Read from several files as though they are glued into a single one

io_chain.py

import io


class IOChain(object):
    def __init__(self, files):
        self.files = iter(files)
        try:
            self._current_file = next(self.files)
        except StopIteration:
            self._current_file = None
    
    def read(self, size=None):
        if size == -1:
            size = None
        if self._current_file is None or size == 0:
            return b''
        data = self._current_file.read(size)
        if size is None or len(data) < size:
            try:
                self._current_file = next(self.files)
            except StopIteration:
                self._current_file = None
        return data + self.read(None if size is None else size - len(data))

  
def test_read_no_files():
    file_chain = IOChain([])
    assert b'' == file_chain.read(50)

def test_read():
    file_chain = IOChain([io.BytesIO(x) for x in (b'123', b'456', b'789')])

    assert b'1' == file_chain.read(1)
    assert b'234' == file_chain.read(3)
    assert b'56789' == file_chain.read()
    assert b'' == file_chain.read(None)
    assert b'' == file_chain.read(-1)


test_read_no_files()
test_read()