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如果函式引數是 None，那麼將使用標識函式：

list(filter(None, [1, 0, 2, [], '', 'a']))  # discards 0, [] and ''   
# Out: [1, 2, 'a']

Python 2.x >= 2.0.1

[i for i in [1, 0, 2, [], '', 'a'] if i] # equivalent list comprehension

Python 3.x >= 3.0.0

(i for i in [1, 0, 2, [], '', 'a'] if i) # equivalent generator expression