Python’s membership operators evaluate whether or not a variable exists in a specified sequence. There are two membership operators, as shown in the table that follows

Membership Operator

Description

in

It evaluates to true if it finds a value in the specified sequence; it evaluates to false otherwise.

not in

It evaluates to true if it does not find a value in the specified sequence; it evaluates to false otherwise.

Next are some examples of Boolean expressions that use membership operators.

  • x in [3, 5, 9]. This can be read as “test if x is equal to 3, or equal to 5, or equal to 9”. It can be written equivalently as

x == 3 or x == 5 or x == 9

  • s in "ace". This can be read as “test if the content of variable s appears in the word “ace” or in other words, test if s is equal to letter “a”, or equal to letter “c”, or equal to letter “e”, or equal to word “ac”, or equal to word “ce”, or equal to word “ace”. It can be written equivalently as

s == "a" or s == "c" or s == "e" or s == "ac" or s == "ce" or s == "ace"

Notice: Please note that the Boolean expression s in "ace" does not check whether or not s is equal to “ae”. It only checks for the presence of the content of variable s within the specified sequence of letters in the word “ace”.

  • s not in ["a", "b"]. This can be read as “test if x is not equal to letter “a”, nor equal to letter “b”. It can be written equivalently as

not(s == "a" or s == "b")

or as

s != "a" and s != "b"

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