I'm new to Python and in the process of rewriting an old Python script, I came across these lines:
value1 = 'some val 1'
value2 = 'some val 2'
some_list = #list of values
if (value1, value2) in some_list:
Does this check if value1 and value2 are in the list?
I googled how to do that and the answers show different approaches, and I didn't see any suggesting using the above code.
Is this doing something else? Should I keep it or change it?
(value1, value2) is a tuple. Your check is seeing if that tuple is in the list. For example:
mylist = [4, 5, (4, 5), 6]
(4, 5) in mylist
>True #because the tuple (4, 5) is in the list
(5, 6)
>False #because although 5 and 6 are both in the list, the tuple is not.
If you want to see if every item in the tuple is in the list, use all() as mentioned by @apero:
mylist = [4, 5, 6, 7]
all(x in mylist for x in (4, 5, 6))
>True
all(x in mylist for x in (4, 5, 8))
>False
You could use all:
>>> lst = [1, 2, 3, 4, 5]
>>> values = (1, 3)
>>> all(value in lst for value in values)
True
>>> values1 = (1, 6)
>>> all(value in lst for value in values1)
False
all evaluates to True only when all the values match the condition, in this case "being in the list". This is a really useful builtin method because it will stop evaluating as soon as 1 of the values does not match the condition. This technique is called short-circuit evaluation.
It behaves like if <test> and <test1> and <test2> .... : which evaluates each test one by one, from left to right and returns False as soon as one of the test doesn't pass.
Another one, using sets:
>>> to_find = {4, 5, 6}
>>> mylist = [4, 5, 6, 7]
>>> to_find.intersection(mylist) == to_find
True
>>> mylist = [5, 6, 7, 8]
>>> to_find.intersection(mylist) == to_find
False
if you want to check if any of the values are in the list:
>>> all(x in some_list for x in [value1, value2])
what you are doing now is checking if the tuple is in the list
(value1, value2)is present in the list. Not the individual values themselves.