Lesson 03 · Basics

Variables and names

A variable is a name that refers to a value. Step through a four-line program, score.py, and watch what = really does: Python works out the right side first, makes an object in memory, then ties the name on the left to it. Two names can share one object, a name can move on to a new one, and a name with the wrong capital letter does not exist at all.

10 min8 steps5 questionsBeginner

Fig 3 · Interactive

Loading figure…

or ← →

Check yourself

5 quick questions

Pick an answer to see why. Your score saves with your progress.

01In Python, what does x = 7 mean?

= is assignment, not a comparison: Python works out the right side, 7, and ties the name x to it. Comparing uses ==.

02After these lines, what does print(a, b) show? a = 3 b = a a = a + 1

b = a ties b to the same 3. Then a + 1 makes a new value, 4, and only a moves on to it. b still refers to 3, so it prints 4 3.

03In score = score + 5 , which part does Python run first?

The right side of = always runs first, using the value score refers to now. Only then is the name score tied to the result.

04Which of these is a valid variable name?

Names may use letters, digits and underscores but cannot start with a digit (2nd_place), contain a space (my total), or be a keyword (for). total_2 is fine.

05A program sets score = 15 and later runs print(Score) . What happens?

Names are case-sensitive, so Score is a different name from score, and nothing ever made Score. Python stops with a NameError (and since 3.10 suggests: Did you mean: 'score'?).

Key terms

Glossary

Variable: A name that refers to a value. In Python the name points at an object; it does not hold the value inside itself.
Assignment (=): The statement name = value. Python works out the right side first, then ties the name on the left to the result.
Object: Python's word for one value in memory, such as the whole number 10. Every object has a type, such as int.
Reassignment: Assigning to a name that already exists, which moves the name on to a new value. Other names are not affected.
Identifier: The technical word for a name: letters, digits and underscores, not starting with a digit.
Keyword: A word Python reserves for its own grammar, such as if, for and while. It cannot be used as a name.
Case-sensitive: Capital and small letters count as different, so score, Score and SCORE are three different names.
NameError: The error Python raises when code uses a name that has not been assigned.

Fine print

Notes and sources

Python calls every value an object. A variable does not hold its value like a box; it is a name tied to an object, and = changes which object a name is tied to.
Whole numbers (int) and strings never change once made, so score + 5 always makes a new object. Two names can share any object, but sharing only shows when an object can change, such as a list.
The is operator asks whether two names refer to the very same object; == asks whether two values are equal.
On macOS and Linux the command is often python3 rather than python; on Windows the py launcher also works. The $ is the terminal's own prompt: you do not type it.
The real NameError message is one line; the figure wraps it. Python also shows the failing line with ^^^^^ under Score, and the ... stands for the full path to the file. The "Did you mean" hint appeared in Python 3.10.
Python style (PEP 8) writes variable names in lower case with words joined by underscores, such as total_score.

Quiz Unlocks after the last step.