Conditionals ↩

• The ‘if’ statement
• The ‘if … else’ statement
• The ‘if … elif … else’ statement
• Nested conditionals
• Conditional expressions

When writing a program, sometimes we need to do different things depending on the context: “if this happens, go this way; if that happens, go that way; if none of the above, go this other way”.

The conditional statements if, elif (else if) and else allow us to branch the control flow of a program.

The ‘if’ statement

The if statement allows us to run a certain piece of code only if a certain condition is met. A simple example:

isRaining = True
if isRaining:
    print("take your rain coat")

Here we are using the if statement to check if the variable is set to True; if it is, we print out some text; otherwise, nothing happens.

The expression after if can be anything: a comparison, a membership test, a function, an object, etc.

age = 16
if age < 18:
    print('you cannot get a driver license yet')

The ‘if … else’ statement

The else statement allows us to run a different piece of code when the condition defined in the if statement is not met:

>>> userName = input("user name: ")
>>> if userName:
...     'hello ' + userName
... else:
...     'error (no name given)'

The example script above will first prompt the user for a name. If one is given, the program will return a greeting; if the question is escaped (by pressing Enter), it will return an error message.

The ‘if … elif … else’ statement

If we need to handle more than just two conditions, we can use one or more elif (else if) statements between if and else:

hour = 13

if 0 < hour < 12:
    print("good morning")

elif 12 <= hour <= 17:
    print("good afternoon")

else:
    print("good evening")

Nested conditionals

Conditionals can be nested and combined with other expressions into larger compound statements:

if font:
    if font.selectedGlyphs:
        for glyph in font.selectedGlyphs:
            glyph.doSomething()
    else:
        print('no glyphs selected')
else:
    print('no font selected')

• The if statement
• if Statements

Conditional expressions

Conditional expressions or ternary operators are a condensed form of conditional statements. They can be used inline to choose one value or another depending on the evaluation of a statement.

When used well, conditional expressions can help to reduce code size and enhance readability. Here’s an example:

sky = 'blue'
value = 10 if sky is 'blue' else 100

The corresponding code using if … else statements would take four lines:

sky = 'blue'
if sky is 'blue':
    value = 10
else:
    value = 100

• Conditional expressions
• PEP 308 – Conditional Expressions