7.3. Mapping Getitem

Key lookup is very efficient O(1)
[...] throws KeyError exception if key not found in dict
.get() returns None if key not found
.get() can have default value, if key not found

7.3.1. Getitem Method

If key exist - return value
If key is missing - raises KeyError exception

>>> data = {'name': 'Alice', 'email': 'alice@example.com'}

If key exist:

>>> data['name']
'Alice'

If key is missing:

>>> data['age']
Traceback (most recent call last):
KeyError: 'age'

7.3.2. Get Method

If key exist - return value
If key is missing - None or default value (if specified)

>>> data = {'name': 'Alice', 'email': 'alice@example.com'}

If key exist:

>>> data.get('name')
'Alice'

If key is missing:

>>> data.get('age')

If key is missing and default value is set:

>>> data.get('age', 'n/a')
'n/a'

7.3.3. Case Study - 1

>>> PL = {
...     'ą': 'a',
...     'ć': 'c',
...     'ę': 'e',
...     'ł': 'l',
...     'ń': 'n',
...     'ó': 'o',
...     'ś': 's',
...     'ż': 'z',
...     'ź': 'z',
... }

When key exists:

>>> letter = 'ą'
>>>
>>> PL[letter]
'a'
>>> PL.get(letter)
'a'

When key does not exist:

>>> letter = 'b'
>>>
>>> PL[letter]
Traceback (most recent call last):
KeyError: 'b'
>>>
>>> PL.get(letter)
>>>
>>> PL.get(letter, letter)
'b'

7.3.4. Case Study - 2

>>> def login(username, password):
...     ...

When all fields exist:

>>> data = {'username': 'mwatney', 'password': 'Ares3'}
>>>
>>> username = data['username']
>>> password = data['password']
>>>
>>> login(username, password)

If some fields are missing (password):

>>> data = {'username': 'mwatney'}

Getitem will raise an exception, which you can catch:

>>> try:
...     username = data['username']
...     password = data['password']
... except KeyError as error:
...     print(f'Cannot login without: {error}')
... else:
...     login(username, password)
...
Cannot login without: 'password'

Get method will need to add if-else to check if value is missing:

>>> username = data.get('username')
>>> password = data.get('password')
>>>
>>> if username is None:
...     print(f'Cannot login without: username')
... elif password is None:
...     print(f'Cannot login without: password')
... else:
...     login(username, password)
...
Cannot login without: password

7.3.5. Use Case - 1

>>> hello = {
...    'English': 'Hello',
...    'German': 'Guten Tag',
...    'Polish': 'Witaj',
... }
>>>
>>>
>>> hello.get('English')
'Hello'
>>>
>>> hello.get('Polish')
'Witaj'

7.3.6. Use Case - 2

>>> MONTHS = {
...     1: 'January',
...     2: 'February',
...     3: 'March',
... }
>>>
>>> MONTHS[1]
'January'
>>>
>>> MONTHS['1']
Traceback (most recent call last):
KeyError: '1'
>>>
>>> MONTHS['01']
Traceback (most recent call last):
KeyError: '01'

7.3.7. Use Case - 3

>>> MONTHS = {
...     1: 'January',
...     2: 'February',
...     3: 'March',
... }
>>>
>>> MONTHS.get(1)
'January'
>>>
>>> MONTHS.get(13)
>>>
>>> MONTHS.get(13, 'invalid month')
'invalid month'

7.3.8. Use Case - 4

>>> 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'
... }
>>>
>>> DATE = '1961-04-12'
>>>
>>> year, month, day = DATE.split('-')
>>>
>>> year
'1961'
>>> month
'04'
>>> day
'12'
>>>
>>> MONTHS[month]
Traceback (most recent call last):
KeyError: '04'
>>>
>>> MONTHS[int(month)]
'April'

7.3.9. Use Case - 4

>>> PAYROLL = [
...     {'name': 'Mark Watney',   'Jan': 2000, 'Feb': 2000, 'Mar': 2000},
...     {'name': 'Melissa Lewis', 'Jan': 3000, 'Feb': 3000, 'Mar': 3000},
...     {'name': 'Rick Martinez', 'Jan': 1000, 'Feb': 2000},
...     {'name': 'Alex Vogel',    'Jan': 2500, 'Feb': 2500, 'Mar': 2500},
... ]

Getitem:

>>> result = f'{"Employee":<15} {"January":>10} {"February":>9} {"March":>6}'
>>>
>>> for employee in PAYROLL:
...     name = employee['name']
...     january = employee['Jan']
...     february = employee['Feb']
...     march = employee['Mar']
...     result += f'{name:<15} {january:>10} {february:>9} {march:>6}'
...
Traceback (most recent call last):
KeyError: 'Mar'

Get method:

>>> result = f'{"Employee":<15} {"January":>10} {"February":>9} {"March":>6}\n'
>>>
>>> for employee in PAYROLL:
...     name = employee['name']
...     january = employee.get('Jan', '-')
...     february = employee.get('Feb', '-')
...     march = employee.get('Mar', '-')
...     result += f'{name:<15} {january:>10} {february:>9} {march:>6}\n'
>>>
>>> print(result)
Employee           January  February  March
Mark Watney           2000      2000   2000
Melissa Lewis         3000      3000   3000
Rick Martinez         1000      2000      -
Alex Vogel            2500      2500   2500

7.3.10. Recap

Key lookup is very efficient O(1)
dict[key] throws KeyError exception if key not found in dict
dict.get() returns None if key not found
dict.get() can have default value, if key not found

7.3.11. Assignments

# %% About
# - Name: Mapping Dict Getitem
# - 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: dict`
# 2. Define `result: str` with value for key 'firstname' from `DATA`
# 3. Use getitem syntax
# 4. Run doctests - all must succeed

# %% Polish
# 1. Użyj `DATA: dict`
# 2. Zdefiniuj `result: str` z wartością dla klucza 'firstname' z `DATA`
# 3. Użyj składni getitem
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Hints
# - `dict[key]`

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

>>> from pprint import pprint

>>> assert type(result) is str, \
'Variable `result` has invalid type, should be str'

>>> pprint(result)
'Mark'
"""

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

# %% Imports

# %% Types
result: str

# %% Data
DATA = {
    'firstname': 'Mark',
    'lastname': 'Watney',
    'group': 'users',
}

# %% Result
result = ...

# %% About
# - Name: Mapping Dict Get
# - 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: dict`
# 2. Define `result: str` with value for key 'firstname'
# 3. Use `get` method
# 4. Run doctests - all must succeed

# %% Polish
# 1. Użyj `DATA: dict`
# 2. Zdefiniuj `result: str` z wartością dla klucza 'firstname'
# 3. Użyj metodę `get`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Hints
# - `dict.get(key)`

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

>>> from pprint import pprint

>>> assert type(result) is str, \
'Variable `result` has invalid type, should be str'

>>> pprint(result)
'Mark'
"""

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

# %% Imports

# %% Types
result: str

# %% Data
DATA = {
    'firstname': 'Mark',
    'lastname': 'Watney',
    'group': 'users',
}

# %% Result
result = ...

# %% About
# - Name: Mapping Dict GetDefault
# - 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: dict`
# 2. Define `result: str` with value for key 'email'
# 3. If value does not exist in dict, return 'n/a'
# 4. Use `get` method with default value
# 5. Run doctests - all must succeed

# %% Polish
# 1. Użyj `DATA: dict`
# 2. Zdefiniuj `result: str` z wartością dla klucza 'email'
# 3. Jeżeli wartość nie istnieje w dict, zwróć 'n/a'
# 4. Użyj metody `get` z wartością domyślną
# 5. Uruchom doctesty - wszystkie muszą się powieść

# %% Hints
# - `dict.get(key, default)`

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

>>> from pprint import pprint

>>> assert type(result) is str, \
'Variable `result` has invalid type, should be str'

>>> pprint(result)
'n/a'
"""

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

# %% Imports

# %% Types
result: str

# %% Data
DATA = {
    'firstname': 'Mark',
    'lastname': 'Watney',
    'group': 'users',
}

# %% Result
result = ...