Python provides logical operators to perform logical (Boolean) operations.(
It is used to describe the relationship between multiple conditions in an if statement.
This section describes the following.
- logical add:
In addition, the following points are explained as cautions.
- Logical operators for objects of type other than bool
orThese return values are not necessarily of type bool.
- Short circuit (short circuit evaluation)
and returns the logical product of two values.
print(True and True) # True print(True and False) # False print(False and True) # False print(False and False) # False
In fact, it is often used not for true or false, but for conditional expressions using comparison operators. For your information, the comparison operators are as follows.
a = 10 print(0 < a) # True print(a < 100) # True print(0 < a and a < 100) # True
and can be concatenated as follows.
print(0 < a < 100) # True
logical add: or
or returns the logical OR of the two values.
print(True or True) # True print(True or False) # True print(False or True) # True print(False or False) # False
not” returns the negation of the value; true and false are reversed.
print(not True) # False print(not False) # True
The order of precedence of these logical operators is as follows: not is the highest.
In the following sample code, the above expression is interpreted as if it were the one below. Since there is no problem with extra parentheses, it may be easier to clearly describe them in cases like this example.
print(True or True and False) # True print(True or (True and False)) # True
If you want to operate or before and, use parentheses().
print((True or True) and False) # False
>These comparison operators have even higher precedence than not. Therefore, parentheses are not necessary for each comparison operation, as was the case in the example above.
print(0 < a and a < 100) # True
See the official documentation below for a summary of operator precedence in Python.
Logical operators for objects of type other than bool
With these logical operators, not only bool types (true, false), but also numbers, strings, lists, etc. are processed as boolean values.
The following objects are considered false in Python's logical operations.
- Constants defined to be false:
- Zero in numeric types:
- Empty sequence or collection:
All other values are considered true.
The function bool() can be used to get the boolean value of an object. Note that the string '0' or 'False' is considered true.
print(bool(10)) # True print(bool(0)) # False print(bool('')) # False print(bool('0')) # True print(bool('False')) # True print(bool()) # False print(bool([False])) # True
To handle '0' or 'false' in a string as false, use distutils.util.strtobool().
and,orThese return values are not necessarily of type bool.
Here is an example of an object other than a bool type, showing the result of each operator on a numeric value.
x = 10 # True y = 0 # False print(x and y) # 0 print(x or y) # 10 print(not x) # False
As you can see from the example above, and and or in Python do not return true or false of type bool, but return the value on the left or right depending on whether it is true or false. The example is numeric, but the same applies to other types such as strings and lists. Incidentally, not returns true or false of type bool.
The definitions of the return values of and and or are as follows.
The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.
The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.
6.11. Boolean operations — Expressions — Python 3.10.1 Documentation
When the values of the left and right expressions are true and false separately, the return values are easy to understand. On the other hand, if both are true or both are false, the return value will be different depending on the order.
If you use it as a conditional expression in an if statement, etc., the result is judged as a boolean value and processed, so you don't need to worry about it, but if you use the return value for further processing, you need to be careful.
x = 10 # True y = 100 # True print(x and y) # 100 print(y and x) # 10 print(x or y) # 10 print(y or x) # 100
x = 0 # False y = 0.0 # False print(x and y) # 0 print(y and x) # 0.0 print(x or y) # 0.0 print(y or x) # 0 print(bool(x and y)) # False
If you want to treat it as true or false, you can do as in the last example.
bool(x and y)
The return values of and and or are summarized in the table below.
|x||y||x and y||x or y|
Short circuit (short circuit evaluation)
As you can see from the table above, if x is false in x and y, or if x is true in x or y, the return value will be x regardless of the value of y.
In such a case, y is not evaluated.
orNote that if you call a function or method on the right side of these processes to do some processing, the process may not be executed depending on the result on the left side.
def test(): print('function is called') return True print(True and test()) # function is called # True print(False and test()) # False print(True or test()) # True print(False or test()) # function is called # True