[New-bugs-announce] [issue46621] Should map(function, iterable, ...) replace StopIteration with RuntimeError?

Peiran Yao report at bugs.python.org
Wed Feb 2 23:46:26 EST 2022

New submission from Peiran Yao <peiran.yao at tuna.tsinghua.edu.cn>:

Currently, StopIteration raised accidentally inside the `function` being applied is not caught by map(). This will cause the iteration of the map object to terminate silently. (Whereas, when some other exception is raised, a traceback is printed pinpointing the cause of the problem.)

Here's a minimal working example:

def take_first(it: Iterable):
    # if `it` is empty, StopIteration will be raised accidentally
    return next(it) 

iterables = [iter([1]), iter([]), iter([2, 3])] # the second one is empty
for i in map(take_first, iterables):

`take_first` function didn't consider the case where `it` is empty. The programmer would expect an uncaught StopIteration, instead of the loop terminating silently after only one iteration.

Similar to the case of generators (described in PEP 497), this behaviour can conceal obscure bugs, and a solution could be catching StopIteration when applying the function, and replacing it with a RuntimeError.

Beside the built-in map(), imap() and imap_unordered() in the concurrent and multiprocessing modules also have similar behaviour.

PEP 479 -- Change StopIteration handling inside generators https://www.python.org/dev/peps/pep-0479/

messages: 412419
nosy: xavieryao
priority: normal
severity: normal
status: open
title: Should map(function, iterable, ...) replace StopIteration with RuntimeError?
type: behavior

Python tracker <report at bugs.python.org>

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