False positive using operator 'AND' while checking keys on dict()

[leonardogalani]

BPO	39149
Nosy	@ronaldoussoren, @ned-deily, @tirkarthi

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-12-28.19:42:13.136>
created_at = <Date 2019-12-28.18:31:50.418>
labels = ['OS-mac', '3.7', 'invalid']
title = "False positive using operator 'AND' while checking keys on dict()"
updated_at = <Date 2019-12-28.20:31:46.370>
user = 'https://bugs.python.org/leonardogalani'

bugs.python.org fields:

activity = <Date 2019-12-28.20:31:46.370>
actor = 'leonardogalani'
assignee = 'none'
closed = True
closed_date = <Date 2019-12-28.19:42:13.136>
closer = 'serhiy.storchaka'
components = ['macOS']
creation = <Date 2019-12-28.18:31:50.418>
creator = 'leonardogalani'
dependencies = []
files = []
hgrepos = []
issue_num = 39149
keywords = []
message_count = 4.0
messages = ['358954', '358956', '358964', '358965']
nosy_count = 4.0
nosy_names = ['ronaldoussoren', 'ned.deily', 'xtreak', 'leonardogalani']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39149'
versions = ['Python 3.7']

[leonardogalani]

using Python 3.7.6 (default, Dec 27 2019, 09:51:07) @ macOs

dict = { 'a': 1, 'b': 2, 'c': 3 }

if you `if 'a' and 'b' and 'c' in dict: print('ok')` you will get a True, since everything is true.

if you `if 'a' and 'g' and 'c' in dict: print('ok')` you also get a True because the last statement is True but the mid statement is false.

To avoid this false positive, you need to be explicit:
if 'a' in dict and 'g' in dict and 'c' in dict: print('ok') you will get a false.

[tirkarthi]

It's intended as non-empty strings evaluate to True so you with 'a' and 'b' and 'c' in dict you are essentially evaluating 'a' and 'b' and ('c' in dict) with brackets precedence i.e. True and True and True . On the other hand 'a' and 'g' and 'c' in dict it's the same with 'g' evaluated to True. I guess you want to check all the keys are present where all is more readable. Some more answers here: https://stackoverflow.com/questions/1285911/how-do-i-check-that-multiple-keys-are-in-a-dict-in-a-single-pass

>>> all(char in dict for char in ['a', 'b', 'c'])
True
>>> all(char in dict for char in ['a', 'b', 'g'])
False

[ned-deily]

P.S. You should also read the "Operator precedence" section for expressions in the Python Language Reference manual which explains that comparison operators bind tighter than Boolean AND operators:

https://docs.python.org/3/reference/expressions.html#operator-precedence

[leonardogalani]

I would totally agree if it wasn't for this:

>>> 'a' and 'b' and 'g' in dict
False

The last evaluation is False, making the whole statement False.

but thanks for the documentation link and stackoverflow suggestion.
it sure does make the code more readable.

Happy new year!