Scope & arguments

Local variables

• A variable made inside a function is local.
• It only exists while the function runs.
• Code outside the function cannot see it.

def make_message():
    note = "secret"
    print(note)

make_message()
# print(note)  would be an error: note is local

Global variables

• A variable made outside all functions is global.
• A function can read a global value.
• To change a global inside a function you need the global keyword — but it is usually cleaner to return a value instead.

Default values

• A parameter can have a default value: def f(x, step=1):.
• If the caller leaves it out, the default is used.
• Parameters with defaults come after the normal ones.

def power(base, exp=2):
    return base ** exp

print(power(5))
print(power(2, 3))

How arguments are passed

• Python passes a reference to the object you give it.
• Numbers and strings are immutable: a function cannot change the caller's value.
• Lists are mutable: changes made inside a function do affect the caller's list.

def try_change(n):
    n = n + 100   # changes only the local copy
    print(n)

x = 5
try_change(x)
print(x)          # x is still 5

In Cambridge pseudocode

• BYVAL passes a copy (caller unchanged); BYREF shares the same data (caller changed).

PROCEDURE Increase(BYREF n : INTEGER)
    n ← n + 1
ENDPROCEDURE

PROCEDURE Show(BYVAL n : INTEGER)
    OUTPUT n
ENDPROCEDURE

Now you try

• The checks call your function with different arguments.
• Press Check answer to test your code.