Why Do Python's In-Place List Methods Return None?

Why Do Python's Mutating List Methods Return None?

Many Python list methods, including append, sort, extend, remove, clear, and reverse, return None after modifying the list. This design choice has sparked questions about its rationale and potential drawbacks.

According to Python's design principles, functions that mutate an object in-place should return None. This emphasizes that the operation is performed on the existing object rather than creating a new one.

Guido van Rossum, Python's creator, explains the reasoning behind this decision:

• Chaining side-effects on a single object is discouraged because it can make code difficult to read.
• The use of None in return statements helps clarify that these methods only modify the original object and don't produce a new value.

While some may argue that this design impedes "chaining" of list processing, Rossum believes that it enhances code readability and makes it clear that subsequent operations are applied to the same object.

He contrasts this with chaining operations that return new values, such as string processing operations, which he considers appropriate for chaining.

In summary, the decision to return None from Python's mutating list methods is based on the design principle of distinguishing between operations that modify an object in-place and those that return a new value.

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