How Does Python\'s List Comprehension with a Variable Prefix Work?

Python for-in Loop with a Variable Prefix

In Python, it's possible to encounter code snippets like the following:

foo = [x for x in bar if x.occupants > 1]

This code snippet raises the question: what does it mean and how does it work?

Understanding List Comprehension

The syntax in question is known as a list comprehension. A list comprehension is a concise way to generate a new list based on an existing list, typically by filtering or transforming its elements.

Syntactic Structure of List Comprehension

A list comprehension follows this general syntactic structure:

[expression for item in iterable if condition]

Explanation of the Given Example

In the given example, the following elements are involved:

• Expression: x (this will become an element of the new list)
• Iterable: bar (the list to be filtered or transformed)
• Condition: x.occupants > 1 (used to filter bar)

How It Works

The list comprehension iterates over each element x in the list bar. For each element x, it checks if the condition x.occupants > 1 is True. If the condition is True, the expression x is evaluated and included in the new list being constructed.

Comparison to a For-in Loop

The list comprehension is equivalent to the following traditional for-in loop:

foo = []
for x in bar:
    if x.occupants > 1:
        foo.append(x)

However, the list comprehension is more concise and readable.

Alternative Way to Understand the Syntax

Alternatively, the list comprehension can be thought of as a shortcut to two built-in functions:

• map(function, iterable) applies the given function to each element in the iterable, returning a list of results.
• filter(condition, iterable) filters the iterable based on the given condition, returning a list of elements that meet the condition.

In the given example, the list comprehension is equivalent to:

foo = map(lambda x: x, filter(lambda x: x.occupants > 1, bar))

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