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

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)):

• To change x in y, define y.__contains__().
• To change for x in y, define y.__iter__().