Python for Programmers: Truthiness

• We can call bool to convert values into booleans. All non-zero numbers (including floats) convert to True. Zero numbers like 0 and 0.0 convert to False. We often say that zero is "falsey" and other numbers are "truthy".

• >

  bool(-3)

  Result:

  True

• >

  bool(10)

  Result:

  True

• >

  bool(0)

  Result:

  False

• >

  bool(0.0)

  Result:

  False

• >

  bool(0.2)

  Result:

  True

• The empty string ("") is falsey, and all non-empty strings are truthy. Remember, a string that contains spaces is still non-empty!

• >

  bool("")

  Result:

  False

• >

  bool("Amir")

  Result:

  True

• >

  bool(" ")

  Result:

  True

• Lists are falsey when empty, and truthy when they contain elements. This holds for Python's other collection data types, like tuples and dictionaries.

• >

  bool(["Amir", "Betty"])

  Result:

  True

• >

  bool([])

  Result:

  False

• Some Python syntax takes a boolean condition, like the x in if x: or while x:. When we use a non-boolean value as the condition, Python automatically converts it into a boolean using the rules that we just saw.

• >

  if -1:
    condition_passed = True
  else:
    condition_passed = False
  condition_passed

  Result:

  True

• >

  if []:
    condition_passed = True
  else:
    condition_passed = False
  condition_passed

  Result:

  False

• Every value in Python is either truthy or falsey, with no exceptions. That means that any Python value can technically serve as the condition in an if.

• While most real-world ifs use booleans directly, relying on other values' truthiness is also idiomatic in Python. In particular, we often rely on lists' truthiness to decide whether the list is empty.

• >

  numbers = [2, 3, 6, 8]
  total = 0
  while numbers:
    total += numbers.pop()
  total

  Result:

  19

• Here's a slight tweak where we explicitly check the list's length, rather than relying on truthiness.

• >

  numbers = [2, 3, 6, 8]
  total = 0
  while len(numbers) > 0:
    total += numbers.pop()
  total

  Result:

  19

• We can also rely on the fact that the number 0 is falsey.

• >

  numbers = [2, 3, 6, 8]
  total = 0
  while len(numbers):
    total += numbers.pop()
  total

  Result:

  19

• Any of these ways work, but checking length through truthiness (while numbers:) is most common in Python code. If you're worried that a particular conditional is unclear, it's always fine to do the comparison explicitly, like if len(numbers) > 0.

• Although there are many cases to consider, "truthiness" does follow a cohesive rule. Values that represent "nothing" are falsey: zero numbers, empty strings, empty lists, and False itself. Python's null value, None, is also falsey, but we cover it in a different lesson. All other Python values are truthy.