Python and Operator

Summary: in this tutorial, you’ll learn about the Python and logical operator and how to use it to control the flow of code.

Introduction to the Python and operator

The Python and operator is a logical operator. Typically, you use the and operator to operate on Boolean values and return a Boolean value.

The and operator returns True if both operands evaluate to True. Otherwise, it returns False.

The following truth table shows the result of the and operator:

XYX and Y
TrueTrueTrue
TrueFalseFalse
FalseFalseFalse
FalseTrueFalse

This table illustrates two important points:

  • If the first operand (X) is True, the result of the and operator depends on the result of the second operand (Y).
  • If the first operand (X) is False, the result of the and operator is always False regardless of the value of the second operand (Y).

The following shows an example of using the and operator:

timeout = False pending_job = True execute_next = timeout and pending_job print(execute_next)
Code language: Python (python)

Output:

False
Code language: Python (python)

In this example, the timeout is False and pending_job is True. Therefore, the result of the expression timeout and pending_job is False.

Python and operator is short-circuiting

The key feature of the and operator is that it short-circuits. It means that if the first operand is False, the and operator won’t evaluate the second operand. The reason is that it already has a conclusion about the outcome, which is False.

The following example results in a ZeroDivisionError:

a = 10 b = 0 c = a / b print(c)
Code language: Python (python)

In this example, since b is zero, the a / b definitely causes the division by zero exception.

However, the following example won’t cause a ZeroDivisionError:

a = 10 b = 0 c = b and a/b print(c)
Code language: Python (python)

Output:

0
Code language: Python (python)

In this example, we used the and operator in the expression:

c = b and a/b
Code language: Python (python)

Since b is zero, which is False, the and operator can conclude the result of the whole expression, which is False regardless of the result of the second part a / b. Therefore, the and operator doesn’t need to evaluate the expression a / b. In fact, it doesn’t do so.

The following example changes the value of b to five:

a = 10 b = 5 c = b and a/b print(c)
Code language: Python (python)

Output:

2.0
Code language: Python (python)

In this example, b is 5 which is True. Since the first operand is True, the value of the whole expression depends on the value of the second operand, which is a / b.

These examples show that the and operator can operate with non-Boolean values and returns a non-value boolean value.

In general, you can use the and operator for the objects:

bool(object1) and bool(object2)
Code language: Python (python)

In fact, you don’t need to use the bool() constructor:

object1 and object2
Code language: Python (python)

In this case, the and opeartor returns the object1 if it’s falsy. Otherwise, it returns the object2.

In other words, the and operator returns the object1 if the object1 is falsy. Otherwise, it evaluates the object2 and returns it.

The expression

c = b and a/b
Code language: Python (python)

is equivalent to the following:

if b:     c = a / b else:     c = b
Code language: Python (python)

By using the and operator, you can control the flow of the program.

Python and operator example

The following defines the avg() function that calculates the average of numbers:

def avg(*numbers): total = sum(numbers) n = len(numbers) if n > 0: return total / n return 0 if __name__ == "__main__": print(avg(1, 2, 3))
Code language: Python (python)

Output:

0
Code language: Python (python)

How it works.

  • First, calculate the sum of the numbers using the sum() function.
  • Second, get the number numbers using the len() function.
  • Third, return the average if the number of numbers is greater than zero, otherwise, return zero.

The main block calculates the average of three numbers 1, 2, and 3, which returns 2.0 as expected.

The following code uses an if statement and returns the average if the number of numbers is greater than zero. Otherwise, it returns zero:

if n > 0: return total / n return 0
Code language: Python (python)

In fact, you can use the and operator to make this code more concise like this:

return n and total / n
Code language: Python (python)

In this case, if n is zero the and operator doesn’t need to evaluate the expression total / n and it returns zero. Otherwise, it evaluates the total / n and returns it.

Here is the new version of the avg() function that uses the and operator:

def avg(*numbers): total = sum(numbers) n = len(numbers) return n and total / n
Code language: Python (python)

Summary

  • The X and Y returns True if both X and Y evaluate to True. Otherwise, it returns False.
  • The X and Y actually returns X if X is falsy. Otherwise, it evaluates Y and returns the result of the evaluation.
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