Calls

Before you can call a function, you have to make it. To do this, run its def statement, either by typing it interactively or by coding it in a module file and importing the file. Once you’ve run the def, you can call the function by passing any two sequence objects in parentheses:

>>> s1 = "SPAM"
>>> s2 = "SCAM"
>>> intersect(s1, s2)            # Strings
['S', 'A', 'M']

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Here, we’ve passed in two strings, and we get back a list containing the characters in common. The algorithm the function uses is simple: “for every item in the first argument, if that item is also in the second argument, append the item to the result.” It’s a little shorter to say that in Python than in English, but it works out the same.

To be fair, our intersect function is fairly slow (it executes nested loops), isn’t really mathematical intersection (there may be duplicates in the result), and isn’t required at all (as we’ve seen, Python’s set data type provides a built-in intersection operation). Indeed, the function could be replaced with a single list comprehension expression, as it exhibits the classic loop collector code pattern:

>>> [x for x in s1 if x in s2]
['S', 'A', 'M']

As a function basics example, though, it does the job—this single piece of code can apply to an entire range of object types, as the next section explains.