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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.

Source​