Conditionals and Boolean Expressions

An if/elif/else chain evaluates conditions in order and executes the first truthy branch.

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "C or below"

Order matters when conditions overlap.

Truth values

None, False, numeric zero, and empty containers are false by default. Most other objects are true; user-defined classes may customize truth testing.

Use an explicit comparison when empty, zero, and missing have different meanings:

if records is None:
    load_records()
elif not records:
    report_empty_dataset()

Comparisons

• == and != compare values.
• is and is not compare identity.
• in and not in test membership.
• Ordered comparisons can be chained: lower <= value < upper.

Do not use is for numeric or string value comparison; interpreter object reuse is not a semantic guarantee.

Short-circuit operators

and and or evaluate left to right and return one of their operands, not necessarily a bool:

display_name = supplied_name or "anonymous"
safe_ratio = denominator and numerator / denominator

The second expression is evaluated only when needed. Use this for guards, but avoid compact expressions whose mixed return types obscure the contract.

Branch design

• Handle invalid or exceptional cases early to reduce nesting.
• Extract a named predicate when a condition encodes domain logic.
• Use a lookup table or polymorphism when a large branch selects behavior by a stable key.
• Use match for structural patterns, not as a compulsory replacement for every simple equality chain.

Expression boundaries

The expression reference places comparisons above not, then and, then or in precedence. Parentheses make a mixed condition easier to read. not always returns a Boolean, and a chained comparison evaluates its middle expression only once.

assert not ""                         # empty string
assert bool("0") and bool("False")   # both nonempty
assert (0 or 7) == 7
assert (0 and 7) == 0
assert (2 < 3 < 4) is True
value = 0
result = value if value is not None else 10
assert result == 0

Use x == "a" or x == "b", or x in {"a", "b"}; x == "a" or "b" is always truthy because "b" is nonempty. A conditional expression yes if condition else no evaluates only the selected result. value or default would replace valid zero values as well as None. Independent if statements can execute several bodies; an if/elif chain selects at most one. Its else handles the case where no condition succeeds, and is optional.

Source

• More Control Flow Tools
• Boolean Operations

My reading · Opened notes are remembered in this browser.