85 lines
2.6 KiB
Markdown
85 lines
2.6 KiB
Markdown
# Python -- Truth Value Testing
|
||
{:.no_toc}
|
||
|
||
<nav markdown="1" class="toc-class">
|
||
* TOC
|
||
{:toc}
|
||
</nav>
|
||
|
||
Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
|
||
|
||
["Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below."](https://docs.python.org/3/library/stdtypes.html#truth-value-testing)
|
||
|
||
# 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
|
||
```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
|
||
```
|
||
|
||
Wrong
|
||
```python
|
||
if a == True:
|
||
pass
|
||
|
||
if a == False:
|
||
pass
|
||
|
||
if a != False:
|
||
pass
|
||
|
||
if a != None:
|
||
pass
|
||
```
|
||
## [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:"](https://docs.python.org/3/library/stdtypes.html#boolean-operations-and-or-not)
|
||
|
||
| | | |
|
||
| ------------- |:-------------:|:-------------:|
|
||
| x or y | if x is false, then y, else x | (1) |
|
||
| x or y | if x is false, then x, else y | (2) |
|
||
| not x | if x is false, then True, else False | (3) |
|
||
|
||
1. This is a short-circuit operator, so it only evaluates the second argument if the first one is false.
|
||
2. This is a short-circuit operator, so it only evaluates the second argument if the first one is true.
|
||
3. 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.
|
||
|
||
## [Comparisons](https://docs.python.org/3/library/stdtypes.html#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 < 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)."
|
||
|
||
"This table summarizes the comparison operations:"
|
||
|
||
| | | |
|
||
| ------------- |:-------------:|:-------------:|
|
||
| < | strictly less than | uses method: \_\_lt\_\_() |
|
||
|<= | less than or equal | uses method: \_\_le\_\_() |
|
||
|> | strictly greater than | uses method: \_\_gt\_\_() |
|
||
|>= | greater than or equal | uses method: \_\_ge\_\_() |
|
||
|== | equal | uses method: \_\_eq\_\_() |
|
||
|!= | not equal | |
|
||
|is | object identity | |
|
||
|is not | negated object identity | |
|
||
|
||
## Resource
|
||
[Built-in Types](https://docs.python.org/3/library/stdtypes.html)
|
||
|
||
|