10.1. Iterator Aboutï

Processes one element at a time
Does not remember previous element
Does not know next element
Can be used only once
Save memory (does not require more memory for processing large data)
Values are computed on demand
No need to store all values in memory
Typical usage: streams, processing larger than memory files or data
Functions (list, dict, tuple, frozenset, set, sum, all, any, etc) will evaluate generator instantly

iterableï: An object supporting iteration. To create iterable class must implement Iterable protocol, to has __iter__() method.
iteratorï: An iterable object. To create iterator class must implement Iterator protocol, to has __iter__() and __next__() method.

10.1.1. Examplesï

reversed(sequence, /)
range(start=0, stop, step=1), count
enumerate(iterable, start=0)
zip(*iterables, strict=False), zip_longest
map(func, iterables*), starmap
filter(func, iterable)
chain(*iterables)
permutations(iterable, r=None)
product(*iterables, repeat=1)
cycle(iterable, /)

10.1.2. Plain Functionï

Plain function returns plain object

>>> def run():
...     return 1

>>> type(run)
<class 'function'>

>>> type(run())
<class 'int'>

10.1.3. Generator Functionï

Generator function returns generator object

>>> def run():
...     yield 1

>>> type(run)
<class 'function'>

>>> type(run())
<class 'generator'>

10.1.4. Yield vs. Returnï

After return function stops
After yield function pauses

We want to return three values from a function. We cannot use return keyword three times, because function will stop being executed after encountering first return:

>>> def run():
...     return 1
...     return 2  # this will never be executed
...     return 3  # this will never be executed

In order to do so, we can return one list of three values:

>>> def run():
...     return [1, 2, 3]

Or we can yield each value:

>>> def run():
...     yield 1
...     yield 2
...     yield 3

10.1.5. Lazy Evaluationï

After yield function pauses
Calling next() resumes function until next yield
After last yield raises StopIteration

>>> def run():
...     yield 1
...     yield 2
...     yield 3
>>>
>>>
>>> result = run()
>>>
>>> next(result)
1
>>>
>>> next(result)
2
>>>
>>> next(result)
3
>>>
>>> next(result)
Traceback (most recent call last):
StopIteration

10.1.6. Instant Evaluationï

Using list() will evaluate generator instantly
Functions (list, tuple, set, dict, sum, min, max, all, any, etc) will evaluate generator instantly

>>> def run():
...     yield 1
...     yield 2
...     yield 3
>>>
>>>
>>> result = run()
>>> list(result)
[1, 2, 3]

10.1.7. Iterateï

>>> def run():
...     yield 1
...     yield 2
...     yield 3
>>>
>>>
>>> for result in run():
...     print(result)
1
2
3

10.1.8. Rationaleï

>>> def a():
...     return 1
...     return 2
...     return 3

>>> def b():
...     return 1, 2, 3

>>> def c():
...     return 1, 2, 3, 4, 5, ..., 1_000_000

>>> def d():
...     yield 1
...     yield 2
...     yield 3
...     yield 4
...     yield 5
...     ...
...     yield 1_000_000

>>> def e():
...     for x in range(1, 1_000_001):
...         yield x

10.1.9. Assignmentsï

# %% About
# - Name: Iterator About Yield
# - Difficulty: easy
# - Lines: 4
# - 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 function `run() -> Iterator[int]`
# 2. Function yields 1
# 3. Function yields 2
# 4. Function yields 3
# 5. Use `yield` keyword
# 6. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj funkcjÄ `run() -> Iterator[int]`
# 2. Funkcja jelduje 1
# 3. Funkcja jelduje 2
# 4. Funkcja jelduje 3
# 5. UÅ¼yj sÅowa kluczowego `yield`
# 6. Uruchom doctesty - wszystkie muszÄ siÄ powieÅÄ

# %% Expected
# >>> result = run()
#
# >>> next(result)
# 1
#
# >>> next(result)
# 2
#
# >>> next(result)
# 3
#
# >>> next(result)
# Traceback (most recent call last):
# StopIteration

# %% 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(run)

>>> result = run()
>>>
>>> next(result)
1
>>> next(result)
2
>>> next(result)
3
>>> next(result)
Traceback (most recent call last):
StopIteration
"""

# %% 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, Iterator
run: Callable[[], Iterator[int]]

# %% Data

# %% Result
def run():
    ...

# %% About
# - Name: Iterator About For-Yield
# - Difficulty: easy
# - Lines: 3
# - 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 function `run() -> Iterator[int]`
# 2. Function yields 1
# 3. Function yields 2
# 4. Function yields 3
# 5. Use `yield` keyword
# 6. Use `for` loop
# 7. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj funkcjÄ `run() -> Iterator[int]`
# 2. Funkcja jelduje 1
# 3. Funkcja jelduje 2
# 4. Funkcja jelduje 3
# 5. UÅ¼yj sÅowa kluczowego `yield`
# 6. UÅ¼yj pÄtli `for`
# 7. Uruchom doctesty - wszystkie muszÄ siÄ powieÅÄ

# %% Expected
# >>> result = run()
#
# >>> next(result)
# 1
#
# >>> next(result)
# 2
#
# >>> next(result)
# 3
#
# >>> next(result)
# Traceback (most recent call last):
# StopIteration

# %% 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(run)

>>> result = run()
>>>
>>> next(result)
1
>>> next(result)
2
>>> next(result)
3
>>> next(result)
Traceback (most recent call last):
StopIteration
"""

# %% 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, Iterator
run: Callable[[], Iterator[int]]

# %% Data

# %% Result
def run():
    ...

# %% About
# - Name: Iterator About Adult-List
# - Difficulty: easy
# - Lines: 6
# - 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 function `adults(users) -> list[str]`
# 2. Function returns firstnames of adult users (age >= 18)
# 3. Do not use `yield` keyword
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj funkcjÄ `adults(users) -> list[str]`
# 2. Funkcja zwraca imiona (firstname) uÅ¼ytkownikÃ³w peÅnoletnich (wiek >= 18)
# 3. Nie uÅ¼ywaj sÅowa kluczowego `yield`
# 4. Uruchom doctesty - wszystkie muszÄ siÄ powieÅÄ

# %% Expected
# >>> list(adults(users))
# ['Alice', 'Bob', 'Carol', 'Dave', 'Eve']

# %% 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(adults)

>>> list(adults(users))
['Alice', 'Bob', 'Carol', 'Dave', 'Eve']
"""

# %% 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
adults: Callable[[list[tuple[str,str,int]]], list[str]]

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

header, *users = DATA

# %% Result
def adults(users):
    ...

# %% About
# - Name: Iterator About Adult-Iterator
# - 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 function `adults(users) -> Iterator[int]`
# 2. Function returns firstnames of adult users (age >= 18)
# 3. Use `yield` keyword
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj funkcjÄ `adults(users) -> Iterator[int]`
# 2. Funkcja zwraca imiona (firstname) uÅ¼ytkownikÃ³w peÅnoletnich (wiek >= 18)
# 3. UÅ¼yj sÅowa kluczowego `yield`
# 4. Uruchom doctesty - wszystkie muszÄ siÄ powieÅÄ

# %% Expected
# >>> list(adults(users))
# ['Alice', 'Bob', 'Carol', 'Dave', 'Eve']

# %% 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(adults)

>>> list(adults(users))
['Alice', 'Bob', 'Carol', 'Dave', 'Eve']
"""

# %% 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, Iterator
adults: Callable[[list[tuple[str,str,int]]], Iterator[int]]

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

header, *users = DATA

# %% Result
def adults(users):
    ...

10.1. Iterator Aboutï

10.1.1. Examplesï

10.1.2. Plain Functionï

10.1.3. Generator Functionï

10.1.4. Yield vs. Returnï

10.1.5. Lazy Evaluationï

10.1.6. Instant Evaluationï

10.1.7. Iterateï

10.1.8. Rationaleï

10.1.9. Assignmentsï