[issue41407] Tricky behavior of builtin-function map

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I concur with Jim and Steven, so we'll mark this a closed.  

If you want to go forward, consider bringing this up on python-ideas.  If it 
gets a favorable reception, this can be re-opened.

--
nosy: +rhettinger
resolution:  -> rejected
stage:  -> resolved
status: pending -> closed

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[issue41407] Tricky behavior of builtin-function map

[Jim Jewett]

Jim Jewett  added the comment:

Why would you raise StopIteration if you didn't want to stop the nearest 
iteration loop?  I agree that the result of your sample code seems strange, but 
that is because it is strange code.

I agree with Steven D'Aprano that changing it would cause more pain than it 
would remove.

Unless it gets a lot more support by the first week of August, I recommend 
closing this request as rejected.

--
nosy: +Jim.Jewett
status: open -> pending

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[issue41407] Tricky behavior of builtin-function map

[Steven D'Aprano]

Steven D'Aprano  added the comment:

Converting *all* exceptions into RuntimeError is certainly not a good idea, 
especially since you include KeyboardInterrupt and other non-errors.

I'm probably going to be on the losing side of this one (I lost the argument 
back when a similar issue was raised about comprehensions), but I argue that 
this is not a bug it is a useful feature.

Having the map function raise StopIteration is a good way for the map to halt 
the loop early, when it detects a special sentinel or condition. A few years 
ago the change to comprehensions broke my code so I changed to map, and now you 
want to break it again :-(

--
nosy: +steven.daprano

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[issue41407] Tricky behavior of builtin-function map

[DarrenDanielDay]

New submission from DarrenDanielDay :

# The following is a tricky example:

# This is an example function that possibly raises `StopIteration`.
# In some cases, this function may raise `StopIteration` in the first iteration.
def raise_stop_iteration(param):
if param == 10:
raise StopIteration
# Suppose this function will do some simple calculation
return -param

# Then when we use builtin-function `map` and for-loop:

print('example 1'.center(30, '='))
for item in map(raise_stop_iteration, range(5)):
print(item)
print('end of example 1'.center(30, '='))

# It works well. The output of example 1 is 0 to -4.
# But the following can be triky:

print('example 2'.center(30, '='))
for item in map(raise_stop_iteration, range(10, 20)):
print(item)
print('end of example 2'.center(30, '='))

# The output of exapmle 2 is just nothing,
# and no errors are reported,
# because `map` simply let the exception spread upward when executing 
`raise_stop_iteration(10)`.
# However, the exception type is StopIteration, so the for-loop catches it and 
breaks the loop,
# assuming this is the end of the loop.
# When the exception raised from buggy mapping function is exactly 
`StopIteration`, 
# for example, functions implemented with builtin-function `next`, 
# it can be confusing to find the real issue.

# I think it might be better improved in this way:

class my_map:
def __init__(self, mapping_function, *iterators):
self.mapping = mapping_function
self.iterators = iterators

def __iter__(self):
for parameter_tuple in zip(*self.iterators):
try:
yield self.mapping(*parameter_tuple)
except BaseException as e:
raise RuntimeError(*e.args) from e

# It works like the map in most cases:

print('example 3'.center(30, '='))
for item in my_map(raise_stop_iteration, range(5)):
print(item)
print('end of example 3'.center(30, '='))

# And then, the crash of the buggy mapping function will be reported:
print('example 4'.center(30, '='))
for item in my_map(raise_stop_iteration, range(10, 20)):
print(item)
print('end of example 4'.center(30, '='))

--
components: Library (Lib)
files: issue.py
messages: 374371
nosy: DarrenDanielDay
priority: normal
severity: normal
status: open
title: Tricky behavior of builtin-function map
type: behavior
versions: Python 3.7
Added file: https://bugs.python.org/file49342/issue.py

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