This rule raises an issue when code uses list(iterable)[0] to retrieve the first element of an iterable.

Why is this an issue?

Using list(iterable)[0] to get the first element of an iterable is inefficient because it materializes the entire iterable into a list in memory before accessing the first element.

This approach has two main problems:

• Memory overhead: The entire iterable is converted to a list, consuming memory proportional to its size. For large iterables or generators, this can waste significant memory.
• Time complexity: Creating the list is an O(n) operation, where n is the number of elements in the iterable. You pay the cost of iterating through all elements even though you only need the first one.

The pattern is particularly problematic with:

• large datasets or database query results
• generator expressions and generator functions
• file readers or network streams
• infinite iterables, which will cause the program to hang or crash

What is the potential impact?

• applications may become unresponsive or crash due to out-of-memory errors when processing large datasets
• execution time increases unnecessarily, degrading user experience in performance-critical paths
• infinite iterables cannot be consumed at all, causing the program to hang
• infrastructure costs grow as data size increases due to wasted memory and CPU cycles

How to fix it

Replace list(iterable)[0] with next(iter(iterable)) to retrieve only the first element without materializing the entire iterable into a list. The iter() function creates an iterator, and next() retrieves the next item from that iterator, in O(1) time and without allocating memory for the entire collection.

Note that the two expressions differ in the exception raised when the iterable is empty: list(iterable)[0] raises IndexError, while next(iter(iterable)) raises StopIteration. If the surrounding code catches IndexError, update the handler after applying this fix.

Code examples

Noncompliant code example

def get_first_user(users):
    return list(users)[0]  # Noncompliant

Compliant solution

def get_first_user(users):
    return next(iter(users))

Resources

Documentation

• Python Documentation - next() built-in function
• Python Documentation - iter() built-in function
• Python Documentation - Iterator

External coding guidelines

• Ruff - RUF015 - Unnecessary iterable allocation for first element