Booleans

There are exactly two literal boolean values in Python: True and False.

However, a large number of Python expressions can evaluate to True or False.

Comparison

Comparison operators are used to test equality (==), inequality (!=) and strict inequality (>, >=, <, <=):

>>> 3 == 3
True
>>> 3 < 3
False
>>> 5 >= 4
True
>>> 'apple' < 'banana'
True

= vs ==

= binds a variable to a value. E.g. x = 5

== compares two values for equality, return True if they are equal, and False otherwise.

Using = where == is expected is a very common error and Python will often flag it.

Comparing non-numeric values

You can compare things that are not numbers, such as strings or user-defined objects. This often requires supplying a comparison criterion or predicate.

By default, strings are compared alphabetically character by character. If you wanted to based the comparison on string length instead, then you would have to be explicit about it: len('apple') < len('banana')

Logical expressions

You can combine multiple boolean expressions using the logical operators and, or and not to produce more complex expressions.

>>> True and True
True
>>> True and False
False
>>> True or False
True
>>> x = 3
>>> x > 0 and x <= 5
True
>>> 0 < x <= 5  # same as above
True
>>> not x > 5
True
>>> age = 25
>>> income = 40_000
>>> if age >= 18 and income > 30_000:   # BOTH must be true
...     print("eligible")
...
eligible
>>> day = "Sun"
>>> if day == "Sat" or day == "Sun":    # EITHER is enough
...     print("weekend")
...
weekend
>>> logged_in = False
>>> if not logged_in:                   # flip a boolean
...     print("please log in")
...
please log in

Truthiness

Python allows you to use any value in conditional statements.

The following values are considered falsy:

zero: 0, 0.0
empty string
empty containers: [], (), {}
None

Everything else (non-zero numbers, non-empty strings, ...) are considered truthy.

>>> name = input("Please enter your name: ")
>>> Please enter your name: Ada
>>> if name:                                    # true if non-empty
...     print("Hi", name)
...
Ada
>>> items = []
>>> if not items:                               # same as `len(items) == 0` but cleaner
...     print("Empty")
...
Empty

Short-circuit evaluation

Python stops evaluation logical expressions as soon as the final outcome can be determined. This allows some useful idioms:

Conditional execution without if statements: age < 18 and print("You are too young")
Default value assignment: user_name = user_input or 'Anonymous User'
Sentinel: guard against errors by ensuring a given operation is allowed. For example, the following code will produce an AttributeError if user is not set:
```
if not user.is_admin():
    print("Access denied")
```
To prevent the AttributeError, ensure user is set:
```
if user is not None and not user.is_admin():
    print("Access denied")
```

Other ways of getting booleans

is, the identity operator. Test if two variables reference the same object. Often used to test for None
Predicates:: functions or methods that return True or False. For example, string values have a method .isdigit() that returns True if all the characters in the string are digits, or False otherwise. Useful for testing valid user inputs. See More on strings