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Boolean Operators in Python (and, or, not)

Modified: | Tags: Python

Python provides Boolean operators, and, or, and not. These are used, for example, when describing the relationship between multiple conditions in an if statement.

Contents

See the following article for bitwise operations on each bit of an integer. Use & and | instead of and and or.

For more details on if statements, see the following article.

and (Logical AND)

and returns the logical AND of two values.

print(True and True)
# True

print(True and False)
# False

print(False and True)
# False

print(False and False)
# False
source: and_or_bool.py

In practice, it is often used not with True and False but with conditional expressions, such as those using comparison operators (<, >, etc.). The same applies to or and not.

a = 10
print(0 < a)
# True

print(a < 100)
# True

print(0 < a and a < 100)
# True
source: and_or_bool.py

Multiple comparisons can be chained as follows.

print(0 < a < 100)
# True
source: and_or_bool.py

or (Logical OR)

or returns the logical OR of two values.

print(True or True)
# True

print(True or False)
# True

print(False or True)
# True

print(False or False)
# False
source: and_or_bool.py

not (Negation)

not returns the negation of a value. True and False are inverted.

print(not True)
# False

print(not False)
# True
source: and_or_bool.py

Precedence of and, or, not

The precedence of Boolean operators is not > and > or (not is the highest precedence).

In the sample code below, the first expression is equivalent to the second one. It does not matter if there are extra parentheses (), so it may be clearer to explicitly write them.

print(True or True and False)
# True

print(True or (True and False))
# True
source: and_or_bool.py

If you want to calculate or before and, enclose or with parentheses.

print((True or True) and False)
# False
source: and_or_bool.py

Comparison operators such as <, > have higher precedence than Boolean operators. This means parentheses () are generally not required to group comparison operations.

print(0 < a and a < 100)
# True
source: and_or_bool.py

However, to clarify operation order or when mixing different types of operators, using parentheses can be helpful.

See the official documentation below for a summary of operator precedence in Python.

Boolean operators with non-bool objects

Boolean operators and, or, not can evaluate non-bool objects, such as numbers, strings, and lists, as truth values.

The following objects are considered false, as in the official documentation above.

  • Constants defined to be false: None and False
  • Zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1)
  • Empty sequences and collections: '', (), [], {}, set(), range(0)

All other objects are considered true.

bool() can be used to explicitly convert objects to bool type, but note that even strings '0' or 'False' are converted to 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
source: and_or_bool.py

To treat strings '0' or 'False' as false, use distutils.util.strtobool(). See the following article for reference.

and and or do not always return bool values

In Python, and and or do not always return bool values (True or False); they return either the left or right value according to their truthiness. On the other hand, not always returns a bool value, inverting the truthiness of its operand.

x = 10  # Evaluated as true
y = 0   # Evaluated as false

print(x and y)
# 0

print(x or y)
# 10

print(not x)
# False
source: and_or_bool.py

The definitions of the return values for 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. Expressions - Boolean operations — Python 3.12.1 documentation

The return values of and and or can be summarized in the following table:

x y x and y x or y
true false y x
false true x y
true true y x
false false x y

When used in conditionals such as if statements, the result is evaluated based on its truthiness, so there is no particular need to worry. However, be cautious when using the return value in subsequent operations.

x = 10   # Evaluated as true
y = 100  # Evaluated as true

print(x and y)
# 100

print(y and x)
# 10

print(x or y)
# 10

print(y or x)
# 100
source: and_or_bool.py
x = 0    # Evaluated as false
y = 0.0  # Evaluated as 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
source: and_or_bool.py

To treat the result as True or False, you can use bool() as shown in the last example.

Short-circuit evaluation

If x is false in x and y or if x is true in x or y, the return value is x regardless of the value of y. In such cases, y is not evaluated.

Note that if you call a function or method on the right side of and or or, it may not be executed if the left side determines the outcome.

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
source: short_circuit.py

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