View source for Truth Value Testing

Questions to [mailto:davrot@uni-bremen.de David Rotermund]

[https://docs.python.org/3/library/stdtypes.html#truth-value-testing “Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below.”]

= Common mistake to avoid =
For True, False, None we use is and is not for comparisons. They are objects and not values thus we need to use is and is not.

Correct<syntaxhighlight lang="python">if a: # "a is True" is shortend to "a" due to the official Python style guidelines
    pass

if a is not True:
    pass

if a is False:
    pass

if a is not False:
    pass

if a is None:
    pass

if a is not None:
    pass</syntaxhighlight>Wrong<syntaxhighlight lang="python">if a == True:
    pass

if a == False:
    pass

if a != False:
    pass

if a != None:
    pass</syntaxhighlight>

== [https://docs.python.org/3/library/stdtypes.html#boolean-operations-and-or-not Boolean Operations - and, or, not] ==
[https://docs.python.org/3/library/stdtypes.html#boolean-operations-and-or-not “These are the Boolean operations, ordered by ascending priority:”]
{| class="wikitable"
|x or y
| style="text-align: center;" |if x is false, then y, else x
| style="text-align: center;" |(1)
|-
|x or y
| style="text-align: center;" |if x is false, then x, else y
| style="text-align: center;" |(2)
|-
|not x
| style="text-align: center;" |if x is false, then True, else False
| style="text-align: center;" |(3)
|}

# This is a short-circuit operator, so it only evaluates the second argument if the first one is false.
# This is a short-circuit operator, so it only evaluates the second argument if the first one is true.
# not has a lower priority than non-Boolean operators, so not a == b is interpreted as not (a == b), and a == not b is a syntax error.

== [https://docs.python.org/3/library/stdtypes.html#comparisons Comparisons] ==
“There are eight comparison operations in Python. They all have the same priority (which is higher than that of the Boolean operations). Comparisons can be chained arbitrarily; for example, x &#x3C; y &#x3C;= z is equivalent to x &#x3C; y and y &#x3C;= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x &#x3C; y is found to be false).”

“This table summarizes the comparison operations:”
{| class="wikitable"
|&#x3C;
| style="text-align: center;" |strictly less than
| style="text-align: center;" |uses method: __lt__()
|-
|&#x3C;=
| style="text-align: center;" |less than or equal
| style="text-align: center;" |uses method: __le__()
|-
|&#x3E;
| style="text-align: center;" |strictly greater than
| style="text-align: center;" |uses method: __gt__()
|-
|&#x3E;=
| style="text-align: center;" |greater than or equal
| style="text-align: center;" |uses method: __ge__()
|-
|==
| style="text-align: center;" |equal
| style="text-align: center;" |uses method: __eq__()
|-
| !=
| style="text-align: center;" |not equal
| style="text-align: center;" |
|-
|is
| style="text-align: center;" |object identity
| style="text-align: center;" |
|-
|is not
| style="text-align: center;" |negated object identity
| style="text-align: center;" |
|}

== Resource ==
[https://docs.python.org/3/library/stdtypes.html Built-in Types]