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

[Vedran Čačić]

Vedran Čačić  added the comment:

Just for the record, I consider PEP 479 one of (very rare) design bugs in 
Python, and would like it reversed some day. (So anything that helps this 
outcome, including -1 on this, is welcome.)

It subverts the natural property of exceptions (that they bubble through frames 
undisturbed until caught) for no benefit at all, and it has made me write 
almost every chained generator since then in a more complex way, adding 
boilerplate code that converts inner StopIteration to return. I'm sure many 
others have done so too. Ceterum censeo PEP479em esse delendam.

--
nosy: +veky

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

[Raymond Hettinger]

Raymond Hettinger  added the comment:

-1 for being a breaking change, for addressing a minor issue that rarely arises 
in real life, and for being a slippery slope.

--
nosy: +rhettinger

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

[Peiran Yao]

New submission from Peiran Yao :

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):
print(i)
```

`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

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