Functions and scope — Python Module 5 | LeadAfrik Public Economics Hub

Functions are how you package up reusable logic. Python's function syntax is clean, supports default arguments, variable-length arguments, and (since 3.5) type hints. Knowing the LEGB scope rule is what separates competent Python from confused Python.

Defining and calling

python

def compound(principal, rate, years):
    return principal * (1 + rate) ** years

compound(1000, 0.10, 5)    # 1610.51

Default arguments

Default arguments let you call a function with fewer arguments. Defaults are evaluated once at function definition time — using a mutable default (like an empty list) is a notorious bug.

python

def compound(principal, rate, years=10):
    return principal * (1 + rate) ** years

compound(1000, 0.10)       # uses years=10
compound(1000, 0.10, 5)    # overrides to 5

The mutable-default trap

def add_rate(rate, history=[]): history.append(rate); return history. The default list is shared across all calls — every call mutates the same list. Fix: use None as the default and create a fresh list inside the function.

Keyword arguments

Named arguments make calls self-documenting. The forced-keyword convention (using * to mark the boundary) is increasingly common in modern Python.

python

def compound(principal, *, rate, years):  # rate and years must be passed by keyword
    return principal * (1 + rate) ** years

compound(1000, rate=0.10, years=5)

*args and **kwargs

*args collects extra positional arguments into a tuple. **kwargs collects extra keyword arguments into a dict. Use them when you don't know in advance how many arguments will be passed.

python

def total(*amounts):
    return sum(amounts)

total(100, 200, 300)        # 600
total(*[10, 20, 30])        # 60 (unpacking a list)

Scope: LEGB

Python looks up names in this order: Local (the current function), Enclosing (any outer function), Global (module level), Built-in. A name not bound locally and not declared global will fall through to enclosing/global scope. This is usually intuitive; the surprises happen when you assign to a global name from inside a function (you must declare 'global x' first).

Exercise

Write a function that takes a list of rates and returns their average.