Code Snippets: Python - If statements with multiple conditions

Python if Statements with Multiple Conditions

When a Python decision depends on more than one condition, combine expressions with and, or, and not. Use parentheses when grouping makes the intended logic clearer.

The basic pattern is:

if condition_a and condition_b:
    print("both conditions are true")
elif condition_a or condition_b:
    print("at least one condition is true")
else:
    print("neither condition is true")

and versus or

Use and when every condition must be true. Use or when at least one condition is enough:

age = 25
has_ticket = True
is_member = False

if age >= 18 and has_ticket:
    print("entry allowed")

if is_member or has_ticket:
    print("discount available")

Python evaluates and before or. When the rule is mixed, parentheses make the grouping explicit:

if (is_member or has_ticket) and age >= 18:
    print("adult member or ticket holder can enter")

Without the parentheses, age >= 18 and is_member or has_ticket is interpreted as (age >= 18 and is_member) or has_ticket, which may allow a ticket holder of any age.

Checking several values

For boolean or integer flags, Python can test truthiness directly. The values 0, None, and False are falsey; non-zero values and True are truthy.

foo = 0
bar = 0
baz = 1

if foo or bar or baz:
    print("at least one flag is true")
else:
    print("all flags are false")

If the values are specifically flags equal to 1, an explicit comparison is also valid:

if foo == 1 or bar == 1 or baz == 1:
    print("at least one flag is 1")

For a collection of values, any() checks whether at least one item is truthy and all() checks whether every item is truthy:

flags = (foo, bar, baz)

if any(flags):
    print("at least one flag is true")

if all(flags):
    print("every flag is true")

Membership checks with in

Use in when a value should match one of several allowed values. This is usually clearer than a long chain of or comparisons:

status = "pending"

if status in {"pending", "approved"}:
    print("status can be processed")

Common mistakes

• Use and and or between complete expressions: age >= 18 and has_ticket.
• Do not write if age == 18 or 21; Python treats 21 as a separate truthy value. Write if age in (18, 21) instead.
• Add parentheses when mixing and and or so the rule is unambiguous.
• Prefer if items: to if len(items) > 0 when checking whether a list, tuple, or set is non-empty.
• Use is None for None, rather than == None.

For more examples of conditional logic in data work, see the guide to conditional columns in pandas.