Question

How does Python (2.6.4, specifically) determine list membership in general? I’ve run some tests to see what it does:

def main():
    obj = fancy_obj(arg='C:\\')
    needle = (50, obj)
    haystack = [(50, fancy_obj(arg='C:\\')), (1, obj,), needle]

    print (1, fancy_obj(arg='C:\\'),) in haystack
    print needle in haystack

if __name__ == '__main__':
    main()

Which yields:

False
True

This tells me that Python is probably checking the object references, which makes sense. Is there something more definitive I can look at?

Answer 1

From (An Unofficial) Python Reference Wiki:

For the list and tuple types, x in y is true if and only if there exists an index i such that x == y[i] is true.

So in your example, if the fancy_obj class stored the value of arg in an instance variable and were to implement an __eq__ method that returned True if the two fancy_objs being compared had the same value for arg then (1, fancy_obj(arg='C:\\'),) in haystack would be True.

The relevant page of the Standard Library reference is: Built-in Types, specifically 5.6 Sequence Types