Why Does `1 in [1,0] == True` Evaluate to False in Python?

Chain of Comparison Operators: Why 1 in [1,0] == True Evaluates to False

In Python, a surprising result can arise when comparing the result of an "in" membership test to True:

<code class="python">1 in [1,0] == True  # Unexpectedly returns False</code>

To understand this behavior, it's essential to recognize that Python employs comparison operator chaining. This means that multiple comparison operators in an expression are evaluated sequentially. In the case of the aforementioned code, the expression is interpreted as:

<code class="python">(1 in [1,0]) == True</code>

Breaking it down further:

1. 1 in [1,0] evaluates to True (as 1 is a member of the list [1,0]).
2. [1,0] == True is evaluated next. Contrary to intuition, this expression returns False.

The Secret Behind [1,0] == True

The reason for this unexpected result lies in the way Python handles boolean values and comparisons. When comparing a list to a boolean value, Python first attempts to convert the list to a boolean. In the case of [1,0], this conversion yields False because a non-empty list is considered True in Python.

The Chain Unfolds

Therefore, the original expression simplifies to:

<code class="python">True == False</code>

which obviously evaluates to False. This explains why 1 in [1,0] == True returns False.

Additional Implications

This operator chaining behavior extends to other comparison operators as well. For example:

<code class="python">a < b < c</code>

translates to:

<code class="python">(a < b) and (b < c)</code>

This ensures that b is evaluated only once.

Conclusion

Understanding the concept of comparison operator chaining is crucial for interpreting the behavior of such expressions. By recognizing that the order of evaluation is left-to-right, we can accurately anticipate the results and avoid any confusion.

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