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Definition

By now, you can probably guess that the solution to this dilemma is to package the for loop inside a function. Doing so offers a number of advantages:

 

Putting the code in a function makes it a tool that you can run as many times as you like.Because callers can pass in arbitrary arguments, functions are general enough to work on any two sequences (or other iterables) you wish to intersect.When the logic is packaged in a function, you only have to change code in one place if you ever need to change the way the intersection works.Coding the function in a module file means it can be imported and reused by any program run on your machine.

In effect, wrapping the code in a function makes it a general intersection utility:

def intersect(seq1, seq2):
    res = []                     # Start empty
    for x in seq1:               # Scan seq1
        if x in seq2:            # Common item?
            res.append(x)        # Add to end
    return res

The transformation from the simple code of Chapter 13 to this function is straightforward; we’ve just nested the original logic under a def header and made the objects on which it operates passed-in parameter names. Because this function computes a result, we’ve also added a return statement to send a result object back to the caller.