Variables & assignment

Names for values, and why they matter

Every nontrivial program builds a result in steps: read an input, transform it, combine it, return it. A variable pins an intermediate value to a name so you can reuse it without recomputing, and so the next person (often you, a week later) can read what the code means. total_price tells a reviewer what a number is. x makes them guess. A good name is the cheapest documentation you will ever write.

Assignment binds a name to a value

Read = as "gets", not "equals":

score = 10        # the name score now refers to 10
name = "Ada"      # name refers to the string "Ada"

The right-hand side is evaluated first, then the name is pointed at the result. That is why rebuilding a value from its old self works:

score = 10
score = score + 5   # RHS 10 + 5 runs first, then score is re-pointed to 15

= is an instruction ("make this name refer to that value"), not a claim that two things are already equal.

A worked example

width = 4
height = 3
area = width * height
print(area)            # 12

width = 10             # reassign width only
print(width * height)  # 30

Notice that area is computed once and stays 12. Rebinding width to 10 does not reach back and update area, because area holds the number that width * height produced at that instant, not a live formula.

Name things clearly

Use lowercase words joined by underscores (snake_case) and pick names that say what the value holds:

total_price = 4.99
items_in_cart = 3

You can compute into a well-named variable and then return it, or return the expression directly when it is a one-liner. Both are clear:

def rectangle_area(width, height):
    area = width * height   # store, then return
    return area

def rectangle_area(width, height):
    return width * height   # return the expression directly

Pitfalls

• Reassignment does not recompute earlier results. As above, area stays 12 after width changes. If you need the updated area, recompute it: area = width * height.
• Using a name before it is assigned raises NameError. The name must be bound on some line that actually runs before you read it, so score = score + 5 fails if score was never given a starting value.

Interview nuance: in Python a variable is a name bound to an object, not a box that stores the value. Assignment never copies the object; it just points a name at it. For numbers and strings this is invisible, but the same rule means two names can refer to the same list, so mutating through one name is visible through the other. Remembering that "assignment rebinds, it does not copy" is what saves you from aliasing bugs later.