cpython/Lib/StringIO.py

"""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"]

EMPTYSTRING = ''

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 += EMPTYSTRING.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 += EMPTYSTRING.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 += EMPTYSTRING.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
        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 += EMPTYSTRING.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(EMPTYSTRING.join(list))

    def flush(self):
        if self.closed:
            raise ValueError, "I/O operation on closed file"

    def getvalue(self):
        if self.buflist:
            self.buf += EMPTYSTRING.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()