Lists, tuples, dicts, sets — Python Module 3 | LeadAfrik Public Economics Hub

Python has four main built-in collection types. Knowing when each one is right — and how to convert between them — is the single most leveraged knowledge in everyday Python.

list — ordered, mutable, the workhorse

python

rates = [0.07, 0.08, 0.085, 0.10, 0.125]
rates.append(0.13)        # add to the end
rates[0]                  # 0.07 (zero-indexed)
rates[-1]                 # 0.13 (last item)
rates[1:3]                # [0.08, 0.085] (slice)
rates.sort()              # sorts in place
len(rates)                # 6

Lists are the default sequence type. Use them whenever order matters and you might add or remove items.

tuple — ordered, immutable

Tuples look like lists but cannot be modified after creation. Use them for fixed records (a coordinate, a (date, value) pair) and as dictionary keys (which lists cannot be).

python

point = (3.14, 2.71)
x, y = point          # tuple unpacking
x, y = y, x           # swap, the Pythonic way

# Returning multiple values
def stats(xs):
    return min(xs), max(xs), sum(xs) / len(xs)

lo, hi, mean = stats([1, 2, 3, 4, 5])

dict — key-value mapping

Dictionaries store key→value associations with O(1) lookup. The most common data structure after lists.

python

shares = {'equity': 0.4, 'fixed_income': 0.5, 'cash': 0.1}
shares['equity']            # 0.4
shares['property'] = 0.05   # add a key
'cash' in shares            # True
for asset, weight in shares.items():
    print(f'{asset}: {weight:.0%}')

set — unordered collection of unique values

Sets are useful when you want fast membership testing or to deduplicate a list. They support mathematical operations (union, intersection, difference).

python

tier_1 = {'KCB', 'Equity', 'Coop', 'NCBA', 'Stanbic'}
tier_2 = {'Family', 'Coop', 'Sidian', 'Equity'}
tier_1 & tier_2             # {'Equity', 'Coop'} (intersection)
tier_1 | tier_2             # union
tier_1 - tier_2             # {'KCB', 'NCBA', 'Stanbic'} (difference)

When to use which

List: ordered, mutable — your default. Tuple: small fixed record, dict key. Dict: key→value lookup. Set: deduplication, fast membership tests. If you find yourself iterating a list to check membership, use a set instead — O(n) becomes O(1).

Exercise

Create a dict mapping three Kenyan asset classes to portfolio weights — equities 40%, fixed income 50%, cash 10%. Then print each line as 'Asset: 40%'.