Why Does `list.append()` Return `False` in a Boolean Context?

Why Does list.append Return False in a Boolean Context?

When dealing with Boolean expressions, it's not uncommon to come across code like this:

u = []
if not u.append(6):
    # Do something...

But why does list.append evaluate to False in a Boolean context? Is it due to a deliberate design decision, or simply a result of C conventions?

The Explanation

Most Python methods that modify a container in place return None, adhering to the principle of command-query separation. According to this principle, methods that modify data should not return any value, as their primary purpose is to alter the object's state. This approach helps maintain a clear distinction between operations that retrieve data and those that change it.

In the case of list.append, it appends an element to the end of the list, mutating it in the process. Therefore, it follows the convention of returning None to indicate that it has successfully added an element to the list. However, in a Boolean context, None is considered False, hence the unexpected result of the aforementioned code snippet.

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