Python language basics 69: filtering comprehensions

Introduction

In the previous post we discussed how to iterate over a dictionary with the concise comprehension syntax. We saw that it wasn’t very different from how list comprehensions are written.

In this post we’ll look at how to attach a conditional clause to a comprehension function.

Filtering comprehensions

Recall the standard “formula” for comprehensions: the standard for-each iteration, e.g. “for capital in capitals” is preceded by a function that operates on each element in the iteration, such as “upper()”:

capitals = ["Tirana", "Skopje", "Moscow", "Budapest", "Sofia", "Belgrade"]
capitals_upper = [capital.upper() for capital in capitals]

The comprehension function can be extended with an if-clause that is also applied to the actual element in the iteration. The if statement is appended to the for-each part of the comprehension formula. Here’s how to exclude capitals with less than 6 letters:

capitals_upper = [capital.upper() for capital in capitals if len(capital) > 6]
print(capitals_upper)

Here’s the result:

[‘BUDAPEST’, ‘BELGRADE’]

…i.e. the capitals with less than 6 chars have not been treated by the iteration function.

The above one-liner can be unpacked into “normal” code as follows:

capitals_upper_long = []
for capital in capitals:
    if len(capital) > 6:
        capitals_upper_long.append(capital.upper())

capitals_upper_long will include the same elements as capitals_upper.

We’ll start looking at something entirely different in the next post: classes.

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