• Enumerate sequence

  • Lazy evaluated

  • enumerate(iterable, start=0)

  • Required iterable - 1 or many sequences or iterator object

  • Optional start=0 - number to starts with, default zero (0)

  • Return an enumerate object

  • The enumerate object yields pairs containing a count (from start, defaults to zero) and a value yielded by the iterable argument

10.4.1. Problem

>>> data = ['Alice', 'Bob', 'Carol']
>>>
>>> result = []
>>> index = 0
>>> for value in data:
...     result.append((index, value))
...     index += 1

10.4.2. Solution

>>> data = ['Alice', 'Bob', 'Carol']
>>>
>>> result = enumerate(data)

10.4.3. Lazy Evaluation

>>> data = ['Alice', 'Bob', 'Carol']
>>> result = enumerate(data)
>>>
>>> next(result)
(0, 'Alice')
>>>
>>> next(result)
(1, 'Bob')
>>>
>>> next(result)
(2, 'Carol')
>>>
>>> next(result)
Traceback (most recent call last):
StopIteration

10.4.4. Iteration

  • Unpacking in a for loop

>>> data = ['Alice', 'Bob', 'Carol']
>>>
>>> for result in enumerate(data):
...     print(result)
(0, 'Alice')
(1, 'Bob')
(2, 'Carol')

>>> data = ['Alice', 'Bob', 'Carol']
>>>
>>> for index, value in enumerate(data):
...     print(f'{index=}, {value=}')
index=0, value='Alice'
index=1, value='Bob'
index=2, value='Carol'

10.4.5. Offset

>>> data = ['Alice', 'Bob', 'Carol']
>>>
>>> for result in enumerate(data, start=1):
...     print(result)
(1, 'Alice')
(2, 'Bob')
(3, 'Carol')

>>> data = ['Alice', 'Bob', 'Carol']
>>>
>>> for result in enumerate(data, start=10):
...     print(result)
(10, 'Alice')
(11, 'Bob')
(12, 'Carol')

10.4.6. As List

>>> data = ['Alice', 'Bob', 'Carol']
>>> result = enumerate(data)
>>>
>>> list(result)
[(0, 'Alice'), (1, 'Bob'), (2, 'Carol')]

10.4.7. As Dict

>>> data = ['Alice', 'Bob', 'Carol']
>>> result = enumerate(data)
>>>
>>> dict(result)
{0: 'Alice', 1: 'Bob', 2: 'Carol'}

10.4.8. Dict Comprehension

>>> data = ['Alice', 'Bob', 'Carol']

As dict:

>>> {index:value for index,value in enumerate(data, start=1)}
{1: 'Alice', 2: 'Bob', 3: 'Carol'}

Reverse dict:

>>> {value:index for index,value in enumerate(data, start=1)}
{'Alice': 1, 'Bob': 2, 'Carol': 3}

Stringify keys:

>>> {f'{index}':value for index,value in enumerate(data, start=1)}
{'1': 'Alice', '2': 'Bob', '3': 'Carol'}

Zero-fill keys:

>>> {f'{index:02}':value for index,value in enumerate(data, start=1)}
{'01': 'Alice', '02': 'Bob', '03': 'Carol'}

10.4.9. Case Study

DATA = """3,4,setosa,virginica,versicolor
5.8,2.7,5.1,1.9,1
5.1,3.5,1.4,0.2,0
5.7,2.8,4.1,1.3,2"""

header, *lines = DATA.splitlines()
nrows, nvalues, *labels = header.split(',')
encoder = dict(enumerate(labels))

encoder
# {0: 'setosa', 1: 'virginica', 2: 'versicolor'}

10.4.10. Assignments

# %% About
# - Name: Iterator Enumerate Dict
# - 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: dict`
# 2. Assign to `result` converted `DATA` to `dict`
# 3. Use `enumerate()` i `dict()`
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj `result: dict`
# 2. Przypisz do `result` przekonwertowane `DATA` do `dict`
# 3. Użyj `enumerate()` i `dict()`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# result
# {0: 'users', 1: 'staff', 2: 'admins'}

# %% Hints
# - `dict()`
# - `enumerate()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

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

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

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

>>> result
{0: 'users', 1: 'staff', 2: 'admins'}
"""

# %% 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[int,str]

# %% Data
DATA = ['users', 'staff', 'admins']

# %% Result
result = ...


# %% About
# - Name: Iterator Enumerate Start
# - 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. Convert `DATA` into dict:
#    - key: int - month number
#    - value: str - month name
# 2. Use `enumerate()` and dict comprehension
# 3. Run doctests - all must succeed

# %% Polish
# 1. Przekonwertuj `DATA` w słownik:
#    - klucz: int - numer miesiąca
#    - wartość: str - nazwa miesiąca
# 2. Użyj `enumerate()` i dict comprehension
# 3. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> result
# {0: 'January',
#  1: 'February',
#  2: 'March',
#  3: 'April',
#  4: 'May',
#  5: 'June',
#  6: 'July',
#  7: 'August',
#  8: 'September',
#  9: 'October',
#  10: 'November',
#  11: 'December'}

# %% Hints
# - `dict()`
# - `enumerate()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> assert type(result)
>>> assert -1 not in result
>>> assert 0 in result
>>> assert 12 not in result
>>> assert 13 not in result
>>> assert result[0] == 'January'

>>> assert all(type(x) is int for x in result.keys())
>>> assert all(type(x) is str for x in result.values())

>>> from pprint import pprint
>>> pprint(result, width=30, sort_dicts=False)
{0: 'January',
 1: 'February',
 2: 'March',
 3: 'April',
 4: 'May',
 5: 'June',
 6: 'July',
 7: 'August',
 8: 'September',
 9: 'October',
 10: 'November',
 11: 'December'}
"""

# %% 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[int,str]

# %% Data
DATA = [
    'January',
    'February',
    'March',
    'April',
    'May',
    'June',
    'July',
    'August',
    'September',
    'October',
    'November',
    'December',
]

# %% Result
result = ...


# %% About
# - Name: Iterator Enumerate Start
# - 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. Convert `DATA` into dict:
#    - key: int - month number
#    - value: str - month name
# 2. Use `enumerate()` and dict comprehension
# 3. Enumerate starting from 1
# 4. Run doctests - all must succeed

# %% Polish
# 1. Przekonwertuj `DATA` w słownik:
#    - klucz: int - numer miesiąca
#    - wartość: str - nazwa miesiąca
# 2. Użyj `enumerate()` i dict comprehension
# 3. Enumerację zacznij od 1
# 4. Uruchom doctesty - wszystkie muszą się powieść

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

# %% Hints
# - `dict()`
# - `enumerate()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> type(result)
<class 'dict'>
>>> 0 not in result
True
>>> 13 not in result
True
>>> result[1] == 'January'
True

>>> assert all(type(x) is int for x in result.keys())
>>> assert all(type(x) is str for x in result.values())

>>> from pprint import pprint
>>> pprint(result, width=30, sort_dicts=False)
{1: 'January',
 2: 'February',
 3: 'March',
 4: 'April',
 5: 'May',
 6: 'June',
 7: 'July',
 8: 'August',
 9: 'September',
 10: 'October',
 11: 'November',
 12: 'December'}
"""

# %% 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[int,str]

# %% Data
DATA = [
    'January',
    'February',
    'March',
    'April',
    'May',
    'June',
    'July',
    'August',
    'September',
    'October',
    'November',
    'December',
]

# %% Result
result = ...


# %% About
# - Name: Iterator Enumerate Start
# - 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. Convert `DATA` into dict:
#    - key: str - month number (convert to str)
#    - value: str - month name
# 2. Use `enumerate()` and dict comprehension
# 3. Enumerate starting from 1
# 4. Run doctests - all must succeed

# %% Polish
# 1. Przekonwertuj `DATA` w słownik:
#    - klucz: str - numer miesiąca (zamień na str)
#    - wartość: str - nazwa miesiąca
# 2. Użyj `enumerate()` i dict comprehension
# 3. Enumerację zacznij od 1
# 4. Uruchom doctesty - wszystkie muszą się powieść

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

# %% Hints
# - dict comprehension
# - `str()`
# - `enumerate()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> type(result)
<class 'dict'>
>>> '0' not in result
True
>>> '13' not in result
True
>>> result['1'] == 'January'
True

>>> assert all(type(x) is str for x in result.keys())
>>> assert all(type(x) is str for x in result.values())

>>> from pprint import pprint
>>> pprint(result, width=30, sort_dicts=False)
{'1': 'January',
 '2': 'February',
 '3': 'March',
 '4': 'April',
 '5': 'May',
 '6': 'June',
 '7': 'July',
 '8': 'August',
 '9': 'September',
 '10': 'October',
 '11': 'November',
 '12': 'December'}
"""

# %% 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 = [
    'January',
    'February',
    'March',
    'April',
    'May',
    'June',
    'July',
    'August',
    'September',
    'October',
    'November',
    'December',
]

# %% Result
result = ...


# %% About
# - Name: Iterator Enumerate ZeroPadded
# - 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. Convert `DATA` into dict:
#    - key: str - month number (convert to str)
#    - value: str - month name
# 2. Use `enumerate()` and dict comprehension
# 3. Enumerate starting from 1
# 4. Month number must be two letter string
#    (zero padded) - np. 01, 02, ..., 09, 10, 11, 12
# 5. Run doctests - all must succeed

# %% Polish
# 1. Przekonwertuj `DATA` w słownik:
#    - klucz: str - numer miesiąca (zamień na str)
#    - wartość: str - nazwa miesiąca
# 2. Użyj `enumerate()` i dict comprehension
# 3. Enumerację zacznij od 1
# 4. Numer miesiąca ma być dwuznakowym stringiem
#    (dopełnionym zerem) - np. 01, 02, ..., 09, 10, 11, 12
# 5. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> result
# {'01': 'January',
#  '02': 'February',
#  '03': 'March',
#  '04': 'April',
#  '05': 'May',
#  '06': 'June',
#  '07': 'July',
#  '08': 'August',
#  '09': 'September',
#  '10': 'October',
#  '11': 'November',
#  '12': 'December'}

# %% Hints
# - dict comprehension
# - `enumerate()`
# - `str.zfill()`
# - `f'{number:02}'`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> type(result)
<class 'dict'>
>>> '00' not in result
True
>>> '13' not in result
True
>>> result['01'] == 'January'
True

>>> assert all(type(x) is str for x in result.keys())
>>> assert all(type(x) is str for x in result.values())
>>> assert all(len(x) == 2 for x in result.keys())

>>> from pprint import pprint
>>> pprint(result, width=30, sort_dicts=False)
{'01': 'January',
 '02': 'February',
 '03': 'March',
 '04': 'April',
 '05': 'May',
 '06': 'June',
 '07': 'July',
 '08': 'August',
 '09': 'September',
 '10': 'October',
 '11': 'November',
 '12': 'December'}
"""

# %% 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 = [
    'January',
    'February',
    'March',
    'April',
    'May',
    'June',
    'July',
    'August',
    'September',
    'October',
    'November',
    'December',
]

# %% Result
result = ...


# %% About
# - Name: Iterator Enumerate Impl
# - Difficulty: medium
# - Lines: 7
# - Minutes: 5

# %% 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. Write own implementation of a built-in `enumerate()` function
# 2. Define function `myenumerate` with parameters:
#    - parameter `iterable: list | tuple`
#    - parameter `start: int`
# 3. Don't validate arguments and assume, that user will:
#    - always pass valid type of arguments
#    - iterable length will always be greater than 0
# 4. Do not use built-in function `enumerate()`
# 5. Use `yield` keyword
# 6. Run doctests - all must succeed

# %% Polish
# 1. Zaimplementuj własne rozwiązanie wbudowanej funkcji `enumerate()`
# 2. Zdefiniuj funkcję `myenumerate` z parametrami:
#    - parametr `iterable: list | tuple`
#    - parametr `start: int`
# 3. Nie waliduj argumentów i przyjmij, że użytkownik:
#    - zawsze poda argumenty poprawnych typów
#    - długość iterable będzie większa od 0
# 4. Nie używaj wbudowanej funkcji `enumerate()`
# 5. Użyj słowa kluczowego `yield`
# 6. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> list(myenumerate(['January', 'February', 'March']))
# [(0, 'January'), (1, 'February'), (2, 'March')]
#
# >>> list(myenumerate(['January', 'February', 'March'], start=1))
# [(1, 'January'), (2, 'February'), (3, 'March')]
#
# >>> dict(myenumerate(['January', 'February', 'March'], start=1))
# {1: 'January', 2: 'February', 3: 'March'}

# %% Hints
# - https://github.com/python/cpython/blob/main/Objects/enumobject.c
# - `range()`
# - `len()`
# - `list.append()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> from inspect import isfunction
>>> assert isfunction(myenumerate)

>>> result = myenumerate(['January', 'February', 'March'])
>>> list(result)
[(0, 'January'), (1, 'February'), (2, 'March')]

>>> result = myenumerate(['January', 'February', 'March'], start=1)
>>> list(result)
[(1, 'January'), (2, 'February'), (3, 'March')]

>>> result = myenumerate(['January', 'February', 'March'], start=1)
>>> dict(result)
{1: 'January', 2: 'February', 3: 'March'}
"""

# %% 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
from typing import Callable
myenumerate: Callable[[tuple|list, int], list[tuple]]

# %% Data

# %% Result
def myenumerate(iterable, start=0):
    ...