“Filtering” means placing a restriction on the elements to be added to a list returned by Python’s comprehension structure or the widely implemented map function. In Python it normally looks like this:
You can map a range of integers to a list:
>>> [i for i in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
And you can filter it, so to include only odd numbers, by placing an if-clause after the for clause:
>>> [i for i in range(10) if i % 2]
[1, 3, 5, 7, 9]
But if you use the fuller if-then syntax to filter or supplement the mapping process — say, to include 0 if an element is not odd — you place it before the for clause:
>>> [i if i % 2 else 0 for i in range(10)]
[0, 1, 0, 3, 0, 5, 0, 7, 0, 9]
Why does Python have two different syntaxes for filtering in a comprehension, one before the for clause and one after?
Answer: It doesn’t. There are two different things happening here.
Only the first example is genuine filtering, which modifies the for clause with an if clause.
The second example is a two-way decision expression, an in-line if-else statement determining the assignment of i at the far left of the comprehension.
Keyword else never occurs in genuine filtering, while it is required in a two-way decision expression. It's coincidental that these two structures both seem to be serving the same function — in reality, they are serving different functions.
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