I want to create a file-like object to be passed to another module. I don't know if that other module will be calling read() or readline() or both. If I were to subclass say io.IOBase and just implement read() would that be sufficient, even if the client is calling readline()?

Bonus question: if my class will be opened in text mode, what's the minimum I can implement to let the superclass handle the unicode decoding?

(This will be an input-only class.)

Meta: I know I could just write all the methods and see what actually gets called, but I'd like to be a bit "future proof" and write to the actual specs, instead of trying things to see if they work, and then having them break in the future someday.

CodePudding user response：

Empirically, implementing read will be enough.

As for text mode, you'd want to just use TextIOWrapper (which technically is meant to be used with a BufferedIOBase, but it looks like things work with IOBase too).

import io


class FooIO(io.IOBase):
    def __init__(self, buffer):
        self.buffer = buffer
        self.pos = 0

    def rewind(self):
        self.pos = 0

    def readable(self) -> bool:
        return True

    def read(self, size=-1):
        if size == -1:
            size = len(self.buffer) - self.pos
        data = self.buffer[self.pos:self.pos   size]
        self.pos  = size
        return data


f = FooIO(b"hello\nworld\nthis\nis\na\ntest")
for x in range(7):
    print(x, f.readline())
f.rewind()

tiow = io.TextIOWrapper(f)
for x in range(7):
    print(x, repr(tiow.readline())))

prints out

0 b'hello\n'
1 b'world\n'
2 b'this\n'
3 b'is\n'
4 b'a\n'
5 b'test'
6 b''
0 'hello\n'
1 'world\n'
2 'this\n'
3 'is\n'
4 'a\n'
5 'test'
6 ''