Local Variables

Probably the most interesting part of this example is its names. It turns out that the variable res inside intersect is what in Python is called a local variable—a name that is visible only to code inside the function def and that exists only while the function runs. In fact, because all names assigned in any way inside a function are classified as local variables by default, nearly all the names in intersect are local variables:

 

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  • res is obviously assigned, so it is a local variable.
  • Arguments are passed by assignment, so seq1 and seq2 are, too.
  • The for loop assigns items to a variable, so the name x is also local.

All these local variables appear when the function is called and disappear when the function exits—the return statement at the end of intersect sends back the result object, but the name res goes away. To fully explore the notion of locals, though, we need to move on to Chapter 17.

 

[35] This code will always work if we intersect files’ contents obtained with file.readlines(). It may not work to intersect lines in open input files directly, though, depending on the file object’s implementation of the in operator or general iteration. Files must generally be rewound (e.g., with a file.seek(0) or another open) after they have been read to end-of-file once. As we’ll see in Chapter 29 when we study operator overloading, classes implement the in operator either by providing the specific __contains__ method or by supporting the general iteration protocol with the __iter__ or older __getitem__ methods; if coded, classes can define what iteration means for their data.