Your First Variable

第一个变量

≈ 10 min · python-variables · Open in WolfCode

A variable is a name you give a value so you can refer to it later. That is the whole idea — but everything else in this course is built on it.

Why we need variables

A computer can compute 3 + 4 just fine. The trouble starts when the same number shows up in five different places. If you change one of them and forget the others, the program breaks. A variable is a name that holds onto a value for you, so you only have to write the value once.

Without a variable · don't do this

print("Hi Alice")
print("Alice's age is 18")
print("Happy birthday, Alice!")
# If Alice changes their name, you have to edit all three lines.

Loading Python runtime (~5 MB, one-time)…

With variables · this is the move

name = "Alice"
age = 18

print("Hi", name)
print(f"{name}'s age is {age}")
print(f"Happy birthday, {name}!")
# Change name once at the top; all three lines update.

Loading Python runtime (~5 MB, one-time)…

The assignment statement

The single most important symbol in this lesson is =. In Python it is the assignment operator. It does not mean "equals" in the math sense — read it as "gets":

age = 18 → "the name age gets the value 18"
name = "Alice" → "the name name gets the string "Alice""

The name always goes on the left, the value on the right. Reversing them is a syntax error:

Try running this — and read the error carefully

18 = age

Loading Python runtime (~5 MB, one-time)…

Naming rules

A name in Python can use letters, digits, and underscores. There are three rules:

Must start with a letter or underscore — not a digit.
No spaces, no hyphens, no @ # $.
Cannot be a Python keyword like if, for, class, return.

By convention in Python, names use snake_case: first_name, total_cents, is_logged_in. Not firstName (that's Java/JavaScript), not FirstName (Python reserves that for class names).

Which of these names are legal?

# Try uncommenting each line one at a time and run.

first_name = "Alice"     # ✓ snake_case
age2 = 18                # ✓ digits allowed after the first character
_private = True          # ✓ underscore allowed

# 2nd_name = "Bob"       # ✗ starts with a digit
# first-name = "Bob"     # ✗ Python reads this as: first MINUS name
# class = "CSC 141"      # ✗ class is a Python keyword
# my name = "Bob"        # ✗ spaces are not allowed

print("All defined names:", first_name, age2, _private)

Loading Python runtime (~5 MB, one-time)…

Variables can be reassigned

The "variable" part of the word is literal: the value behind a name can change. The new value simply replaces the old one.

Python · runnable

score = 0
print("Start:", score)

score = score + 10     # read right to left: "old score + 10, store back in score"
print("After hit:", score)

score = score + 25
print("After bonus:", score)

Loading Python runtime (~5 MB, one-time)…

Check yourself

x = 10 then x = 20 then print(x)

After this code runs, what does `print(x)` show?

Now you try

Open this lesson in WolfCode: the file 01-first-variable.py has age = ? waiting for a value. Assign your own age, then run the lesson with Ctrl+Enter.