Why Does `0 < 0 == 0` Evaluate to False in Python?

Understanding Chained Comparisons: Why 0 < 0 == 0 Evaluates to False in Python

The Python code snippet from the standard library raises a question about the evaluation of the expression 0 < 0 == 0, which returns False unexpectedly. This article will delve into the concept of chained comparisons in Python to provide an explanation for this behavior.

Chained Comparisons in Python

Python allows multiple relational operators to be chained together, enclosed in a single expression. Chained comparisons make it convenient to express comparisons of values within a range. For instance, instead of writing (0 < x) and (x <= 5), you can write the concise form 0 < x <= 5.

Evaluation Process

The key to understanding why 0 < 0 == 0 evaluates to False is to recognize Python's special case handling for chained comparisons. Python evaluates chained comparisons from right to left, with the exception of the initial comparison, which is evaluated first.

In the expression 0 < 0 == 0, the initial comparison is 0 < 0, which evaluates to False. The remaining comparisons, 0 == 0, are irrelevant as a False value precedes them. Therefore, the expression evaluates to False as a whole.

Breaking the Chain

Parentheses can be used to force the evaluation of specific relational operators before others, thereby breaking the chained comparison. For instance:

• (0 < 0) == 0 evaluates to True because the parentheses ensure that 0 < 0 is evaluated first, resulting in a True value.
• 0 < (0 == 0) also evaluates to True as the comparison 0 == 0 is evaluated first, yielding True.

Conclusion

Python's chained comparisons simplify the expression of range comparisons. However, it's important to understand the evaluation process to avoid unexpected results. By default, chained comparisons are evaluated from right to left, with the initial comparison always being evaluated first. Parentheses can be used to group comparisons and force specific evaluation orders.

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