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17. Variables and Assignment Rules

1. Introduction​

A variable is a name that stores a value in memory.
In Python, you don’t need to declare a variable type — Python figures it out automatically.
This is called dynamic typing.

2. Creating Variables​

Assign a value to a variable with the = operator:

x = 10
name = "Alice"
pi = 3.14

3. Variable Naming Rules​

  • Must start with a letter or underscore (_).
  • Can contain letters, numbers, and underscores.
  • Case-sensitive (Name and name are different).
  • Cannot be a Python keyword (if, while, class, etc.).

✅ Valid examples:

user_name = "Bob"
age2 = 25
_price = 99.99

❌ Invalid examples:

2cool = "nope"      # cannot start with a number
first-name = "Eve"  # hyphen not allowed
class = "Physics"   # reserved keyword

4. Reassigning Variables​

You can change a variable’s value at any time:

x = 5
x = "hello"

Here, x first held a number, then a string.

5. Multiple Assignments​

You can assign multiple variables at once:

a, b, c = 1, 2, 3

Or give multiple variables the same value:

x = y = z = 0

6. Constants in Python​

Python does not have true constants, but by convention:

  • Use ALL_CAPS names for values that should not change.
PI = 3.14159
MAX_USERS = 100

7. Checking Variable Type​

Use the built-in type() function:

x = 42
print(type(x))   # <class 'int'>

8. Deleting Variables​

You can delete a variable with del:

x = 10
del x

Trying to use x now will give an error.

9. Best Practices​

  • Use descriptive names (age instead of a).
  • Follow lowercase_with_underscores convention.
  • Avoid single-letter names except in loops (i, j).

10. Next Steps​

✅ You now understand how variables work and the rules for naming them.
In the next chapter, we’ll explore Python’s basic data types.