The builtin iter(callable, sentinel) from pep 234 is not implemented.

The use case is something like this.

with open('file.bin', 'rb') as f:
    for chunk in iter(lambda: f.read(128), b''):
        process_chunk(chunk)

The two parameter form allows an easy way to make a callable into an iterable by using a sentinel.

It allows for replacing the do while workarounds with a simple iterator.

with open('file.bin', 'rb') as f:
    chunk = f.read(128)
    while chunk != b'':
        process_chunk(chunk)
        chunk = f.read(128)

Credit to this stack overflow answer from jfs
https://stackoverflow.com/a/20014805