Master Python Variables and Constants: Key Differences and Practical Examples

1. Difference Between Variables and Constants

In programming, a variable holds a value that can change during execution, while a constant’s value remains unchanged.

Both allocate memory space when created.

2. Variables in Python

1. No Type Declaration Needed

Python’s dynamic nature allows variables to be created without specifying a type. For example:

a = 4
b = "haha"
c = []
d = 9-5

2. Assignment with “=”

The “=” operator assigns a value to a variable, not a mathematical equality.

a = 100

3. Variables Must Be Assigned Before Use

Using a variable before assignment raises a NameError.

# a          # NameError
a = 1        # OK
c.append(1)  # NameError because c is undefined

4. Variables Have No Intrinsic Data Type

The type belongs to the object the variable references.

a = 1
a = "haha"
a = [1, 2, 3]
a = {"k1":"v1"}

5. Assignment Evaluates Right‑to‑Left

The right‑hand side is evaluated first, then the result is stored in the left‑hand variable.

a = 1
b = 2
c = a + b   # c becomes 3
print(c)

6. Multiple Assignment

Python can assign the same value to several variables or unpack tuples.

a = b = c = 1
a, b, c = 1, 2, 3

Remember that “=” is assignment, not mathematical equality.

x = 1
x = x + 2   # x becomes 3

When executing a = 'ABC', Python creates a string object and a variable that references it. Assigning one variable to another makes the second reference the same object, but reassigning the first does not affect the second.

a = 'Jack'
b = a
a = 'Tom'
print(b)   # Jack
print(a)   # Tom

All objects are references; variables are just names pointing to objects.

3. Constants

Constants are conventionally written in uppercase, e.g., PI = 3.14159265359, but Python does not enforce immutability.

They are usually defined at the top of a module and used globally.

4. Summary

This article introduced the basic differences between variables and constants in Python, demonstrated variable usage with numerous examples, and highlighted that everything in Python is an object reference.