[issue24434] ItemsView.__contains__ does not mimic dict_items

[Caleb Levy]

Changes by Caleb Levy caleb.l...@berkeley.edu:


--
components: Library (Lib)
nosy: clevy, rhettinger, stutzbach
priority: normal
severity: normal
status: open
title: ItemsView.__contains__ does not mimic dict_items
type: behavior
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4, Python 3.5, Python 3.6

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[issue24434] ItemsView.__contains__ does not mimic dict_items

[Caleb Levy]

New submission from Caleb Levy:

The current implementation ItemsView.__contains__ reads

class ItemsView(MappingView, Set):
...
def __contains__(self, item):
key, value = item
try:
v = self._mapping[key]
except KeyError:
return False
else:
return v == value
...

This poses several problems. First, any non-iterable or iterable not having 
exactly two elements will raise an error instead of returning false. 

Second, an ItemsView object is roughly the same as a set of tuple-pairs hashed 
by the first element. Thus, for example,

[a, 1] in d.items()

will return False for any dict d, yet in the current ItemsView implementation, 
this is True.

The patch changes behavior to immediately return false for non-tuple items and 
tuples not of length 2, avoiding unnecessary exceptions. It also adds tests to 
collections which fail under the old behavior and pass with the update.

--
keywords: +patch
Added file: http://bugs.python.org/file39682/ItemsView_contains.patch

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[issue24434] ItemsView.__contains__ does not mimic dict_items

[Caleb Levy]

Caleb Levy added the comment:

@serhiy.storchaka: I don't think that will work.

First of all,

x, y = item

will raise a ValueError if fed an iterable whose length is not exactly 2, so 
you would have to check for that. Moreover, if item is something like a dict, 
for example, then:

{a: 1, b: 2} in DictLikeMapping(a=b)

could return True, which I don't think would be expected behavior.

I'm not terribly fond of the instance check myself, but in this case I can't 
see any other way to do it: the built in dict_items necessarily consists of 
*tuples* of key-value pairs.

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[issue24434] ItemsView.__contains__ does not mimic dict_items

[Caleb Levy]

Caleb Levy added the comment:

Sorry; that should be DictLikeMapping(a=b).items(), where DictLikeMapping is 
defined in the patch unit tests.

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