Comparison and Boolean Logic

A comparison is an expression that produces a value, the same way 3 + 4 produces 7. The value it produces is either True or False. By the end of this lesson you can build such expressions, combine them with and, or and not, and print the result.

A comparison produces a value

Six operators compare two values and hand back a boolean.

The result is an ordinary value, so you can store it:

bool is a type of its own, alongside int and str. It has exactly two values and they are spelled with a capital letter: True and False. Lowercase true is a NameError, and the text "True" in quotes is a string, not a boolean.

One = assigns a value to a name; two == compare two values. Mixing them up is the oldest beginner bug there is.

Text compares too, and it is picky

Text and numbers cannot be ordered against each other:

A line you read with input() is always text, so convert it with int() before you compare it as a number.

in: is one piece of text inside another?

in looks for the piece anywhere in the text; == demands the whole thing. Different questions, different answers.

and, or, not

A      B      A and B   A or B
True   True   True      True
True   False  False     True
False  True   False     True
False  False  False     False

not grabs only what comes directly after it. To flip a whole combined expression, wrap that expression in parentheses first:

Stored booleans combine exactly like the comparisons themselves:

Short-circuit evaluation

Python reads and and or left to right and stops the moment the answer is settled.

Neither inner print runs. That is what makes the guard count != 0 and total / count > 5 safe: when count is 0 the left side is already False, so the division never happens.

== versus is

== asks whether two values are equal. is asks whether two names refer to the very same object. Use == for the comparisons in this course, and keep is for the one idiom value is None.

Common mistakes

• or where a range needs and: n >= 50 or n <= 200 is True for every number on earth, because each number satisfies at least one half. The range test needs and.
• > where the boundary counts: with "between 50 and 200, both ends included", n > 50 prints False for exactly 50, which is supposed to be inside the range.
• in where == was meant: "go" in "golden" is True, but "go" == "golden" is False. The loose question says True where the strict one says False.
• Comparing a raw input line as a number: without int(), the comparison raises TypeError and your program dies before printing anything.

Your exercise

Print the four boolean checks described below, one per line. The starter reads both lines and prints a placeholder on each output line; replace each placeholder with the comparison it names. The one that catches most people is the fourth: not needs parentheses around the range it flips.