How to avoid buffering in the Python fileinput library

Question

I've seen this question asked here, but the answers given did not work in my case and was marked duplicate.

• python -u does not work for stdin in Python 3.
• sys.stdin = sys.stdin.detach() throws a ValueError: underlying buffer has been detached.
• None of these work for non-stdin inputs and other files being used as stream.
• Adding a hook does not work: FileInput(openhook=hook_nobuf) and using open(buffering=0) in the hook.

I dug in the source code (/usr/lib/python3.2/fileinput.py) and saw that readlines(bufsize) was being used internally to load a buffer. No shell or other piping shenanigans.

Answer 1

What worked for me was simply setting FileInput(bufsize=1). The file.readlines() documentation does state "The optional size argument, if given, is an approximate bound on the total number of bytes in the lines returned." In practice, I get exactly one new line every time rather than having to fill a buffer.

with fileinput.input(bufsize=1) as f:
    for line in f:
        print("One line in, one line out!")