Confusion related to Python's `in` operator [duplicate]

Question

I found strange behavior with Python's in operator

d = {}
'k' in d == False # False! 

I thought it's because of precedence:

('k' in d) == False # True, it's okay
'k' in (d == False) # Error, it's also okay

But, what precedence evaluates the following expression then?

d = {}
'k' in d == False

If it's because of wrong precedence why it doesn't fire an error like if:

'k' in (d == False)

In other words, what happens under the hood of Python with this expression?

'k' in d == False

Answer 1

in is considered a comparison operator, and so it is subject to comparison chaining.

'k' in d == False

is equivalent to

'k' in d and d == False

because both in and == are comparison operators.

You virtually never need direct comparison to Boolean literals, though. The "correct" expression here is 'k' not in d.

---

For reference, this is described in the Python documentation, under 6.10. Comparisons:

comparison    ::=  or_expr (comp_operator or_expr)*
comp_operator ::=  "<" | ">" | "==" | ">=" | "<=" | "!="
                   | "is" ["not"] | ["not"] "in"

and

Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is found to be false).