Comparisons and logic

Picture a ticket checker at the door of a cinema showing an 18+ film. The checker asks questions answered with "yes" or "no": do you have a ticket, are you 18 yet. You get in only when both answers are "yes".

A program constantly needs the same kind of answers: whether a user is an adult, whether a number falls within a range. Such an answer comes from a comparison, and its result has type bool:

age = 20
adult = age >= 18
teen = 13 <= age <= 19
print(adult, teen)  # True False
print(age == 20, age != 20)  # True False

Let's go through the program line by line:

1. adult = age >= 18 puts the answer to "is twenty at least eighteen?" into adult: True.
2. teen = 13 <= age <= 19 checks whether age falls within the range from 13 to 19. Twenty does not, so teen is False.
3. The last line prints True False: age is 20, so "is not 20" is false.

Comparisons

There are six comparison operators:

• == means equal;
• != means not equal;
• < and <= mean less than, less than or equal;
• > and >= mean greater than, greater than or equal.

The double sign == checks equality, and a single = assigns a value. An int and a float compare by value: 1 == 1.0 gives True. A string never equals a number: "5" == 5 gives False.

Comparisons can be chained, as in math. The chained comparison 13 <= age <= 19 means "13 <= age, and also age <= 19". Comparisons run after arithmetic: n % 2 == 0 means (n % 2) == 0.

and, or and not

Logical operators join conditions, the way the checker joins the questions. There are three of them.

and

a and b holds when both operands hold. That is how the checker lets you in: you have a ticket, and you are 18.

or

a or b holds when at least one operand holds. That is how the cashier gives a discount: the viewer is a student or a retiree.

not

not a turns True into False and back. not runs first, then and, and or comes last.

Short circuit

Once the checker sees there is no ticket, they no longer ask about age: the answer is clear. Python does the same. and and or evaluate the left operand first, and if it already settles the answer, the right one is not evaluated at all.

For and that is a false left operand, for or a true one. This rule is called short-circuit evaluation:

def is_positive(n):
    print("check", n, end=" ")
    return n > 0


print(is_positive(-1) and is_positive(5))  # check -1 False
print(is_positive(5) or is_positive(-1))  # check 5 True

Let's see what got printed:

1. is_positive(-1) prints check -1 and returns False.
2. The left operand of and is false, so is_positive(5) is not called, and print outputs False.
3. In the second line, is_positive(5) returns True, and or no longer calls is_positive(-1).

In each line the function was called once: the right operand was not needed.

What and and or return

Here the checker analogy ends. The checker answers only "yes" or "no", but and and or in Python return one of their operands.

An operand of and and or need not be a bool. Then zero and the empty string "" count as false, and other numbers and non-empty strings count as true. The result is the operand where evaluation stopped:

print(0 or 7, "" or "guest", 5 or 7)  # 7 guest 5
print(0 and 7, 5 and 7)  # 0 7
print(not 5, not "")  # False True

The rules are:

• or returns the first true operand or, if there is none, the last one;
• and returns the first false operand or, if all are true, the last one;
• not always returns True or False.

That is why "" or "guest" gives guest: this is how a default value replaces an empty string. Interviewers like to ask this: "What does 0 or 7 return?" The right answer is 7, not True.

Common mistake

The check "the answer is 1 or 2" is written the way it is said:

answer = 3
is_valid = answer == 1 or 2  # ✗
print(is_valid)  # 2

Python reports no error. The expression reads as (answer == 1) or 2: the left side gave False, and or returned the right operand 2. Each side needs its own comparison: answer == 1 or answer == 2 gives False.

Key points

• A comparison gives a bool; == checks equality, and = assigns.
• and and or do not evaluate the right operand if the left one settles the answer.
• or returns the first true operand, and the first false one; if there is none, both return the last one.