Why Do Python's List Modification Methods Return `None`?

Why Do Python List Methods Return None Instead of the List?

Modifying list methods like append, sort, extend, remove, clear, and reverse typically return None rather than the modified list itself. This design decision is rooted in Python's overarching principle that functions that modify objects in-place should return None.

The Rationale

According to Guido van Rossum, the Python architect, this choice serves to emphasize that a new object is not being created. By not returning the modified list, it discourages the use of chained side effects, such as:

x.compress().chop(y).sort(z)

which can be confusing and hamper readability. Instead, Python prefers the more explicit form:

x.compress()
x.chop(y)
x.sort(z)

This separation makes it clear that each method is acting on the same object.

Chaining Operations

While chaining side-effect calls is discouraged, Python allows it for operations that return new values, such as string processing operations:

y = x.rstrip("\n").split(":").lower()

Considerations

This design decision has drawbacks. It prevents intuitive "chaining" of list processing, such as:

mylist.reverse().append('a string')[:someLimit]

Alternatives like list comprehension and other data structures can provide similar functionality without breaking the "no side-effect returns" convention.

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