More Conditional Expressions

Note

Source: Adapted from the C# edition (decisions/comparisonops.rst). Membership (in/not in) and identity (is/is not) operators are Python-specific additions; string comparison examples are original.

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.):

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

not in is the negation:

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

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:

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

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

Comparison with Strings

String comparison uses lexicographic (dictionary) order:

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

Chained Comparisons Revisited

Python’s chained comparisons work with all comparison operators:

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

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