Why is it a mistake to assign the result of list.append()?

• append
• None
• list

New users will often write code like:

seq = []
for i in range(10):
    seq = seq.append(i)

… And become confused when it crashes with AttributeError: 'NoneType' object has no attribute 'append'.
They assume that list.append returns a reference to the original list, but it actually returns None. The solution is to call .append without assigning the result to anything.

seq = []
for i in range(10):
    seq.append(i)

• Python Append to a list returns none
• Why does append() always return None in Python?
• How to append to the end of an empty list?
• Why does list.append evaluate to false in a boolean context?
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