More Conditional Expressions

We introduced the basic comparison operators in the previous section. Here we look at a few more expressions that produce Boolean results.

Membership Testing

The in operator tests whether a value appears in a sequence (string, list, tuple, etc.). Press Try it live and change the operands:

Try it live ▶

>>> "lo" in "hello"
True
>>> "xyz" in "hello"
False
>>> 3 in [1, 2, 3, 4]
True

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not in is the negation. Try it live:

Try it live ▶

>>> "xyz" not in "hello"
True

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Identity: is and is not

The is operator tests whether two names refer to the same object, not just equal values. Its most common use is testing for None. Try it live:

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>>> x = None
>>> x is None
True
>>> x is not None
False

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Use is None rather than == None — it is more precise and Pythonic.

Comparison with Strings

String comparison uses lexicographic (dictionary) order. Try it live:

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>>> "apple" < "banana"
True
>>> "Z" < "a"    # uppercase letters come before lowercase in ASCII
True
>>> "cat" == "cat"
True

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Chained Comparisons Revisited

Python’s chained comparisons work with all comparison operators. Try it live:

Try it live ▶

>>> score = 85
>>> 0 <= score <= 100
True
>>> 'a' <= 'c' <= 'z'
True

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Each expression below evaluates to True or False. Press Try it live, then change the operands and operators to test your understanding:

Try it live ▶

>>> "lo" in "hello"        # membership
True
>>> 3 in [1, 2, 3, 4]
True
>>> "apple" < "banana"     # lexicographic order
True
>>> "Z" < "a"              # uppercase sorts before lowercase
True
>>> score = 85
>>> 0 <= score <= 100      # chained comparison
True

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Be cautious when mixing is in a chain — it is unusual and can be confusing. For numeric range checks, chaining is clear and idiomatic.