"""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.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).
"""
class StringIO:
def __init__(self, buf = ''):
self.buf = buf
self.len = len(buf)
self.buflist = []
self.pos = 0
self.closed = 0
self.softspace = 0
def close(self):
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 = self.buf + ''.join(self.buflist)
self.buflist = []
if mode == 1:
pos = pos + self.pos
elif mode == 2:
pos = 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 = 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 = 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):
lines = []
line = self.readline()
while line:
lines.append(line)
line = self.readline()
return lines
def write(self, s):
if self.closed:
raise ValueError, "I/O operation on closed file"
if not s: return
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 = self.buf + ''.join(self.buflist)
self.buflist = []
self.buflist = [self.buf[:self.pos], s, self.buf[newpos:]]
self.buf = ''
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):
if self.buflist:
self.buf = 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()
|