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Use if to test a condition, elif to test additional alternatives, and else to handle everything left over. Python checks the branches in order and runs the suite belonging to the first true condition; in an if/elif/else chain, only one suite is selected.
Write a basic if-else statement
An if statement begins with the keyword, a condition, and a colon. Put the statements to run on the next line and indent them consistently. An optional else clause provides a fallback when the condition is false.
age = 17
if age >= 18:
print("Adult")
else:
print("Not an adult")Here, Python tests whether age >= 18 is true. Because age is 17, the indented else suite runs. The Python tutorial explains that an if statement can have zero or more elif parts and an optional else part: Python Tutorial: More Control Flow Tools.
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Use elif when you need to check another condition if the earlier one was false. Python evaluates the chain from top to bottom, stopping at the first true condition. If none is true and an else is present, its suite runs.
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score = 82
default_grade = "Below B"
if score >= 90:
grade = "A"
elif score >= 80:
grade = "B"
else:
grade = default_grade
print(grade)The result is B: the first condition is false, and the second is true. When conditions overlap, arrange them so a more specific or higher-threshold case is checked before a broader one. Otherwise, the broader condition might match first and prevent a later branch from being reached.
Choose a value with a conditional expression
If each branch only chooses one value, you can use Python’s conditional expression: value_if_true if condition else value_if_false.
age = 17
label = "adult" if age >= 18 else "minor"Python evaluates the condition first, then evaluates and returns the value on the matching side. Unlike a statement, a conditional expression produces a value, so it can be assigned to a variable or used within another expression. The language reference defines this form as x if C else y: Python Language Reference: Conditional Expressions.
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Quick Recap
Fix common syntax and logic errors
- Missing colon: End every
if,elif, andelseheader with:. - Inconsistent indentation: Indent each branch’s body consistently so Python can identify which statements belong to it.
- More than one else at the same level: An
ifchain has at most oneelse, and it comes last. Useeliffor additional conditions. - Conditions in the wrong order: If a broad condition is true for values that should match a narrower case, put the narrower test first.
- Assignment instead of comparison: Use
==to test equality in a condition;=assigns a value to a variable. - Unnecessarily compact logic: Replace a hard-to-read conditional expression with a regular
if/elsestatement.
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