r"""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). | |
""" | |
try: | |
from errno import EINVAL | |
except ImportError: | |
EINVAL = 22 | |
__all__ = ["StringIO"] | |
def _complain_ifclosed(closed): | |
if closed: | |
raise ValueError, "I/O operation on closed file" | |
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 not isinstance(buf, basestring): | |
buf = str(buf) | |
self.buf = buf | |
self.len = len(buf) | |
self.buflist = [] | |
self.pos = 0 | |
self.closed = False | |
self.softspace = 0 | |
def __iter__(self): | |
return self | |
def next(self): | |
"""A file object is its own iterator, for example iter(f) returns f | |
(unless f is closed). When a file is used as an iterator, typically | |
in a for loop (for example, for line in f: print line), the next() | |
method is called repeatedly. This method returns the next input line, | |
or raises StopIteration when EOF is hit. | |
""" | |
_complain_ifclosed(self.closed) | |
r = self.readline() | |
if not r: | |
raise StopIteration | |
return r | |
def close(self): | |
"""Free the memory buffer. | |
""" | |
if not self.closed: | |
self.closed = True | |
del self.buf, self.pos | |
def isatty(self): | |
"""Returns False because StringIO objects are not connected to a | |
tty-like device. | |
""" | |
_complain_ifclosed(self.closed) | |
return False | |
def seek(self, pos, mode = 0): | |
"""Set the file's current position. | |
The mode argument is optional and defaults to 0 (absolute file | |
positioning); other values are 1 (seek relative to the current | |
position) and 2 (seek relative to the file's end). | |
There is no return value. | |
""" | |
_complain_ifclosed(self.closed) | |
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): | |
"""Return the file's current position.""" | |
_complain_ifclosed(self.closed) | |
return self.pos | |
def read(self, n = -1): | |
"""Read at most size bytes from the file | |
(less if the read hits EOF before obtaining size bytes). | |
If the 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. | |
""" | |
_complain_ifclosed(self.closed) | |
if self.buflist: | |
self.buf += ''.join(self.buflist) | |
self.buflist = [] | |
if n is None or 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): | |
r"""Read one entire line from the file. | |
A trailing newline character is kept in the string (but may be absent | |
when a file ends with an incomplete line). If the 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 only when EOF is encountered immediately. | |
Note: Unlike stdio's fgets(), the returned string contains null | |
characters ('\0') if they occurred in the input. | |
""" | |
_complain_ifclosed(self.closed) | |
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 and length > 0: | |
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): | |
"""Read until EOF using readline() and return a list containing the | |
lines thus read. | |
If the optional sizehint argument is present, instead of reading up | |
to EOF, whole lines totalling approximately sizehint bytes (or more | |
to accommodate a final whole line). | |
""" | |
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): | |
"""Truncate the file's size. | |
If the optional size argument is present, the file is truncated to | |
(at most) that size. The size defaults to the current position. | |
The current file position is not changed unless the position | |
is beyond the new file size. | |
If the specified size exceeds the file's current size, the | |
file remains unchanged. | |
""" | |
_complain_ifclosed(self.closed) | |
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] | |
self.len = size | |
def write(self, s): | |
"""Write a string to the file. | |
There is no return value. | |
""" | |
_complain_ifclosed(self.closed) | |
if not s: return | |
# Force s to be a string or unicode | |
if not isinstance(s, basestring): | |
s = str(s) | |
spos = self.pos | |
slen = self.len | |
if spos == slen: | |
self.buflist.append(s) | |
self.len = self.pos = spos + len(s) | |
return | |
if spos > slen: | |
self.buflist.append('\0'*(spos - slen)) | |
slen = spos | |
newpos = spos + len(s) | |
if spos < slen: | |
if self.buflist: | |
self.buf += ''.join(self.buflist) | |
self.buflist = [self.buf[:spos], s, self.buf[newpos:]] | |
self.buf = '' | |
if newpos > slen: | |
slen = newpos | |
else: | |
self.buflist.append(s) | |
slen = newpos | |
self.len = slen | |
self.pos = newpos | |
def writelines(self, iterable): | |
"""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 readlines(); writelines() does not add | |
line separators.) | |
""" | |
write = self.write | |
for line in iterable: | |
write(line) | |
def flush(self): | |
"""Flush the internal buffer | |
""" | |
_complain_ifclosed(self.closed) | |
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. | |
""" | |
_complain_ifclosed(self.closed) | |
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 =', repr(f.readline()) | |
print 'Position =', f.tell() | |
line = f.readline() | |
print 'Second line =', repr(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.truncate(length/2) | |
f.seek(0, 2) | |
print 'Truncated length =', f.tell() | |
if f.tell() != length/2: | |
raise RuntimeError, 'truncate did not adjust length' | |
f.close() | |
if __name__ == '__main__': | |
test() |