Adapt an iterator to behave like a file-like object in Python

The “correct” way to do this is inherit from a standard Python io abstract base class. However it doesn’t appear that Python allows you to provide a raw text class, and wrap this with a buffered reader of any kind.

The best class to inherit from is TextIOBase. Here’s such an implementation, handling readline, and read while being mindful of performance. (gist)

import io

class StringIteratorIO(io.TextIOBase):

    def __init__(self, iter):
        self._iter = iter
        self._left=""

    def readable(self):
        return True

    def _read1(self, n=None):
        while not self._left:
            try:
                self._left = next(self._iter)
            except StopIteration:
                break
        ret = self._left[:n]
        self._left = self._left[len(ret):]
        return ret

    def read(self, n=None):
        l = []
        if n is None or n < 0:
            while True:
                m = self._read1()
                if not m:
                    break
                l.append(m)
        else:
            while n > 0:
                m = self._read1(n)
                if not m:
                    break
                n -= len(m)
                l.append(m)
        return ''.join(l)

    def readline(self):
        l = []
        while True:
            i = self._left.find('\n')
            if i == -1:
                l.append(self._left)
                try:
                    self._left = next(self._iter)
                except StopIteration:
                    self._left=""
                    break
            else:
                l.append(self._left[:i+1])
                self._left = self._left[i+1:]
                break
        return ''.join(l)