TopolearnBranching is the first step from a calculator to a program, and truthiness is used everywhere.
Comparisons (==, !=, <, <=, >, >=) return True or False. They can be chained: 0 <= x < 10 means 0 <= x and x < 10. Combine conditions with the words and, or and not.
An if statement runs its indented block when the condition is true; elif adds more branches and else catches the rest. Only the first matching branch runs.
Every object has a truth value, so you can write if items: instead of if len(items) > 0:. These are falsy: False, None, 0, 0.0, empty strings, and empty containers ([], {}, (), set()). Almost everything else is truthy.
and and or short-circuit and return one of their operands, not necessarily a bool. a or b returns a if it is truthy, otherwise b; that is why name = given or "anonymous" works as a default. a and b returns a if it is falsy, otherwise b, so user and user.name is safe when user might be None.
For a simple choice between two values, the conditional expression reads like English: label = "even" if n % 2 == 0 else "odd".
Use == to compare values and is only to check identity, which in practice means x is None. Two equal strings or numbers may or may not be the same object, so is on them is a bug waiting to happen.
Example
def classify(temp):
if temp < 0:
return "freezing"
elif temp < 18:
return "cold"
elif temp <= 25:
return "pleasant"
else:
return "hot"
print(classify(-3), classify(21)) # freezing pleasant
print(0 <= 5 < 10) # True
items = []
if not items:
print("nothing to do") # nothing to do
name = "" or "anonymous"
print(name) # anonymous
n = 7
print("even" if n % 2 == 0 else "odd") # oddRemember
- Empty containers, 0, '' and None are falsy
and/orshort-circuit and return an operand- Use
isonly forNone; use==for values
Watch out for
- Writing
if x == True:orif len(x) > 0:instead ofif x:. - Using
orfor defaults when 0 or '' are valid values, since they are falsy too.