Python - checking if an element is in two lists at the same time

Question

In Python, to check if an element is in two lists, we do

if elem in list1 and elem in list2:

Can we do the following for this purpose?

if elem in (list1 and list2):

Answer 1

No, you cannot.

list1 and list2 means "list1 if it's empty, list2 otherwise". So, this will not check what you're trying to check.

Try it in the interactive interpreter and see.

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The simple way to do this is the code you already have:

if elem in list1 and elem in list2:

It works, it's easy to read, and it's obvious to write. If there's an obvious way to do something, Python generally tries to avoid adding synonyms that don't add any benefit. ("TOOWTDI", or "There should be one-- and preferably only one --obvious way to do it.")

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If you're looking for an answer that's better in some particular way, instead of just different, there are different options depending on what you want.

For example, if you're going to be doing this check often:

elems_in_both_lists = set(list1) & set(list2)

Now you can just do:

if elem in elems_in_both_lists:

This is simpler, and it's also faster.

Answer 2

No, the statement

if elem in (list1 and list2):

would not work for this specified purpose. What the Python interpreter does is first check list1, if found empty (i.e - False), it just returns the empty list (Why? - False and anything will always result in a false, so, why check further? ) and if not empty (i.e evaluated to True), it returns list2 (Why? - If first value is True, the result of the expression depends on the second value, if it is False, the expression evaluates to False, else, True.) , so the above code becomes if elem in list1 or if elem in list2 depending on your implementation. This is known as short circuiting.

The Wiki page on Short Circuiting might be a helpful read.

Example -

>>> list1 = [1, 2]
>>> list2 = [3, 4]
>>> list1 and list2
[3, 4]

>>> list1 = []
>>> list2 = [3, 4]
>>> list1 and list2
[]