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We have a variable and we want to check if its value is permissible. Let’s say that the user enters the name of a day and we want to check if it’s Saturday or Sunday. It seems obvious in Python, but it’s not:

if day.lower() == 'saturday' or 'sunday': # It’s weekend!

The boolean or operator has lower precedence than the comparison operator (==), which means that the comparison is evaluated before the boolean operator. So if we place parentheses around the higher-precedence operator to avoid confusion, the above statement looks like this:

if (day.lower() == 'saturday') or 'sunday': ...

The value of an or expression is true if any of its operands are true. The non-empty 'sunday' string is always interpreted as true. Thus, the condition is always true, regardless of the day. It’s good to have a never-ending weekend.

Perhaps, then, we should adjust the precedence by enclosing the or expression in the parentheses?

if day.lower() == ('saturday' or 'sunday'): ...

No, again. The value of the or operator is first True operand, or False if both operands are false. Because 'saturday' is a non-empty string, it’s interpreted as true. The expression in the parentheses is always 'saturday' and never 'sunday'. The expression treats Sunday as a weekday. We’d rather have a never-ending weekend than a one-day weekend.

The correct way to go is to check if the day is in a collection of suitable days using the membership operator, in. The collection can be a list, a tuple, or even a dictionary, but we get optimal performance by using sets.

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