Preview

The whole resource, exactly as a class sees it. Answers and marking are held back.

Variable scope

A variable's scope is the part of a program that can access it. A local variable belongs to one subprogram call, while a global variable can be accessed more widely. Two variables may have the same name but still be separate because they belong to different scopes.

By the end, you will be able to

identify which local and global variable each statement accesses
predict shadowing without treating same-named variables as shared
explain side effects and drawbacks of excessive global state
replace hidden global state with parameters and returned values

Worked scope reading

Read a name from the inside out. If a function assigns to score without selecting the global variable, that call creates or changes its local score. A separate global score can still exist outside the function. Returning the local result does not itself overwrite the global; the caller decides where to store the returned value.

A local variable is created for the call and is no longer available after that call ends. A global variable normally exists for the lifetime of the program. Therefore a complete prediction tracks score in global scope, score in the current function call and the destination of the returned result separately.

Worked example

score = 10

def add_bonus(points):
    score = points + 2
    return score

new_score = add_bonus(5)
print(score)
print(new_score)

Worked model: draw the boundary

The assignment inside add_bonus creates/uses the local score. It does not cross the function boundary to replace global score. The output is therefore 10 then 7.

For every scope trace, write variable name and scope, for example score(global) and score(add_bonus). This small label prevents accidental merging.

Multiple choice1 mark

What does the supplied Python program print?

A7 then 7
B10 then 12
C10 then 7
DIt always raises an error

Written answer3 marks

Explain the difference between the global score and the local score in the supplied program.

Refer to where each can be accessed and which assignment changes.

Students type their answer here.

Remove unnecessary global state

Refactor the function so it takes the current score and bonus as parameters and returns the updated score. It must not use a global declaration.

Coding task3 marks

def updated_score(current, bonus):
    pass

Written answer4 marks

A different version stores tax_rate = 0.2 locally inside final_price(). Rewrite the short design so tax_rate is a global variable that the function reads, then state one benefit and one drawback of that change.

Show the global declaration/assignment and the function use. Keep the comparison balanced: wider access may avoid repeated parameters, but creates a hidden dependency and may keep data alive for longer.

Students type their answer here.

Written answer4 marks

Explain two drawbacks of excessive global variables in a larger program.

For each, give a mechanism and its consequence for development or execution.

Students type their answer here.

Checkpoint

Complete the scope explanation from memory. There is no answer bank, and correctness is withheld until teacher review.

Fill in the blanks4 marks

A checkpoint gap 1 variable is accessible only within its declared subprogram or block. A checkpoint gap 2 variable can be accessed more widely. A local name can checkpoint gap 3 a global name, and unexpected shared-state changes are checkpoint gap 4.

Review your understanding

Before submitting, check that you can explain the main distinction in your own words, apply it in an unfamiliar context and justify the resulting behaviour or consequence.