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325 lines
10 KiB
Python
325 lines
10 KiB
Python
r"""File-like objects that read from or write to a string buffer.
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This implements (nearly) all stdio methods.
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f = StringIO() # ready for writing
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f = StringIO(buf) # ready for reading
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f.close() # explicitly release resources held
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flag = f.isatty() # always false
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pos = f.tell() # get current position
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f.seek(pos) # set current position
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f.seek(pos, mode) # mode 0: absolute; 1: relative; 2: relative to EOF
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buf = f.read() # read until EOF
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buf = f.read(n) # read up to n bytes
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buf = f.readline() # read until end of line ('\n') or EOF
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list = f.readlines()# list of f.readline() results until EOF
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f.truncate([size]) # truncate file at to at most size (default: current pos)
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f.write(buf) # write at current position
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f.writelines(list) # for line in list: f.write(line)
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f.getvalue() # return whole file's contents as a string
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Notes:
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- Using a real file is often faster (but less convenient).
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- There's also a much faster implementation in C, called cStringIO, but
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it's not subclassable.
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- fileno() is left unimplemented so that code which uses it triggers
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an exception early.
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- Seeking far beyond EOF and then writing will insert real null
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bytes that occupy space in the buffer.
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- There's a simple test set (see end of this file).
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"""
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try:
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from errno import EINVAL
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except ImportError:
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EINVAL = 22
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__all__ = ["StringIO"]
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def _complain_ifclosed(closed):
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if closed:
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raise ValueError, "I/O operation on closed file"
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class StringIO:
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"""class StringIO([buffer])
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When a StringIO object is created, it can be initialized to an existing
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string by passing the string to the constructor. If no string is given,
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the StringIO will start empty.
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The StringIO object can accept either Unicode or 8-bit strings, but
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mixing the two may take some care. If both are used, 8-bit strings that
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cannot be interpreted as 7-bit ASCII (that use the 8th bit) will cause
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a UnicodeError to be raised when getvalue() is called.
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"""
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def __init__(self, buf = ''):
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# Force self.buf to be a string or unicode
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if not isinstance(buf, basestring):
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buf = str(buf)
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self.buf = buf
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self.len = len(buf)
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self.buflist = []
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self.pos = 0
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self.closed = False
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def __iter__(self):
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return self
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def __next__(self):
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"""A file object is its own iterator, for example iter(f) returns f
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(unless f is closed). When a file is used as an iterator, typically
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in a for loop (for example, for line in f: print line), the __next__()
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method is called repeatedly. This method returns the next input line,
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or raises StopIteration when EOF is hit.
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"""
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_complain_ifclosed(self.closed)
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r = self.readline()
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if not r:
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raise StopIteration
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return r
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def close(self):
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"""Free the memory buffer.
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"""
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if not self.closed:
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self.closed = True
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del self.buf, self.pos
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def isatty(self):
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"""Returns False because StringIO objects are not connected to a
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tty-like device.
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"""
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_complain_ifclosed(self.closed)
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return False
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def seek(self, pos, mode = 0):
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"""Set the file's current position.
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The mode argument is optional and defaults to 0 (absolute file
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positioning); other values are 1 (seek relative to the current
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position) and 2 (seek relative to the file's end).
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There is no return value.
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"""
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_complain_ifclosed(self.closed)
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if self.buflist:
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self.buf += ''.join(self.buflist)
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self.buflist = []
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if mode == 1:
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pos += self.pos
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elif mode == 2:
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pos += self.len
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self.pos = max(0, pos)
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def tell(self):
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"""Return the file's current position."""
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_complain_ifclosed(self.closed)
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return self.pos
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def read(self, n=None):
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"""Read at most size bytes from the file
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(less if the read hits EOF before obtaining size bytes).
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If the size argument is negative or omitted, read all data until EOF
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is reached. The bytes are returned as a string object. An empty
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string is returned when EOF is encountered immediately.
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"""
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_complain_ifclosed(self.closed)
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if self.buflist:
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self.buf += ''.join(self.buflist)
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self.buflist = []
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if n is None:
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n = -1
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if n < 0:
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newpos = self.len
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else:
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newpos = min(self.pos+n, self.len)
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r = self.buf[self.pos:newpos]
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self.pos = newpos
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return r
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def readline(self, length=None):
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r"""Read one entire line from the file.
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A trailing newline character is kept in the string (but may be absent
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when a file ends with an incomplete line). If the size argument is
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present and non-negative, it is a maximum byte count (including the
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trailing newline) and an incomplete line may be returned.
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An empty string is returned only when EOF is encountered immediately.
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Note: Unlike stdio's fgets(), the returned string contains null
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characters ('\0') if they occurred in the input.
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"""
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_complain_ifclosed(self.closed)
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if self.buflist:
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self.buf += ''.join(self.buflist)
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self.buflist = []
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i = self.buf.find('\n', self.pos)
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if i < 0:
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newpos = self.len
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else:
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newpos = i+1
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if length is not None:
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if self.pos + length < newpos:
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newpos = self.pos + length
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r = self.buf[self.pos:newpos]
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self.pos = newpos
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return r
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def readlines(self, sizehint = 0):
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"""Read until EOF using readline() and return a list containing the
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lines thus read.
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If the optional sizehint argument is present, instead of reading up
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to EOF, whole lines totalling approximately sizehint bytes (or more
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to accommodate a final whole line).
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"""
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total = 0
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lines = []
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line = self.readline()
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while line:
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lines.append(line)
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total += len(line)
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if 0 < sizehint <= total:
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break
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line = self.readline()
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return lines
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def truncate(self, size=None):
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"""Truncate the file's size.
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If the optional size argument is present, the file is truncated to
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(at most) that size. The size defaults to the current position.
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The current file position is not changed unless the position
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is beyond the new file size.
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If the specified size exceeds the file's current size, the
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file remains unchanged.
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"""
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_complain_ifclosed(self.closed)
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if size is None:
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size = self.pos
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elif size < 0:
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raise IOError(EINVAL, "Negative size not allowed")
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elif size < self.pos:
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self.pos = size
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self.buf = self.getvalue()[:size]
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self.len = size
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def write(self, s):
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"""Write a string to the file.
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There is no return value.
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"""
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_complain_ifclosed(self.closed)
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if not s: return
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# Force s to be a string or unicode
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if not isinstance(s, basestring):
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s = str(s)
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spos = self.pos
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slen = self.len
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if spos == slen:
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self.buflist.append(s)
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self.len = self.pos = spos + len(s)
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return
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if spos > slen:
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self.buflist.append('\0'*(spos - slen))
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slen = spos
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newpos = spos + len(s)
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if spos < slen:
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if self.buflist:
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self.buf += ''.join(self.buflist)
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self.buflist = [self.buf[:spos], s, self.buf[newpos:]]
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self.buf = ''
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if newpos > slen:
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slen = newpos
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else:
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self.buflist.append(s)
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slen = newpos
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self.len = slen
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self.pos = newpos
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def writelines(self, iterable):
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"""Write a sequence of strings to the file. The sequence can be any
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iterable object producing strings, typically a list of strings. There
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is no return value.
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(The name is intended to match readlines(); writelines() does not add
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line separators.)
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"""
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write = self.write
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for line in iterable:
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write(line)
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def flush(self):
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"""Flush the internal buffer
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"""
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_complain_ifclosed(self.closed)
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def getvalue(self):
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"""
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Retrieve the entire contents of the "file" at any time before
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the StringIO object's close() method is called.
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The StringIO object can accept either Unicode or 8-bit strings,
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but mixing the two may take some care. If both are used, 8-bit
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strings that cannot be interpreted as 7-bit ASCII (that use the
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8th bit) will cause a UnicodeError to be raised when getvalue()
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is called.
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"""
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if self.buflist:
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self.buf += ''.join(self.buflist)
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self.buflist = []
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return self.buf
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# A little test suite
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def test():
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import sys
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if sys.argv[1:]:
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file = sys.argv[1]
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else:
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file = '/etc/passwd'
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lines = open(file, 'r').readlines()
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text = open(file, 'r').read()
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f = StringIO()
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for line in lines[:-2]:
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f.write(line)
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f.writelines(lines[-2:])
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if f.getvalue() != text:
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raise RuntimeError, 'write failed'
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length = f.tell()
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print('File length =', length)
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f.seek(len(lines[0]))
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f.write(lines[1])
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f.seek(0)
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print('First line =', repr(f.readline()))
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print('Position =', f.tell())
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line = f.readline()
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print('Second line =', repr(line))
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f.seek(-len(line), 1)
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line2 = f.read(len(line))
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if line != line2:
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raise RuntimeError, 'bad result after seek back'
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f.seek(len(line2), 1)
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list = f.readlines()
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line = list[-1]
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f.seek(f.tell() - len(line))
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line2 = f.read()
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if line != line2:
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raise RuntimeError, 'bad result after seek back from EOF'
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print('Read', len(list), 'more lines')
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print('File length =', f.tell())
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if f.tell() != length:
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raise RuntimeError, 'bad length'
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f.truncate(length/2)
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f.seek(0, 2)
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print('Truncated length =', f.tell())
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if f.tell() != length/2:
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raise RuntimeError, 'truncate did not adjust length'
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f.close()
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if __name__ == '__main__':
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test()
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