5.9 Comparisons

Contrary to C, all comparison operations in Python have the same priority, which is lower than that of any arithmetic, shifting or bitwise operation. Also contrary to C, expressions like a < b < c have the interpretation that is conventional in mathematics:

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

Comparisons yield integer values: 1 for true, 0 for false.

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).

Formally, if a, b, c, ..., y, z are expressions and opa, opb, ..., opy are comparison operators, then a opa b opb c ...y opy z is equivalent to a opa b and b opb c and ... y opy z, except that each expression is evaluated at most once.

Note that a opa b opb c doesn't imply any kind of comparison between a and c, so that, e.g., x < y > z is perfectly legal (though perhaps not pretty).

The forms <> and != are equivalent; for consistency with C, != is preferred; where != is mentioned below <> is also acceptable. At some point in the (far) future, <> may become obsolete.

The operators "<", ">", "==", ">=", "<=", and "!=" compare the values of two objects. The objects needn't have the same type. If both are numbers, they are coverted to a common type. Otherwise, objects of different types always compare unequal, and are ordered consistently but arbitrarily.

(This unusual definition of comparison was used to simplify the definition of operations like sorting and the in and not in operators. In the future, the comparison rules for objects of different types are likely to change.)

Comparison of objects of the same type depends on the type:

The operators in and not in test for sequence membership: if y is a sequence, x in y is true if and only if there exists an index i such that x = y[i]. x not in y yields the inverse truth value. The exception TypeError is raised when y is not a sequence, or when y is a string and x is not a string of length one.5.3

The operators is and is not test for object identity: x is y is true if and only if x and y are the same object. x is not y yields the inverse truth value.


... lists.5.2
This is expensive since it requires sorting the keys first, but it is about the only sensible definition. An earlier version of Python compared dictionaries by identity only, but this caused surprises because people expected to be able to test a dictionary for emptiness by comparing it to {}.
... one.5.3
The latter restriction is sometimes a nuisance.

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