Search code examples
pythonloopspython-3.xsemantics

Python 'in' keyword in expression vs. in for loop

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.

Solution

  • 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)):