7.1. Mapping Dict

• dict are key-value storage (HashMap)

• Mutable - can add, remove, and modify items

• Since Python 3.7: dict keeps order of elements

• Before Python 3.7: dict order is not ensured!!

Example:

>>> user = {'username': 'alice', 'password': 'secret'}
>>> person = {'firstname': 'Alice', 'lastname': 'Apricot', 'age': 20}
>>> measurement = {'parameter': 'co2', 'value': 500, 'unit': 'ppm'}
>>> health = {'name': 'Alice', 'bp': '120/80', 'hr': 70, 'height': 170, 'weight': 55.5}
>>> product = {'name': 'MyProduct', 'usd': 100, 'eur': 90, 'pln': 400}

How are dictionaries implemented in CPython? [1]

CPython's dictionaries are implemented as resizable hash tables. Compared to B-trees, this gives better performance for lookup (the most common operation by far) under most circumstances, and the implementation is simpler.

Dictionaries work by computing a hash code for each key stored in the dictionary using the hash() built-in function. The hash code varies widely depending on the key and a per-process seed; for example, "Python" could hash to -539294296 while "python", a string that differs by a single bit, could hash to 1142331976. The hash code is then used to calculate a location in an internal array where the value will be stored. Assuming that you're storing keys that all have different hash values, this means that dictionaries take constant time – O(1), in Big-O notation – to retrieve a key.

7.1.1. Define Empty

• {} is faster

• dict() is more verbose

>>> data = {}
>>> data = dict()

7.1.2. Define With Elements

• Two ways of creating dict with elements

• Comma after last element is optional

>>> data = {
...     'firstname': 'Alice',
...     'lastname': 'Apricot',
... }

>>> data = dict(
...     firstname='Alice',
...     lastname='Apricot',
... )

7.1.3. Define from List of Pairs

Pair:

>>> pair1 = ('firstname', 'Alice')
>>> pair2 = ('lastname', 'Apricot')

List of pairs:

>>> pairs = [
...     ('firstname', 'Alice'),
...     ('lastname', 'Apricot'),
... ]

Convert list of pairs to dict:

>>> dict(pairs)
{'firstname': 'Alice', 'lastname': 'Apricot'}

7.1.4. Duplicates

Duplicating items are overridden by latter:

>>> data = {
...     'firstname': 'Alice',
...     'firstname': 'Bob',
... }
>>>
>>> data
{'firstname': 'Bob'}

7.1.5. Dict vs. Set

• Both set and dict keys must be hashable

• Both set and dict uses the same { and } braces

• Despite similar syntax, they are different types

set() values behaves similar to dict() keys. This is one of the reasons why they both have the same brackets:

>>> data = {'Alice', 'Bob', 'Carol'}  # set
>>> data = {'firstname': 'Alice', 'lastname': 'Apricot'}  # dict

However both structures are ment to store different types of data: sets store values, such as usernames, and dicts store key-value pairs, such as user details:

Set:

>>> data = {'Alice', 'Bob', 'Carol'}  # ok
>>> data = {'Alice': None, 'Bob': None, 'Carol': None}  # bad

Dict:

>>> data = {'Alice', 'Apricot', 20}  # bad
>>> data = {'firstname': 'Alice', 'lastname': 'Apricot', 'age': 20}  # ok

7.1.6. Length

>>> data = {
...     'firstname': 'Alice',
...     'lastname': 'Apricot',
... }
>>>
>>>
>>> len(data)
2

7.1.7. Use Case - 1

>>> MONTHS = {
...     1: 'January',
...     2: 'February',
...     3: 'March',
...     4: 'April',
...     5: 'May',
...     6: 'June',
...     7: 'July',
...     8: 'August',
...     9: 'September',
...     10: 'October',
...     11: 'November',
...     12: 'December'
... }

7.1.8. Use Case - 2

• GIT - version control system

>>> git = {
...    'ce16a8ce': 'commit/1',
...    'cae6b510': 'commit/2',
...    '895444a6': 'commit/3',
...    'aef731b5': 'commit/4',
...    '4a92bc79': 'branch/master',
...    'b3bbd85a': 'tag/v1.0',
... }

7.1.9. Recap

• dict are key-value storage (HashMap)

• Mutable - you can add, modify and remove items

• Empty dict: data = {} or data = dict()

7.1.10. References

[1]

van Rossum, G. et al. How are dictionaries implemented in CPython? Year: 2022. Retrieved: 2022-09-25. URL: https://docs.python.org/3/faq/design.html#why-must-dictionary-keys-be-immutable

7.1.11. Assignments

# %% About
# - Name: Mapping Dict Create
# - Difficulty: easy
# - Lines: 3
# - Minutes: 2

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Define `result: dict` with:
#    - key `firstname` with value 'Alice'
#    - key `lastname` with value 'Apricot'
#    - key `age` with value 20
# 2. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj `result: dict` z:
#    - kluczem `firstname` o wartości 'Alice'
#    - kluczem `lastname` o wartości 'Apricot'
#    - kluczem `age` o wartości 20
# 2. Uruchom doctesty - wszystkie muszą się powieść

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'

>>> assert type(result) is dict, \
'Variable `result` has an invalid type; expected: `dict`.'

>>> assert 'firstname' in result.keys(), \
'Value `firstname` is not in the result keys'

>>> assert 'lastname' in result.keys(), \
'Value `lastname` is not in the result keys'

>>> assert 'age' in result.keys(), \
'Value `age` is not in the result keys'

>>> assert 'Alice' in result.values(), \
'Value `Alice` is not in the result values'

>>> assert 'Apricot' in result.values(), \
'Value `Apricot` is not in the result values'

>>> assert 20 in result.values(), \
'Value `20` is not in the result values'
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports

# %% Types
result: dict[str,str]

# %% Data

# %% Result
result = ...

# %% About
# - Name: Mapping Dict ListOfPairs
# - Difficulty: easy
# - Lines: 1
# - Minutes: 2

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Use `DATA: list[tuple]`
# 2. Define `result: dict` with `DATA` converted to `dict`
# 3. Run doctests - all must succeed

# %% Polish
# 1. Użyj `DATA: list[tuple]`
# 2. Zdefiniuj `result: dict` z przekonwertowanym `DATA` do `dict`
# 3. Uruchom doctesty - wszystkie muszą się powieść

# %% Example
# >>> result
# {'firstname': 'Alice',
#  'lastname': 'Apricot',
#  'age': 20}

# %% Hints
# - `dict()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'

>>> from pprint import pprint

>>> assert type(result) is dict, \
'Variable `result` has an invalid type; expected: `dict`.'

>>> assert all(type(x) is str for x in result.keys()), \
'Variable `result.keys()` has elements of an invalid type; all items should be: `str`.'

>>> assert 'firstname' in result.keys()
>>> assert 'lastname' in result.keys()
>>> assert 'age' in result.keys()

>>> assert 'Alice' in result.values()
>>> assert 'Apricot' in result.values()
>>> assert 20 in result.values()

>>> pprint(result, width=40, sort_dicts=False)
{'firstname': 'Alice',
 'lastname': 'Apricot',
 'age': 20}
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports

# %% Types
result: dict[str,str]

# %% Data
DATA = [
    ('firstname', 'Alice'),
    ('lastname', 'Apricot'),
    ('age', 20),
]

# %% Result
result = ...