Environments and Scope: The Rule Behind UnboundLocalError

Why can a function freely read a global variable, yet raise UnboundLocalError the instant you try to assign to that same name?

Finger exercises

Short drills on what this video just taught. Write the code, run the checks, and reveal the answer only if you are stuck.

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Exercise 1 of 5

Arguments map to parameters by position

Define a function subtract(a, b) that returns a - b. Two values are given: x = 10 and y = 3. Call your function so that it computes 10 minus 3 — think about which argument lands on which parameter — and assign the returned value to diff.

x = 10
y = 3

def subtract(a, b):
    ...  # your code here

diff = ...  # call subtract to compute 10 - 3, then store the result

x = 10
y = 3

def subtract(a, b):
    ...  # your code here

diff = ...  # call subtract to compute 10 - 3, then store the result

Python runs in your browser

⌘/Ctrl + Enter runs

is a number

subtracts in the right order

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