Conditions

Note

Source: Adapted from the C# edition (decisions/conditions.rst). Chained comparisons and the truthiness discussion are Python-specific features drawn from the SE4ML Python chapter (chapter_python.rst, lines 872–905).

So far all our programs have executed instructions sequentially. But many problems require making a decision: do one thing if some condition is true, something else otherwise.

A condition is an expression whose value is either True or False. These are Python’s Boolean values (named after 19th-century mathematician George Boole), and their type is bool.

>>> 2 < 5
True
>>> 3 > 7
False
>>> x = 11
>>> x > 10
True
>>> 2 * x < x
False

Comparison Operators

The basic comparison operators that produce Boolean results are:

Operator	Meaning
==	Equal to
!=	Not equal to
<	Less than
>	Greater than
<=	Less than or equal to
>=	Greater than or equal to

Warning

Use == for comparison, = for assignment. Writing if x = 5: is a SyntaxError in Python.

The bool Type

True and False are built-in constants, not strings. You can also create Boolean values with the bool() conversion function:

>>> bool(0)
False
>>> bool(1)
True
>>> bool("")
False
>>> bool("hello")
True

Any non-zero number and any non-empty sequence is truthy; zero, None, and empty sequences are falsy. This matters in if statements.

Chained Comparisons

Python allows comparisons to be chained — a convenience not in C#:

>>> x = 5
>>> 0 < x < 10
True
>>> 0 < x < 4
False

The expression 0 < x < 10 is equivalent to 0 < x and x < 10. Chaining reads naturally and avoids repeating the middle variable.