I understand what the in operator does in this code:
some_list = [1, 2, 3, 4, 5]
print(2 in some_list)
I also do understand that i will take on each value of the list in this code:
for i in [1, 2, 3, 4, 5]:
print(i)
I am curious if the in operator used in the for loop is the same as the in operator used in the first code.
They are the same concept but not the same operators.
In the print(2 in some_list) example, in is an operator that handles several different situations. The Python docs for the in operator give the details, which I paraphrase as follows: x in y calls y.__contains__(x) if y has a __contains__ member function. Otherwise, x in y tries iterating through y.__iter__() to find x, or calls y.__getitem__(x) if __iter__ doesn't exist. The complexity is to provide consistent membership testing for older code as well as newer code — __contains__ is what you want if you're implementing your own classes.
In the for loop, in is just a marker that separates the loop-index variable from whatever you're looping over. The Python docs for the for loop discuss the semantics, which I paraphrase as follows: whatever comes after in is evaluated at the beginning of a loop to provide an iterator. The loop body then runs for each element of the iterator (barring break or other control-flow changes). The for statement doesn't worry about __contains__ or __getitem__.
Edit @Kelvin makes a good point: you can change the behaviour of in with respect to your own new-style classes (class foo(object)):
x in y, define y.__contains__(). for x in y, define y.__iter__().No, it is not the same. The in test like in your first example is a test for membership and returns a truth value. This in might be thought of as the sequential analogue of == or is (depending on how __contains__ is implemented).
The in in your second example is part of the iteration grammar; it temporarily binds your selected variable i to each item in the iterable. This in might be thought of as the sequential analogue of =, the assignment operator.
No, although they both use the same word they do different things. in is in both cases a syntax structure, e.g. it is not a name of a object and can't be changed. You can see here and here the syntactic definition of each one. As you can see the names are hardcoded and have no relationship.
