Why is the boolean expression “1 in (1, 2, 3) == True” False? [duplicate]

问题

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Why does (1 in [1,0] == True) evaluate to False? [duplicate] (1 answer)

Why does the expression 0 < 0 == 0 return False in Python? (9 answers)

Closed 3 years ago.

Why does the statement 1 in (1, 2, 3) == True return False in Python? Is the operator priority in Python ambiguous?

回答1:

Because, per the documentation on operator precedence:

Note that comparisons, membership tests, and identity tests, all have the same precedence and have a left-to-right chaining feature as described in the Comparisons section.

The Comparisons section shows an example of the chaining:

Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent to x < y and y <= z

So:

1 in (1, 2, 3) == True

is interpreted as:

(1 in (1, 2, 3)) and ((1, 2, 3) == True)

If you override this chaining by adding parentheses, you get the expected behaviour:

>>> (1 in (1, 2, 3)) == True
True

Note that, rather than comparing truthiness by equality to True or False, you should just use e.g. if thing: and if not thing:.

来源：https://stackoverflow.com/questions/41075735/why-is-the-boolean-expression-1-in-1-2-3-true-false