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:

 

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  • 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.