What is key=lambda

링크텍스트
참고

>>> def lowercased(word): return word.lower()
...
>>> lowercased('Some')
'some'
>>> sorted(['Some', 'words', 'sort', 'differently'], key=lowercased)
['differently', 'Some', 'sort', 'words']

we could not inline the def lowercased() line into the sorted() expression:

>>> sorted(['Some', 'words', 'sort', 'differently'], key=def lowercased(word): return word.lower())
  File "<stdin>", line 1
    sorted(['Some', 'words', 'sort', 'differently'], key=def lowercased(word): return word.lower())
                                                           ^
SyntaxError: invalid syntax

>>> sorted(['Some', 'words', 'sort', 'differently'], key=lambda word: word.lower())
['differently', 'Some', 'sort', 'words']

lambda는 하나의 표현식으로만 제한되며 그 결과는 반환 값이다