"""File-like objects that read from or write to a string buffer. This implements (nearly) all stdio methods. f = StringIO() # ready for writing f = StringIO(buf) # ready for reading f.close() # explicitly release resources held flag = f.isatty() # always false pos = f.tell() # get current position f.seek(pos) # set current position f.seek(pos, mode) # mode 0: absolute; 1: relative; 2: relative to EOF buf = f.read() # read until EOF buf = f.read(n) # read up to n bytes buf = f.readline() # read until end of line ('\n') or EOF list = f.readlines()# list of f.readline() results until EOF f.truncate([size]) # truncate file at to at most size (default: current pos) f.write(buf) # write at current position f.writelines(list) # for line in list: f.write(line) f.getvalue() # return whole file's contents as a string Notes: - Using a real file is often faster (but less convenient). - There's also a much faster implementation in C, called cStringIO, but it's not subclassable. - fileno() is left unimplemented so that code which uses it triggers an exception early. - Seeking far beyond EOF and then writing will insert real null bytes that occupy space in the buffer. - There's a simple test set (see end of this file). """ import types try: from errno import EINVAL except ImportError: EINVAL = 22 __all__ = ["StringIO"] class StringIO: """class StringIO([buffer]) When a StringIO object is created, it can be initialized to an existing string by passing the string to the constructor. If no string is given, the StringIO will start empty. The StringIO object can accept either Unicode or 8-bit strings, but mixing the two may take some care. If both are used, 8-bit strings that cannot be interpreted as 7-bit ASCII (that use the 8th bit) will cause a UnicodeError to be raised when getvalue() is called. """ def __init__(self, buf = ''): # Force self.buf to be a string or unicode if type(buf) not in types.StringTypes: buf = str(buf) self.buf = buf self.len = len(buf) self.buflist = [] self.pos = 0 self.closed = 0 self.softspace = 0 def __iter__(self): return iter(self.readline, '') def close(self): """Free the memory buffer.""" if not self.closed: self.closed = 1 del self.buf, self.pos def isatty(self): if self.closed: raise ValueError, "I/O operation on closed file" return 0 def seek(self, pos, mode = 0): if self.closed: raise ValueError, "I/O operation on closed file" if self.buflist: self.buf += ''.join(self.buflist) self.buflist = [] if mode == 1: pos += self.pos elif mode == 2: pos += self.len self.pos = max(0, pos) def tell(self): if self.closed: raise ValueError, "I/O operation on closed file" return self.pos def read(self, n = -1): if self.closed: raise ValueError, "I/O operation on closed file" if self.buflist: self.buf += ''.join(self.buflist) self.buflist = [] if n < 0: newpos = self.len else: newpos = min(self.pos+n, self.len) r = self.buf[self.pos:newpos] self.pos = newpos return r def readline(self, length=None): if self.closed: raise ValueError, "I/O operation on closed file" if self.buflist: self.buf += ''.join(self.buflist) self.buflist = [] i = self.buf.find('\n', self.pos) if i < 0: newpos = self.len else: newpos = i+1 if length is not None: if self.pos + length < newpos: newpos = self.pos + length r = self.buf[self.pos:newpos] self.pos = newpos return r def readlines(self, sizehint = 0): total = 0 lines = [] line = self.readline() while line: lines.append(line) total += len(line) if 0 < sizehint <= total: break line = self.readline() return lines def truncate(self, size=None): if self.closed: raise ValueError, "I/O operation on closed file" if size is None: size = self.pos elif size < 0: raise IOError(EINVAL, "Negative size not allowed") elif size < self.pos: self.pos = size self.buf = self.getvalue()[:size] def write(self, s): if self.closed: raise ValueError, "I/O operation on closed file" if not s: return # Force s to be a string or unicode if type(s) not in types.StringTypes: s = str(s) if self.pos > self.len: self.buflist.append('\0'*(self.pos - self.len)) self.len = self.pos newpos = self.pos + len(s) if self.pos < self.len: if self.buflist: self.buf += ''.join(self.buflist) self.buflist = [] self.buflist = [self.buf[:self.pos], s, self.buf[newpos:]] self.buf = '' if newpos > self.len: self.len = newpos else: self.buflist.append(s) self.len = newpos self.pos = newpos def writelines(self, list): self.write(''.join(list)) def flush(self): if self.closed: raise ValueError, "I/O operation on closed file" def getvalue(self): """ Retrieve the entire contents of the "file" at any time before the StringIO object's close() method is called. The StringIO object can accept either Unicode or 8-bit strings, but mixing the two may take some care. If both are used, 8-bit strings that cannot be interpreted as 7-bit ASCII (that use the 8th bit) will cause a UnicodeError to be raised when getvalue() is called. """ if self.buflist: self.buf += ''.join(self.buflist) self.buflist = [] return self.buf # A little test suite def test(): import sys if sys.argv[1:]: file = sys.argv[1] else: file = '/etc/passwd' lines = open(file, 'r').readlines() text = open(file, 'r').read() f = StringIO() for line in lines[:-2]: f.write(line) f.writelines(lines[-2:]) if f.getvalue() != text: raise RuntimeError, 'write failed' length = f.tell() print 'File length =', length f.seek(len(lines[0])) f.write(lines[1]) f.seek(0) print 'First line =', `f.readline()` here = f.tell() line = f.readline() print 'Second line =', `line` f.seek(-len(line), 1) line2 = f.read(len(line)) if line != line2: raise RuntimeError, 'bad result after seek back' f.seek(len(line2), 1) list = f.readlines() line = list[-1] f.seek(f.tell() - len(line)) line2 = f.read() if line != line2: raise RuntimeError, 'bad result after seek back from EOF' print 'Read', len(list), 'more lines' print 'File length =', f.tell() if f.tell() != length: raise RuntimeError, 'bad length' f.close() if __name__ == '__main__': test()