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:

- Numbers are compared arithmetically.
- Strings are compared lexicographically using the numeric equivalents
(the result of the built-in function
`ord()`) of their characters. - Tuples and lists are compared lexicographically using comparison of
corresponding items.
- Mappings (dictionaries) are compared through lexicographic
comparison of their sorted (key, value) lists.
^{5.2} - Most other types compare unequal unless they are the same object;
the choice whether one object is considered smaller or larger than
another one is made arbitrarily but consistently within one
execution of a program.

The operators `in` and `not in` test for sequence
membership: if `y` is a sequence,

is
true if and only if there exists an index `x` in `y``i` such that

.
`x` = `y`[`i`]

yields the inverse truth value. The
exception `x` not in `y``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:

is true if and only if `x` is `y``x` and `y`
are the same object.

yields the inverse
truth value.
`x` is not `y`

- ... 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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