Programmer's Picnic · Python Foundations · 0 → ∞

Python Dictionaries: From 0 to Infinity

Organise data under meaningful names and build small record-processing programs.

Free course · 65–90 minutes reading and practice · No installation beyond Python 3

Start at Level 0 ↓Try code in Python Editor ↗Collections overview

How to learn

Read one level, type the code into the editor, predict the output, then run it. Change one line and observe the result before proceeding. All code examples work with standard Python 3 (the dictionary union example requires 3.9 or newer).

The course moves from first principles to realistic code, common mistakes, performance and a mini-project.

Level 0 / 10

0 · Keys and values

A dictionary maps unique hashable keys to values. It is mutable and maintains insertion order in modern Python (3.7+).

Python 3 · runnable example

student = {'name': 'Asha', 'marks': 89}
print(student['name'])
print(len(student))
print(type(student).__name__)

Expected output

Asha
2
dict

Remember: A key occurs only once. Writing it again replaces the prior value.

Level 1 / 10

1 · Safe lookups

Accessing a missing key with [] raises KeyError. get() returns None or a supplied default when the key is absent.

Python 3 · runnable example

record = {'name': 'Ravi'}
print(record.get('marks'))
print(record.get('marks', 0))
print('name' in record)

Expected output

None
0
True

Remember: Choose [] when absence is a programming error; use get() for optional fields.

Level 2 / 10

2 · Insert, update and delete

Assigning to a key inserts or replaces a value. update() changes several fields. pop() removes and returns a value.

Python 3 · runnable example

stock = {'pen': 10}
stock['pen'] += 5
stock['book'] = 4
stock.update({'eraser': 8})
removed = stock.pop('book')
print(stock)
print(removed)

Expected output

{'pen': 15, 'eraser': 8}
4

Remember: pop('missing', default) avoids KeyError; clear() empties the dictionary.

Level 3 / 10

3 · Iterate over items

A loop over a dict yields keys. items() yields key-value pairs; values() yields values. Keep the order predictable by sorting if needed.

Python 3 · runnable example

marks = {'Asha': 80, 'Ravi': 75}
for name, score in marks.items():
    print(f'{name}: {score}')
print(sum(marks.values()))

Expected output

Asha: 80
Ravi: 75
155

Remember: Do not add or delete keys while iterating over the same dictionary.

Level 4 / 10

4 · Nested dictionaries

Store meaningful records as dictionaries inside a larger dictionary. get() can help with missing optional fields.

Python 3 · runnable example

students = {
    'S01': {'name': 'Asha', 'marks': 90},
    'S02': {'name': 'Ravi', 'marks': 73}
}
print(students['S02']['name'])
print(students['S01']['marks'])

Expected output

Ravi
90

Remember: For more structured applications, consider dataclasses, database tables or typed models.

Level 5 / 10

5 · Dictionary comprehensions

Transform an iterable into a key-value mapping, optionally filtering or converting values.

Python 3 · runnable example

marks = {'Asha': 90, 'Ravi': 38, 'Meera': 75}
passed = {name: score for name, score in marks.items() if score >= 40}
print(passed)

Expected output

{'Asha': 90, 'Meera': 75}

Remember: If generated keys repeat, later values overwrite earlier ones.

Level 6 / 10

6 · Count occurrences

Counter is a dictionary-like class designed for frequencies. Use it instead of manual counting when possible.

Python 3 · runnable example

from collections import Counter
items = ['pen', 'book', 'pen', 'eraser', 'pen']
counts = Counter(items)
print(counts['pen'])
print(counts.most_common(2))

Expected output

3
[('pen', 3), ('book', 1)]

Remember: Counter returns 0 for missing keys when accessed with [] instead of KeyError.

Level 7 / 10

7 · setdefault and defaultdict

Use setdefault() to create a missing key with a default. defaultdict(list) is convenient for grouping many records.

Python 3 · runnable example

from collections import defaultdict
by_city = defaultdict(list)
for name, city in [('Asha', 'Varanasi'), ('Ravi', 'Ranchi'), ('Meera', 'Varanasi')]:
    by_city[city].append(name)
print(by_city['Varanasi'])

Expected output

['Asha', 'Meera']

Remember: Accessing a missing defaultdict key creates a value; this side effect may be surprising.

Level 8 / 10

8 · Combine dictionaries and views

Use {**a, **b} in older Python or a | b in Python 3.9+ to make a new dictionary. Later values win for duplicate keys.

Python 3 · runnable example

a = {'pen': 10, 'book': 5}
b = {'book': 8, 'eraser': 2}
c = a | b
print(c)
print(list(c.keys()))

Expected output

{'pen': 10, 'book': 8, 'eraser': 2}
['pen', 'book', 'eraser']

Remember: a |= b updates a itself. keys(), values() and items() are dynamic view objects.

Level 9 / 10

9 · Valid keys and performance

Dictionary keys must be hashable, so strings, numbers and suitable tuples work. Lists cannot be keys. Typical lookup and insertion are O(1) average.

Python 3 · runnable example

prices = {('Varanasi', 'pen'): 12}
print(prices.get(('Varanasi', 'pen')))
print(('Patna', 'pen') in prices)

Expected output

12
False

Remember: A dictionary preserves insertion order, but that does not mean its keys are alphabetically sorted.

Level 10 / 10

10 · Mini-project: inventory report

Maintain stock and display reorder items using clear business rules. Turn the print loop into a function for reuse.

Python 3 · runnable example

stock = {'rice': 12, 'dal': 4, 'oil': 2}
reorder_level = 5
for item, qty in stock.items():
    status = 'REORDER' if qty < reorder_level else 'OK'
    print(f'{item}: {qty} — {status}')

Expected output

rice: 12 — OK
dal: 4 — REORDER
oil: 2 — REORDER

Remember: Extension: process a delivery by increasing stock only when its quantity is nonnegative.

Practice lab

Test your understanding

Predict or explain the answer before opening the solution. Try rewriting one of the examples without copying it.

1. What happens when data['missing'] is used?

Answer: KeyError

Try data.get('missing', default) for optional keys.

2. What happens when a dict comprehension produces duplicate keys?

Answer: Later values replace earlier values.

Keys are unique.

3. What does a | b do for dictionaries in Python 3.9+?

Answer: Creates a merged dictionary, preferring b on duplicate keys.

a and b remain unchanged.

Take it further: expand the mini-project with an input() prompt, a reusable function, input validation and three tests (normal, empty and boundary cases).

When to use each Python collection

Comparison of Python built-in data types
Type	Ordered?	Mutable?	Duplicates?	Best for
str	Yes	No	Yes	Text
list	Yes	Yes	Yes	Changing sequences
tuple	Yes	No*	Yes	Fixed records
set	No guaranteed order	Yes	No	Unique membership
dict	Insertion order	Yes	Keys unique	Named lookups

* A tuple itself cannot be reassigned, but a mutable item stored inside it can still change.

Python official built-in types reference ↗

Programmer's Picnic · Developed and maintained by Champak Roy · Practice with real Python, one step at a time.