly joined them into a tuple; this no longer works in Python 2.0. Use of this misfeature has been deprecated since Python 1.4. \item[(2)] Raises an exception when \var{x} is not an iterable object. \item[(3)] Raises \exception{ValueError} when \var{x} is not found in \var{s}. \item[(4)] When a negative index is passed as the first parameter to the \method{insert()} method, the new element is prepended to the sequence. \item[(5)] The \method{pop()} method is only supported by the list and array types. The optional argument \var{i} defaults to \code{-1}, so that by default the last item is removed and returned. \item[(6)] The \method{sort()} and \method{reverse()} methods modify the list in place for economy of space when sorting or reversing a large list. To remind you that they operate by side effect, they don't return the sorted or reversed list. \item[(7)] The \method{sort()} method takes an optional argument specifying a comparison function of two arguments (list items) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument. Note that this slows the sorting process down considerably; for example, to sort a list in reverse order it is much faster to call \method{sort()} followed by \method{reverse()} than to use \method{sort()} with a comparison function that reverses the ordering of the elements. As an example of using the \var{cmpfunc} argument to the \method{sort()} method, consider sorting a list of sequences by the second element of that list: \begin{verbatim} def mycmp(a, b): return cmp(a[1], b[1]) mylist.sort(mycmp) \end{verbatim} A more time-efficient approach for reasonably-sized data structures can often be used: \begin{verbatim} tmplist = [(x[1], x) for x in mylist] tmplist.sort() mylist = [x for (key, x) in tmplist] \end{verbatim} \item[(8)] Whether the \method{sort()} method is stable is not defined by the language (a sort is stable if it guarantees not to change the relative order of elements that compare equal). In the C implementation of Python, sorts were stable only by accident through Python 2.2. Code that intends to be portable across implementations and versions must not rely on stability. \item[(9)] While a list is being sorted, the effect of attempting to mutate, or even inspect, the list is undefined. \end{description} \subsection{Mapping Types \label{typesmapping}} \obindex{mapping} \obindex{dictionary} A \dfn{mapping} object maps values of one type (the key type) to arbitrary objects. Mappings are mutable objects. There is currently only one standard mapping type, the \dfn{dictionary}. A dictionary's keys are almost arbitrary values. The only types of values not acceptable as keys are values containing lists or dictionaries or other mutable types that are compared by value rather than by object identity. Numeric types used for keys obey the normal rules for numeric comparison: if two numbers compare equal (e.g. \code{1} and \code{1.0}) then they can be used interchangeably to index the same dictionary entry. Dictionaries are created by placing a comma-separated list of \code{\var{key}: \var{value}} pairs within braces, for example: \code{\{'jack': 4098, 'sjoerd': 4127\}} or \code{\{4098: 'jack', 4127: 'sjoerd'\}}. The following operations are defined on mappings (where \var{a} and \var{b} are mappings, \var{k} is a key, and \var{v} and \var{x} are arbitrary objects): \indexiii{operations on}{mapping}{types} \indexiii{operations on}{dictionary}{type} \stindex{del} \bifuncindex{len} \withsubitem{(dictionary method)}{ \ttindex{clear()} \ttindex{copy()} \ttindex{has_key()} \ttindex{items()} \ttindex{keys()} \ttindex{update()} \ttindex{values()} \ttindex{get()} \ttindex{setdefault()} \ttindex{popitem()} \ttindex{iteritems()} \ttindex{iterkeys()} \ttindex{itervalues()}} \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} \lineiii{len(\var{a})}{the number of items in \var{a}}{} \lineiii{\var{a}[\var{k}]}{the item of \var{a} with key \var{k}}{(1)} \lineiii{\var{a}[\var{k}] = \var{v}} {set \code{\var{a}[\var{k}]} to \var{v}} {} \lineiii{del \var{a}[\var{k}]} {remove \code{\var{a}[\var{k}]} from \var{a}} {(1)} \lineiii{\var{a}.clear()}{remove all items from \code{a}}{} \lineiii{\var{a}.copy()}{a (shallow) copy of \code{a}}{} \lineiii{\var{a}.has_key(\var{k})} {\code{1} if \var{a} has a key \var{k}, else \code{0}} {} \lineiii{\var{k} \code{in} \var{a}} {Equivalent to \var{a}.has_key(\var{k})} {(2)} \lineiii{\var{k} not in \var{a}} {Equivalent to \code{not} \var{a}.has_key(\var{k})} {(2)} \lineiii{\var{a}.items()} {a copy of \var{a}'s list of (\var{key}, \var{value}) pairs} {(3)} \lineiii{\var{a}.keys()}{a copy of \var{a}'s list of keys}{(3)} \lineiii{\var{a}.update(\var{b})} {\code{for k in \var{b}.keys(): \var{a}[k] = \var{b}[k]}} {} \lineiii{\var{a}.values()}{a copy of \var{a}'s list of values}{(3)} \lineiii{\var{a}.get(\var{k}\optional{, \var{x}})} {\code{\var{a}[\var{k}]} if \code{\var{k} in \var{a}}, else \var{x}} {(4)} \lineiii{\var{a}.setdefault(\var{k}\optional{, \var{x}})} {\code{\var{a}[\var{k}]} if \code{\var{k} in \var{a}}, else \var{x} (also setting it)} {(5)} \lineiii{\var{a}.popitem()} {remove and return an arbitrary (\var{key}, \var{value}) pair} {(6)} \lineiii{\var{a}.iteritems()} {return an iterator over (\var{key}, \var{value}) pairs} {(2), (3)} \lineiii{\var{a}.iterkeys()} {return an iterator over the mapping's keys} {(2), (3)} \lineiii{\var{a}.itervalues()} {return an iterator over the mapping's values} {(2), (3)} \end{tableiii} \noindent Notes: \begin{description} \item[(1)] Raises a \exception{KeyError} exception if \var{k} is not in the map. \item[(2)] \versionadded{2.2} \item[(3)] Keys and values are listed in random order. If \method{items()}, \method{keys()}, \method{values()}, \method{iteritems()}, \method{iterkeys()}, and \method{itervalues()} are called with no intervening modifications to the dictionary, the lists will directly correspond. This allows the creation of \code{(\var{value}, \var{key})} pairs using \function{zip()}: \samp{pairs = zip(\var{a}.values(), \var{a}.keys())}. The same relationship holds for the \method{iterkeys()} and \method{itervalues()} methods: \samp{pairs = zip(\var{a}.itervalues(), \var{a}.iterkeys())} provides the same value for \code{pairs}. Another way to create the same list is \samp{pairs = [(v, k) for (k, v) in \var{a}.iteritems()]}. \item[(4)] Never raises an exception if \var{k} is not in the map, instead it returns \var{x}. \var{x} is optional; when \var{x} is not provided and \var{k} is not in the map, \code{None} is returned. \item[(5)] \function{setdefault()} is like \function{get()}, except that if \var{k} is missing, \var{x} is both returned and inserted into the dictionary as the value of \var{k}. \item[(6)] \function{popitem()} is useful to destructively iterate over a dictionary, as often used in set algorithms. \end{description} \subsection{File Objects \label{bltin-file-objects}} File objects\obindex{file} are implemented using C's \code{stdio} package and can be created with the built-in constructor \function{file()}\bifuncindex{file} described in section \ref{built-in-funcs}, ``Built-in Functions.''\footnote{\function{file()} is new in Python 2.2. The older built-in \function{open()} is an alias for \function{file()}.} They are also returned by some other built-in functions and methods, such as \function{os.popen()} and \function{os.fdopen()} and the \method{makefile()} method of socket objects. \refstmodindex{os} \refbimodindex{socket} When a file operation fails for an I/O-related reason, the exception \exception{IOError} is raised. This includes situations where the operation is not defined for some reason, like \method{seek()} on a tty device or writing a file opened for reading. Files have the following methods: \begin{methoddesc}[file]{close}{} Close the file. A closed file cannot be read or written anymore. Any operation which requires that the file be open will raise a \exception{ValueError} after the file has been closed. Calling \method{close()} more than once is allowed. \end{methoddesc} \begin{methoddesc}[file]{flush}{} Flush the internal buffer, like \code{stdio}'s \cfunction{fflush()}. This may be a no-op on some file-like objects. \end{methoddesc} \begin{methoddesc}[file]{isatty}{} Return true if the file is connected to a tty(-like) device, else false. \note{If a file-like object is not associated with a real file, this method should \emph{not} be implemented.} \end{methoddesc} \begin{methoddesc}[file]{fileno}{} \index{file descriptor} \index{descriptor, file} Return the integer ``file descriptor'' that is used by the underlying implementation to request I/O operations from the operating system. This can be useful for other, lower level interfaces that use file descriptors, such as the \refmodule{fcntl}\refbimodindex{fcntl} module or \function{os.read()} and friends. \note{File-like objects which do not have a real file descriptor should \emph{not} provide this method!} \end{methoddesc} \begin{methoddesc}[file]{read}{\optional{size}} Read at most \var{size} bytes from the file (less if the read hits \EOF{} before obtaining \var{size} bytes). If the \var{size} argument is negative or omitted, read all data until \EOF{} is reached. The bytes are returned as a string object. An empty string is returned when \EOF{} is encountered immediately. (For certain files, like ttys, it makes sense to continue reading after an \EOF{} is hit.) Note that this method may call the underlying C function \cfunction{fread()} more than once in an effort to acquire as close to \var{size} bytes as possible. Also note that when in non-blocking mode, less data than what was requested may be returned, even if no \var{size} parameter was given. \end{methoddesc} \begin{methoddesc}[file]{readline}{\optional{size}} Read one entire line from the file. A trailing newline character is kept in the string\footnote{ The advantage of leaving the newline on is that an empty string can be returned to mean \EOF{} without being ambiguous. Another advantage is that (in cases where it might matter, for example. if you want to make an exact copy of a file while scanning its lines) you can tell whether the last line of a file ended in a newline or not (yes this happens!). } (but may be absent when a file ends with an incomplete line). If the \var{size} argument is present and non-negative, it is a maximum byte count (including the trailing newline) and an incomplete line may be returned. An empty string is returned when \EOF{} is hit immediately. \note{Unlike \code{stdio}'s \cfunction{fgets()}, the returned string contains null characters (\code{'\e 0'}) if they occurred in the input.} \end{methoddesc} \begin{methoddesc}[file]{readlines}{\optional{sizehint}} Read until \EOF{} using \method{readline()} and return a list containing the lines thus read. If the optional \var{sizehint} argument is present, instead of reading up to \EOF, whole lines totalling approximately \var{sizehint} bytes (possibly after rounding up to an internal buffer size) are read. Objects implementing a file-like interface may choose to ignore \var{sizehint} if it cannot be implemented, or cannot be implemented efficiently. \end{methoddesc} \begin{methoddesc}[file]{xreadlines}{} Equivalent to \function{xreadlines.xreadlines(\var{file})}.\refstmodindex{xreadlines} (See the \refmodule{xreadlines} module for more information.) \versionadded{2.1} \end{methoddesc} \begin{methoddesc}[file]{seek}{offset\optional{, whence}} Set the file's current position, like \code{stdio}'s \cfunction{fseek()}. The \var{whence} argument is optional and defaults to \code{0} (absolute file positioning); other values are \code{1} (seek relative to the current position) and \code{2} (seek relative to the file's end). There is no return value. Note that if the file is opened for appending (mode \code{'a'} or \code{'a+'}), any \method{seek()} operations will be undone at the next write. If the file is only opened for writing in append mode (mode \code{'a'}), this method is essentially a no-op, but it remains useful for files opened in append mode with reading enabled (mode \code{'a+'}). \end{methoddesc} \begin{methoddesc}[file]{tell}{} Return the file's current position, like \code{stdio}'s \cfunction{ftell()}. \end{methoddesc} \begin{methoddesc}[file]{truncate}{\optional{size}} Truncate the file's size. If the optional \var{size} argument present, the file is truncated to (at most) that size. The size defaults to the current position. Availability of this function depends on the operating system version (for example, not all \UNIX{} versions support this operation). \end{methoddesc} \begin{methoddesc}[file]{write}{str} Write a string to the file. There is no return value. Due to buffering, the string may not actually show up in the file until the \method{flush()} or \method{close()} method is called. \end{methoddesc} \begin{methoddesc}[file]{writelines}{sequence} Write a sequence of strings to the file. The sequence can be any iterable object producing strings, typically a list of strings. There is no return value. (The name is intended to match \method{readlines()}; \method{writelines()} does not add line separators.) \end{methoddesc} Files support the iterator protocol. Each iteration returns the same result as \code{\var{file}.readline()}, and iteration ends when the \method{readline()} method returns an empty string. File objects also offer a number of other interesting attributes. These are not required for file-like objects, but should be implemented if they make sense for the particular object. \begin{memberdesc}[file]{closed} Boolean indicating the current state of the file object. This is a read-only attribute; the \method{close()} method changes the value. It may not be available on all file-like objects. \end{memberdesc} \begin{memberdesc}[file]{mode} The I/O mode for the file. If the file was created using the \function{open()} built-in function, this will be the value of the \var{mode} parameter. This is a read-only attribute and may not be present on all file-like objects. \end{memberdesc} \begin{memberdesc}[file]{name} If the file object was created using \function{open()}, the name of the file. Otherwise, some string that indicates the source of the file object, of the form \samp{<\mbox{\ldots}>}. This is a read-only attribute and may not be present on all file-like objects. \end{memberdesc} \begin{memberdesc}[file]{softspace} Boolean that indicates whether a space character needs to be printed before another value when using the \keyword{print} statement. Classes that are trying to simulate a file object should also have a writable \member{softspace} attribute, which should be initialized to zero. This will be automatic for most classes implemented in Python (care may be needed for objects that override attribute access); types implemented in C will have to provide a writable \member{softspace} attribute. \note{This attribute is not used to control the \keyword{print} statement, but to allow the implementation of \keyword{print} to keep track of its internal state.} \end{memberdesc} \subsection{Other Built-in Types \label{typesother}} The interpreter supports several other kinds of objects. Most of these support only one or two operations. \subsubsection{Modules \label{typesmodules}} The only special operation on a module is attribute access: \code{\var{m}.\var{name}}, where \var{m} is a module and \var{name} accesses a name defined in \var{m}'s symbol table. Module attributes can be assigned to. (Note that the \keyword{import} statement is not, strictly speaking, an operation on a module object; \code{import \var{foo}} does not require a module object named \var{foo} to exist, rather it requires an (external) \emph{definition} for a module named \var{foo} somewhere.) A special member of every module is \member{__dict__}. This is the dictionary containing the module's symbol table. Modifying this dictionary will actually change the module's symbol table, but direct assignment to the \member{__dict__} attribute is not possible (you can write \code{\var{m}.__dict__['a'] = 1}, which defines \code{\var{m}.a} to be \code{1}, but you can't write \code{\var{m}.__dict__ = \{\}}). Modules built into the interpreter are written like this: \code{}. If loaded from a file, they are written as \code{}. \subsubsection{Classes and Class Instances \label{typesobjects}} \nodename{Classes and Instances} See chapters 3 and 7 of the \citetitle[../ref/ref.html]{Python Reference Manual} for these. \subsubsection{Functions \label{typesfunctions}} Function objects are created by function definitions. The only operation on a function object is to call it: \code{\var{func}(\var{argument-list})}. There are really two flavors of function objects: built-in functions and user-defined functions. Both support the same operation (to call the function), but the implementation is different, hence the different object types. The implementation adds two special read-only attributes: \code{\var{f}.func_code} is a function's \dfn{code object}\obindex{code} (see below) and \code{\var{f}.func_globals} is the dictionary used as the function's global namespace (this is the same as \code{\var{m}.__dict__} where \var{m} is the module in which the function \var{f} was defined). Function objects also support getting and setting arbitrary attributes, which can be used to, e.g. attach metadata to functions. Regular attribute dot-notation is used to get and set such attributes. \emph{Note that the current implementation only supports function attributes on user-defined functions. Function attributes on built-in functions may be supported in the future.} Functions have another special attribute \code{\var{f}.__dict__} (a.k.a. \code{\var{f}.func_dict}) which contains the namespace used to support function attributes. \code{__dict__} and \code{func_dict} can be accessed directly or set to a dictionary object. A function's dictionary cannot be deleted. \subsubsection{Methods \label{typesmethods}} \obindex{method} Methods are functions that are called using the attribute notation. There are two flavors: built-in methods (such as \method{append()} on lists) and class instance methods. Built-in methods are described with the types that support them. The implementation adds two special read-only attributes to class instance methods: \code{\var{m}.im_self} is the object on which the method operates, and \code{\var{m}.im_func} is the function implementing the method. Calling \code{\var{m}(\var{arg-1}, \var{arg-2}, \textrm{\ldots}, \var{arg-n})} is completely equivalent to calling \code{\var{m}.im_func(\var{m}.im_self, \var{arg-1}, \var{arg-2}, \textrm{\ldots}, \var{arg-n})}. Class instance methods are either \emph{bound} or \emph{unbound}, referring to whether the method was accessed through an instance or a class, respectively. When a method is unbound, its \code{im_self} attribute will be \code{None} and if called, an explicit \code{self} object must be passed as the first argument. In this case, \code{self} must be an instance of the unbound method's class (or a subclass of that class), otherwise a \code{TypeError} is raised. Like function objects, methods objects support getting arbitrary attributes. However, since method attributes are actually stored on the underlying function object (\code{meth.im_func}), setting method attributes on either bound or unbound methods is disallowed. Attempting to set a method attribute results in a \code{TypeError} being raised. In order to set a method attribute, you need to explicitly set it on the underlying function object: \begin{verbatim} class C: def method(self): pass c = C() c.method.im_func.whoami = 'my name is c' \end{verbatim} See the \citetitle[../ref/ref.html]{Python Reference Manual} for more information. \subsubsection{Code Objects \label{bltin-code-objects}} \obindex{code} Code objects are used by the implementation to represent ``pseudo-compiled'' executable Python code such as a function body. They differ from function objects because they don't contain a reference to their global execution environment. Code objects are returned by the built-in \function{compile()} function and can be extracted from function objects through their \member{func_code} attribute. \bifuncindex{compile} \withsubitem{(function object attribute)}{\ttindex{func_code}} A code object can be executed or evaluated by passing it (instead of a source string) to the \keyword{exec} statement or the built-in \function{eval()} function. \stindex{exec} \bifuncindex{eval} See the \citetitle[../ref/ref.html]{Python Reference Manual} for more information. \subsubsection{Type Objects \label{bltin-type-objects}} Type objects represent the various object types. An object's type is accessed by the built-in function \function{type()}. There are no special operations on types. The standard module \module{types} defines names for all standard built-in types. \bifuncindex{type} \refstmodindex{types} Types are written like this: \code{}. \subsubsection{The Null Object \label{bltin-null-object}} This object is returned by functions that don't explicitly return a value. It supports no special operations. There is exactly one null object, named \code{None} (a built-in name). It is written as \code{None}. \subsubsection{The Ellipsis Object \label{bltin-ellipsis-object}} This object is used by extended slice notation (see the \citetitle[../ref/ref.html]{Python Reference Manual}). It supports no special operations. There is exactly one ellipsis object, named \constant{Ellipsis} (a built-in name). It is written as \code{Ellipsis}. \subsubsection{Internal Objects \label{typesinternal}} See the \citetitle[../ref/ref.html]{Python Reference Manual} for this information. It describes stack frame objects, traceback objects, and slice objects. \subsection{Special Attributes \label{specialattrs}} The implementation adds a few special read-only attributes to several object types, where they are relevant: \begin{memberdesc}[object]{__dict__} A dictionary or other mapping object used to store an object's (writable) attributes. \end{memberdesc} \begin{memberdesc}[object]{__methods__} \deprecated{2.2}{Use the built-in function \function{dir()} to get a list of an object's attributes. This attribute is no longer available.} \end{memberdesc} \begin{memberdesc}[object]{__members__} \deprecated{2.2}{Use the built-in function \function{dir()} to get a list of an object's attributes. This attribute is no longer available.} \end{memberdesc} \begin{memberdesc}[instance]{__class__} The class to which a class instance belongs. \end{memberdesc} \begin{memberdesc}[class]{__bases__} The tuple of base classes of a class object. If there are no base classes, this will be an empty tuple. \end{memberdesc}