Why Do Multiple `or` Operators in Python Produce Unexpected Boolean Results?

Why Multiple Or Operators Produce Unexpected Results

In Python, evaluating an expression involving multiple or operators can be misleading. For instance, consider the following line of code:

if name == "Kevin" or "Jon" or "Inbar":

This line intends to grant access to unauthorized users, but in reality, it allows all users. To understand why, we need to examine the operator's behavior.

The Problem with Multiple Ors

In English, "or" connects multiple clauses, indicating that at least one must be true. However, in Python, an expression like "A or B or C" is parsed as "(A or B) or C." This means that only the first expression is evaluated. If it is true, the entire expression is true.

Solution 1: Multiple Equality Operators

To correctly compare "name" to each authorized user, use multiple equality operators:

if name == "Kevin" or name == "Jon" or name == "Inbar":

Solution 2: Set Membership

An alternative solution is to use a set of authorized names and check for membership:

authorized = {"Kevin", "Jon", "Inbar"}
if name in authorized:

Solution 3: Any Operator

The any() function can also be used to iterate over authorized names and return True if any of them match "name":

authorized = ["Kevin", "Jon", "Inbar"]
if any(name == auth for auth in authorized):

Performance

Among the three solutions, set membership offers the best performance, followed by multiple equality operators. Using any() is the least efficient.

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