Two argument form of iter() is not implemented.

[hardkrash]

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

[mikewadsten]

Sort of related to #4400.

[dpgeorge]

Yes, this is simply not implemented, mainly because the same thing can be achieved in pure Python. As a workaround the following can be used:

def iter2(c, s):
    while True:
        v = c()
        if v == s:
            raise StopIteration
        yield v

(This would be simpler with the walrus operator!)

[hardkrash]

While I agree on the fact that this can be worked around in many ways. it is unfortunate that this is not explicitly called out in the documentation as a difference from python's core syntax.

This would be a compatibility wrapper needed to make the two argument form of iter work without changing source code.

from compat import iter

Where compat.py has something like this.

S = some_singleton
_orig_iter = iter
def _compat_iter(c, s=S):
    if s is S:
        for x in _orig_iter(c):
            yield x
    else:
        while True:
            v = c()
            if v == s:
                raise StopIteration
            yield v

iter = _compat_iter

Would adding 2 argument form to the builtin iter function be an acceptable patch?

[dpgeorge]

it is unfortunate that this is not explicitly called out in the documentation as a difference from python's core syntax.

It's not really a syntax difference, rather the fact that MicroPython implements a subset of the Python builtins. It could be added to the docs via tests/cpydiff (which automatically generates differences for the docs).

Would adding 2 argument form to the builtin iter function be an acceptable patch?

Implemented in C I think it'd be a large patch and therefore unlikely to be made part of default MicroPython builds. It could be made optional like the 2-arg form of next(), but I'm just not sure how generally useful it would be (it hasn't been requested so far).

[dpgeorge]

This would be a good candidate for #5025.

[hex007]

Found this not implemented the hard way. next called with two params can just throw Not implemented exception would be much easier to understand than

TypeError: function takes 1 positional arguments but 2 were given