9.1. Unpack Getitem

  • Allows to get element from ordered sequence

  • Works with ordered sequences: str, list, tuple

  • Index must be int (positive, negative or zero)

  • Positive Index starts with 0

  • Negative index starts with -1

>>> data = ['red', 'green', 'blue']
>>>
>>> data[0]
'red'

9.1.1. Positive Index

  • Ascending order

  • Index must be less than length of an object

  • If index does not exist, then IndexError exception will be raised

SetUp:

>>> data = ['red', 'green', 'blue']

You can ask iterable about element at index, mind that Python starts indexing from 0:

>>> data[0]
'red'
>>>
>>> data[1]
'green'
>>>
>>> data[2]
'blue'

Index must be less than length of an object. If index does not exist, then IndexError exception will be raised:

>>> data[3]
Traceback (most recent call last):
IndexError: list index out of range

9.1.2. Negative Index

  • Descending order

  • Starts with -1

  • Number 0 is equal to -0

  • Negative index starts from the end and go right to left

SetUp:

>>> data = ['red', 'green', 'blue']

Negative index starts from the end and go right to left:

>>> data[-1]
'blue'
>>>
>>> data[-2]
'green'
>>>
>>> data[-3]
'red'

Number 0 is equal to -0:

>>> data[-0]
'red'
>>>
>>> -0 == 0
True

Index must be less than length of an object. If index does not exist, then IndexError exception will be raised:

>>> data[-4]
Traceback (most recent call last):
IndexError: list index out of range

9.1.3. Index Type

  • Index must int (positive, negative or zero)

SetUp:

>>> data = ['red', 'green', 'blue']

Index must int (positive, negative or zero):

>>> data[0]
'red'
>>>
>>> data[1]
'green'
>>>
>>> data[-1]
'blue'

Any other type, even float or str, will not work:

>>> data[1.0]
Traceback (most recent call last):
TypeError: list indices must be integers or slices, not float
>>>
>>> data['one']
Traceback (most recent call last):
TypeError: list indices must be integers or slices, not str

9.1.4. Getitem from str

Get Item from str:

>>> data = 'red'
>>>
>>>
>>> data[0]
'r'
>>>
>>> data[1]
'e'
>>>
>>> data[2]
'd'
>>>
>>> data[-0]
'r'
>>>
>>> data[-1]
'd'
>>>
>>> data[-2]
'e'

9.1.5. Getitem from list

Getitem from list:

>>> data = ['red', 'green', 'blue']

Positive:

>>> data[1]
'green'
>>>
>>> data[2]
'blue'

Zero:

>>> data[0]
'red'
>>>
>>> data[-0]
'red'

Negative:

>>> data[-1]
'blue'
>>>
>>> data[-2]
'green'
>>>
>>> data[-3]
'red'

9.1.6. Getitem from tuple

Getitem from tuple:

>>> data = ('red', 'green', 'blue')

Positive:

>>> data[1]
'green'
>>>
>>> data[2]
'blue'

Zero:

>>> data[0]
'red'
>>>
>>> data[-0]
'red'

Negative:

>>> data[-1]
'blue'
>>>
>>> data[-2]
'green'
>>>
>>> data[-3]
'red'

9.1.7. Getitem from set

Getitem from set is impossible. set is unordered data structure:

>>> data = {'a', 'b', 'c', 'd'}
>>>
>>>
>>> data[0]
Traceback (most recent call last):
TypeError: 'set' object is not subscriptable
>>>
>>> data[1]
Traceback (most recent call last):
TypeError: 'set' object is not subscriptable
>>>
>>> data[2]
Traceback (most recent call last):
TypeError: 'set' object is not subscriptable
>>>
>>> data[-0]
Traceback (most recent call last):
TypeError: 'set' object is not subscriptable
>>>
>>> data[-1]
Traceback (most recent call last):
TypeError: 'set' object is not subscriptable
>>>
>>> data[-2]
Traceback (most recent call last):
TypeError: 'set' object is not subscriptable

9.1.8. Getitem from dict

  • Getitem with index on dict is not possible

Non-integer keys:

>>> data = {'a': 0, 'b': 1, 'c': 2}
>>>
>>> data[0]
Traceback (most recent call last):
KeyError: 0
>>>
>>> data['a']
0

Integer keys:

>>> data = {0: 'a', 1: 'b', 2: 'c'}
>>>
>>> data[0]
'a'
>>>
>>> data['a']
Traceback (most recent call last):
KeyError: 'a'
>>>
>>> data[-1]
Traceback (most recent call last):
KeyError: -1

9.1.9. Getitem from list[list]

Get elements from list of list:

>>> data = [
...     [1, 2, 3],
...     [4, 5, 6],
...     [7, 8, 9],
... ]
>>>
>>> data[0]
[1, 2, 3]
>>>
>>> data[0][1]
2

9.1.10. Getitem from list[tuple]

Get elements from list of tuple:

>>> data = [
...     ('alice', 'secret'),
...     ('bob', 'qwerty'),
...     ('carol', '123456'),
... ]
>>>
>>> data[0]
('alice', 'secret')
>>>
>>> data[0][0]
'alice'
>>>
>>> data[0][0][0]
'a'

9.1.11. Getitem from list[dict]

>>> data = [
...     {'username': 'alice', 'password': 'secret'},
...     {'username': 'bob', 'password': 'qwerty'},
...     {'username': 'carol', 'password': '123456'},
... ]
>>>
>>>
>>> data[0]
{'username': 'alice', 'password': 'secret'}
>>>
>>> data[0]['username']
'alice'
>>>
>>> data[0]['username'][0]
'a'

9.1.12. Getitem from list[Sequence]

  • list of mixed Sequence, such as: list, tuple or str

Get elements from list of sequences:

>>> data = [[1, 2, 3],
...         (4, 5, 6),
...         {7, 8, 9}]

List:

>>> data[0]
[1, 2, 3]
>>>
>>> data[0][1]
2

Tuple:

>>> data[1]
(4, 5, 6)
>>>
>>> data[1][0]
4

Set:

>>> data[2]
{8, 9, 7}
>>>
>>> data[2][0]
Traceback (most recent call last):
TypeError: 'set' object is not subscriptable

9.1.13. Use Case - 1

>>> crew = {
...    0: 'alice',
...    1: 'bob',
...    2: 'carol',
... }
>>>
>>>
>>> crew[0]
'alice'
>>>
>>> crew[1]
'bob'
>>>
>>> crew[2]
'carol'
>>>
>>> crew[-0]
'alice'
>>>
>>> crew[-1]
Traceback (most recent call last):
KeyError: -1

9.1.14. Use Case - 2

>>> calendarium = {
...    1961: 'First Human Space Flight',
...    1969: 'First Step on the Moon',
... }
>>>
>>>
>>> calendarium[1961]
'First Human Space Flight'
>>>
>>> calendarium['1961']
Traceback (most recent call last):
KeyError: '1961'

9.1.15. Assignments

# %% About
# - Name: Unpack Getitem Header
# - 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. Define `result: tuple` with `DATA` header (row with index 0)
# 2. Use getitem, i.e.: `list[index]`
# 3. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj `result: tuple` z nagłówkiem `DATA` (wiersz o indeksie 0)
# 2. Użyj getitem, tj. `list[index]`
# 3. Uruchom doctesty - wszystkie muszą się powieść

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

>>> assert result is not Ellipsis, \
'Assign your result to variable `result`'
>>> assert type(result) is tuple, \
'Variable `result` has invalid type, should be tuple'
>>> assert len(result) == 3, \
'Variable `result` length should be 3'

>>> result
('firstname', 'lastname', 'age')
"""

# %% 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: tuple[str,str,str]

# %% Data
DATA = [
    ('firstname', 'lastname', 'age'),
    ('Alice', 'Apricot', 30),
    ('Bob', 'Banana', 31),
    ('Carol', 'Corn', 32),
    ('Dave', 'Durian', 33),
    ('Eve', 'Elderberry', 34),
    ('Mallory', 'Melon', 15),
]

# %% Result
result = ...


# %% About
# - Name: Unpack Getitem Positive
# - 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_a: tuple` with row from `DATA` at index 2
# 2. Define `result_b: tuple` with row from `DATA` at index 4
# 3. Define `result_c: tuple` with row from `DATA` at index 6
# 4. Use getitem, i.e.: `list[index]`
# 5. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj `result_a: tuple` z wierszem z `DATA` o indeksie 2
# 2. Zdefiniuj `result_b: tuple` z wierszem z `DATA` o indeksie 4
# 3. Zdefiniuj `result_c: tuple` z wierszem z `DATA` o indeksie 6
# 4. Użyj getitem, tj. `list[index]`
# 5. Uruchom doctesty - wszystkie muszą się powieść

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

>>> assert result_a is not Ellipsis, \
'Assign your result to variable `result_a`'
>>> assert type(result_a) is tuple, \
'Variable `result_a` has invalid type, should be tuple'
>>> assert len(result_a) == 3, \
'Variable `result_a` length should be 3'

>>> assert result_b is not Ellipsis, \
'Assign your result to variable `result_b`'
>>> assert type(result_b) is tuple, \
'Variable `result_b` has invalid type, should be tuple'
>>> assert len(result_b) == 3, \
'Variable `result_b` length should be 3'

>>> assert result_c is not Ellipsis, \
'Assign your result to variable `result_c`'
>>> assert type(result_c) is tuple, \
'Variable `result_c` has invalid type, should be tuple'
>>> assert len(result_c) == 3, \
'Variable `result_c` length should be 3'

>>> result_a
('Bob', 'Banana', 31)

>>> result_b
('Dave', 'Durian', 33)

>>> result_c
('Mallory', 'Melon', 15)
"""

# %% 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_a: tuple[str,str,int]
result_b: tuple[str,str,int]
result_c: tuple[str,str,int]

# %% Data
DATA = [
    ('firstname', 'lastname', 'age'),
    ('Alice', 'Apricot', 30),
    ('Bob', 'Banana', 31),
    ('Carol', 'Corn', 32),
    ('Dave', 'Durian', 33),
    ('Eve', 'Elderberry', 34),
    ('Mallory', 'Melon', 15),
]

# %% Result
result_a = ...
result_b = ...
result_c = ...


# %% About
# - Name: Unpack Getitem Negative
# - 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_a: tuple` with row from `DATA` at index -5
# 2. Define `result_b: tuple` with row from `DATA` at index -3
# 3. Define `result_c: tuple` with row from `DATA` at index -1
# 4. Use getitem, i.e.: `list[index]`
# 5. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj `result_a: tuple` z wierszem z `DATA` o indeksie -5
# 2. Zdefiniuj `result_b: tuple` z wierszem z `DATA` o indeksie -3
# 3. Zdefiniuj `result_c: tuple` z wierszem z `DATA` o indeksie -1
# 4. Użyj getitem, tj. `list[index]`
# 5. Uruchom doctesty - wszystkie muszą się powieść

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

>>> assert result_a is not Ellipsis, \
'Assign your result to variable `result_a`'
>>> assert type(result_a) is tuple, \
'Variable `result_a` has invalid type, should be tuple'
>>> assert len(result_a) == 3, \
'Variable `result_a` length should be 3'

>>> assert result_b is not Ellipsis, \
'Assign your result to variable `result_b`'
>>> assert type(result_b) is tuple, \
'Variable `result_b` has invalid type, should be tuple'
>>> assert len(result_b) == 3, \
'Variable `result_b` length should be 3'

>>> assert result_c is not Ellipsis, \
'Assign your result to variable `result_c`'
>>> assert type(result_c) is tuple, \
'Variable `result_c` has invalid type, should be tuple'
>>> assert len(result_c) == 3, \
'Variable `result_c` length should be 3'

>>> result_a
('Bob', 'Banana', 31)

>>> result_b
('Dave', 'Durian', 33)

>>> result_c
('Mallory', 'Melon', 15)
"""

# %% 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_a: tuple[str,str,int]
result_b: tuple[str,str,int]
result_c: tuple[str,str,int]

# %% Data
DATA = [
    ('firstname', 'lastname', 'age'),
    ('Alice', 'Apricot', 30),
    ('Bob', 'Banana', 31),
    ('Carol', 'Corn', 32),
    ('Dave', 'Durian', 33),
    ('Eve', 'Elderberry', 34),
    ('Mallory', 'Melon', 15),
]

# %% Result
result_a = ...
result_b = ...
result_c = ...


# %% About
# - Name: Unpack Getitem Select
# - Difficulty: easy
# - Lines: 4
# - Minutes: 3

# %% 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: list` with rows from `DATA` at indexes: 0, 1, -1
# 2. Use getitem, i.e.: `list[index]`
# 3. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj `result: list` z wierszami z `DATA` o indeksach: 0, 1, -1
# 2. Użyj getitem, tj. `list[index]`
# 3. Uruchom doctesty - wszystkie muszą się powieść

# %% Hints
# - `list.append()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 12), \
'Python 3.12+ required'

>>> from pprint import pprint

>>> assert result is not Ellipsis, \
'Assign your result to variable `result`'
>>> assert type(result) is list, \
'Variable `result` has invalid type, should be list'
>>> assert len(result) == 3, \
'Variable `result` length should be 3'

>>> pprint(result)
[('firstname', 'lastname', 'age'),
 ('Alice', 'Apricot', 30),
 ('Mallory', 'Melon', 15)]
"""

# %% 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
type header = tuple[str,str,str]
type row = tuple[str,str,int]
result: list[header|row]

# %% Data
DATA = [
    ('firstname', 'lastname', 'age'),
    ('Alice', 'Apricot', 30),
    ('Bob', 'Banana', 31),
    ('Carol', 'Corn', 32),
    ('Dave', 'Durian', 33),
    ('Eve', 'Elderberry', 34),
    ('Mallory', 'Melon', 15),
]

# %% Result
result = ...


# %% About
# - Name: Unpack Getitem Header/Data
# - Difficulty: easy
# - Lines: 11
# - Minutes: 3

# %% 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 `header: tuple[str]` with a header (first line of `DATA`)
# 2. Define `rows: list[tuple]` with rows (all the other lines of `DATA`)
# 3. Use getitem, i.e.: `list[index]`
# 4. Do not use slice, i.e.: `list[start:stop:step]`
# 5. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj `header: tuple[str]` z nagłówkiem (pierwsza linia `DATA`)
# 2. Zdefiniuj `rows: list[tuple]` z wierszami (wszystkie inne linie `DATA`)
# 3. Użyj getitem, tj. `list[index]`
# 4. Nie używaj slice, tj. `list[start:stop:step]`
# 5. Uruchom doctesty - wszystkie muszą się powieść

# %% Hints
# - `list.append()`

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

>>> from pprint import pprint

>>> assert header is not Ellipsis, \
'Assign your result to variable `header`'
>>> assert type(header) is tuple, \
'Variable `header` has invalid type, should be tuple'
>>> assert header not in rows, \
'Header should not be in `rows`'

>>> assert rows is not Ellipsis, \
'Assign your result to variable `rows`'
>>> assert all(type(x) is tuple for x in rows), \
'All elements in `rows` should be tuple'

>>> content = open(__file__).read()
>>> assert 'DATA'+'[1:]' not in content, \
'You should not use `slice`'
>>> assert 'DATA'+'[1:11]' not in content, \
'You should not use `slice`'

>>> pprint(header)
('firstname', 'lastname', 'age')

>>> pprint(rows)
[('Alice', 'Apricot', 30),
 ('Bob', 'Banana', 31),
 ('Carol', 'Corn', 32),
 ('Dave', 'Durian', 33),
 ('Eve', 'Elderberry', 34),
 ('Mallory', 'Melon', 15)]
"""

# %% 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
header = tuple[str,str,str]
rows = list[tuple[str,str,int]]

# %% Data
DATA = [
    ('firstname', 'lastname', 'age'),
    ('Alice', 'Apricot', 30),
    ('Bob', 'Banana', 31),
    ('Carol', 'Corn', 32),
    ('Dave', 'Durian', 33),
    ('Eve', 'Elderberry', 34),
    ('Mallory', 'Melon', 15),
]

# %% Result
header = ...
rows = ...