Variables in Python#

🐍 Data Analysis Using Python 🐍 Python Fundamentals Lesson 005

Important

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Naming values for reuse#

A program that could not remember values would be useless; variables are how Python remembers. A variable is a named container for a value, letting you store data, refer to it by name, and change it as the program runs. This lesson covers variables in Python — assigning, reassigning, and using them — the foundation of holding data in code.

Assignment#

A variable is created by assigning a value to a name with =:

sales = 1000
region = "North"
tax_rate = 0.08

The name on the left is bound to the value on the right; afterwards, the name stands for the value:

total = sales + sales * tax_rate
print(total)              # 1080.0
print(region)             # North

The = here is assignment, not mathematical equality — it means “let this name refer to this value,” a distinction worth keeping clear.

Reassignment#

A variable’s value can change — reassigning binds the name to a new value:

count = 5
count = count + 1         # count is now 6
count += 1                # shorthand for count = count + 1; now 7

The last form, +=, is an augmented assignment — a common shorthand for updating a variable based on its current value (-=, *=, /= work similarly). Reassignment is what lets a variable track a changing value as a program runs.

Why variables matter#

Variables serve the same purposes as naming anything: reuse (compute a value once, use it many times), clarity (a well-named variable documents what a value means — tax_rate is clearer than 0.08 scattered through code), and changeability (update a value in one place and everything using it updates). This is the abstraction-and-naming principle from the foundations, in code: meaningful names for values make programs readable and maintainable, exactly as meaningful column names make data readable.

The caveat#

Variables have subtleties that catch beginners. A variable must be assigned before it is used — referencing a name Python has not seen raises an error. Reassignment means a variable’s value depends on when you look (its value is whatever was last assigned), which matters especially in notebooks where cells run out of order — a variable can hold a surprising value if cells ran in an unexpected sequence. And Python variables are case-sensitive (Sales ≠ sales), a frequent source of “undefined name” errors. These are learned by writing code and reading the errors, which say precisely what went wrong. The next lesson covers naming variables well.

Hint