how to conceptualize grouping in python logic statments

Question

No idea why this happens. Can anyone explain?

>>> foo = ['a', 'b', 'c']
>>> bar = [1, 2, 3]

>>> 'a' in (foo or bar)
True
>>> 'a' in (bar or foo)
False

I understand that python reads left to right, and that I should write out

>>> 'a' in foo or 'a' in bar

but what is going on in my test example? Why do I get True and False respectively?

What is most important thing to understand is the expression in the parens is evaluated first and the in check is done on the result of that. Without the parens `'a' in bar or foo -> ['a', 'b', 'c'] ` — Padraic Cunningham, Sep 20 '16 at 23:13

score 1 · Accepted Answer · answered Sep 20 '16 at 22:56

Since foo is true, foo or bar returns foo:

>>> foo = ['a', 'b', 'c']
>>> bar = [1, 2, 3]
>>> (foo or bar)
['a', 'b', 'c']

In a logical-or statement, evaluation can stop when the first true quantity is found. So, once python evaluates foo as true, there is no need for it to consider bar. Further, in a logical-or statement, python does not return True: it returns the first item that evaluates to True.

Likewise, since bar is true, bar or foo returns bar. Order matters:

>>> bar or foo
[1, 2, 3]

As another example:

>>> False or 3 or 6
3

Moinuddin Quadri · Answer 2 · 2016-09-20T23:11:46.370

1

The reason lies in how a or b works in Python. If a is having value or is True, a will be returned else value of b is returned. In your case:

>>> foo or bar
['a', 'b', 'c']
>>> bar or foo
[1, 2, 3]

If you want to check the values in both the list, firstly make a single list with elements from both list by using + as:

>>> foo + bar
['a', 'b', 'c', 1, 2, 3]

Then you can make your condition as:

>>> 'a' in (foo + bar)
True
>>> 'a' in (bar + foo)
True

Alternatively, you can also do it using or (without concatenating lists) as:

>>> 'a' in foo or 'a' in bar
True

edited Sep 20 '16 at 23:11

answered Sep 20 '16 at 23:03

Moinuddin Quadri

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You don't want to create a new list when you can just use or. – Padraic Cunningham Sep 20 '16 at 23:09
@PadraicCunningham: I think I was not clear with my words. Updated the answer – Moinuddin Quadri Sep 20 '16 at 23:12
I mean, it makes more sense to use `if "a" in foo if "a" in bar` , `bar + foo` would create a new list for every in check – Padraic Cunningham Sep 20 '16 at 23:15
Even I was also thinking about the same. Already updated the answer. `a+b` logic is based on what I think OP wanted to achieve. – Moinuddin Quadri Sep 20 '16 at 23:15
You could also `any("a" in l for l in (foo, bar))` which would be nicer if you had more than two iterables to check. – Padraic Cunningham Sep 20 '16 at 23:19
In fact there are multiple ways to achieve that. One more: `'a' in itertools.chain(foo, bar)`. – Moinuddin Quadri Sep 20 '16 at 23:23
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/123822/discussion-between-moinuddin-quadri-and-padraic-cunningham). – Moinuddin Quadri Sep 20 '16 at 23:44

how to conceptualize grouping in python logic statments

2 Answers2