Logical and Membership Operators

15 min · Beginner

Real rules need more than one check. Python combines checks with plain English words, and and / or have a trick of their own: they hand back one of the values.

and, or and not

Logical operator is one of the words and, or and not. They combine or flip conditions, and they decide by whether a value is true or false, not by its bits.

Python writes the three logic operators as words, not as &&, || and !.

Python

is_admin = Truehas_token = Falseprint(is_admin and has_token)print(is_admin or has_token)print(not has_token)

and is true only if both sides are. or is true if at least one side is. not flips the value.

Truthiness

Truthiness is whether Python treats a value as true or false when a condition asks. Zero, empty text and None are false. Almost everything else is true.

Python can use any value as a yes or no. A few values count as no; everything else counts as yes.

Python

print(bool(0))print(bool(""))print(bool(None))print(bool("0"))print(bool(-3))

The false values are 0, 0.0, the empty string "", None and False. "0" is text with one character, so it is true.

Short-circuiting and the value that comes back

Short-circuit evaluation is Python stopping early in and and or. In a and b, if a is false then b is never evaluated. In a or b, if a is true, b is skipped.

Python stops as soon as the answer is known. And it returns the value it stopped at, not just True or False.

Python

name = ""print(name or "guest")print("Asha" or "guest")print("Asha" and 42)print(0 and 42)

Example 1 · Guarding a division

Python

divisor = 0result = divisor != 0 and 100 / divisor > 2print(result)

Output

False

What this code does

divisor != 0 is False, so Python stops there.
The division is never run, so there is no ZeroDivisionError.

Example 2 · A default display name

Python

user_name = ""display_name = user_name or "Anonymous"print(display_name)

Output

Anonymous

What this code does

An empty string is false, so or moves on and returns the second value.
A non-empty name would have been returned unchanged.

or returns the first true value, or the last one. and returns the first false value, or the last one. Once the answer is known, the right side is never read.

Membership: in and not in

Membership operator is in or not in. It tests whether a value is found inside something that holds values, such as a piece of text, and gives back True or False.

To ask whether some text appears inside other text, use in. It reads like English.

Python

email = "developer@mentornxt.com"print("@" in email)print("admin" not in email)print("mentor" in email)

Example 3 · Valid file name

Python

file_name = "report.pdf"is_pdf = ".pdf" in file_nameis_temp = "temp" in file_nameprint(is_pdf and not is_temp)

Output

True

What this code does

".pdf" in file_name is True.
not is_temp is True, because the name has no "temp".
not is worked out first, then and, then or.

in is true if the left text is found anywhere inside the right text. not in is the opposite.

Next step

Check your understanding

9 quick questions

Logical and Membership Operators

15 min · Beginner

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Real rules need more than one check. Python combines checks with plain English words, and and / or have a trick of their own: they hand back one of the values.

and, or and not

Logical operator is one of the words and, or and not. They combine or flip conditions, and they decide by whether a value is true or false, not by its bits.

Python writes the three logic operators as words, not as &&, || and !.

Python

is_admin = Truehas_token = Falseprint(is_admin and has_token)print(is_admin or has_token)print(not has_token)

and is true only if both sides are. or is true if at least one side is. not flips the value.

Truthiness

Truthiness is whether Python treats a value as true or false when a condition asks. Zero, empty text and None are false. Almost everything else is true.

Python can use any value as a yes or no. A few values count as no; everything else counts as yes.

Python

print(bool(0))print(bool(""))print(bool(None))print(bool("0"))print(bool(-3))

The false values are 0, 0.0, the empty string "", None and False. "0" is text with one character, so it is true.

Short-circuiting and the value that comes back

Short-circuit evaluation is Python stopping early in and and or. In a and b, if a is false then b is never evaluated. In a or b, if a is true, b is skipped.

Python stops as soon as the answer is known. And it returns the value it stopped at, not just True or False.

Python

name = ""print(name or "guest")print("Asha" or "guest")print("Asha" and 42)print(0 and 42)

Example 1 · Guarding a division

Python

divisor = 0result = divisor != 0 and 100 / divisor > 2print(result)

Output

False

What this code does

divisor != 0 is False, so Python stops there.
The division is never run, so there is no ZeroDivisionError.

Example 2 · A default display name

Python

user_name = ""display_name = user_name or "Anonymous"print(display_name)

Output

Anonymous

What this code does

An empty string is false, so or moves on and returns the second value.
A non-empty name would have been returned unchanged.

or returns the first true value, or the last one. and returns the first false value, or the last one. Once the answer is known, the right side is never read.

Membership: in and not in

Membership operator is in or not in. It tests whether a value is found inside something that holds values, such as a piece of text, and gives back True or False.

To ask whether some text appears inside other text, use in. It reads like English.

Python

email = "developer@mentornxt.com"print("@" in email)print("admin" not in email)print("mentor" in email)

Example 3 · Valid file name

Python

file_name = "report.pdf"is_pdf = ".pdf" in file_nameis_temp = "temp" in file_nameprint(is_pdf and not is_temp)

Output

True

What this code does

".pdf" in file_name is True.
not is_temp is True, because the name has no "temp".
not is worked out first, then and, then or.

in is true if the left text is found anywhere inside the right text. not in is the opposite.

Next step

Check your understanding

9 quick questions