[New-bugs-announce] [issue23049] Fix functools.reduce code equivalent.

[Terry J. Reedy]

New submission from Terry J. Reedy:

from functools import reduce
def add(a,b): return a+b
reduce(add, {})
>>>
Traceback (most recent call last):
  File "C:\Programs\Python34\tem.py", line 3, in <module>
    reduce(add, {})
TypeError: reduce() of empty sequence with no initial value

However, the reduce-equivalent code in the doc sets a bad example and forgets to account for empty iterators.

def reduce(function, iterable, initializer=None):
    it = iter(iterable)
    if initializer is None:
        value = next(it)
    else: ...

So it lets the StopIteration escape (a bad practice that can silently break iterators).  The code should be

def reduce(function, iterable, initializer=None):
    it = iter(iterable)
    if initializer is None:
        try:
            value = next(it)
        except StopIteration:
            raise TypeError("reduce() of empty sequence with no initial value") from None
    else: ...

(patch coming)

----------
assignee: docs at python
components: Documentation, Interpreter Core
messages: 232626
nosy: docs at python, terry.reedy
priority: normal
severity: normal
stage: needs patch
status: open
title: Fix functools.reduce code equivalent.
type: behavior
versions: Python 3.4, Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23049>
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