10.6. Iterator Chain

• Lazy evaluated

• itertools.chain(*iterables)

10.6.1. Solution

>>> from itertools import chain
>>>
>>>
>>> keys = ['a', 'b', 'c']
>>> values = [1, 2, 3]
>>>
>>> for x in chain(keys, values):
...     print(x)
a
b
c
1
2
3

10.6.2. Use Case - 1

>>> from itertools import chain
>>>
>>>
>>> def a():
...     yield 1
...     yield 2
...     yield 3
>>>
>>> def b():
...     yield 10
...     yield 20
...     yield 30
>>>
>>>
>>> data = chain(a(), b())
>>>
>>> next(data)
1
>>>
>>> next(data)
2
>>>
>>> next(data)
3
>>>
>>> next(data)
10
>>>
>>> next(data)
20
>>>
>>> next(data)
30
>>>
>>> next(data)
Traceback (most recent call last):
StopIteration

10.6.3. Use Case - 2

>>> def square(x):
...     return x ** 2
>>>
>>> def cube(x):
...     return x ** 3
>>>
>>>
>>> data = [1, 2, 3]
>>>
>>> result = chain(
...     map(square, data),
...     map(cube, data)
... )
>>>
>>> for x in result:
...     print(x)
...
1
4
9
1
8
27

10.6.4. Use Case - 3

>>> from pathlib import Path
>>>
>>>
>>> files = Path('/tmp').rglob('*.txt')
>>>
>>> files
<map object at 0x104b6e560>
>>>
>>> sorted(files)
[PosixPath('/tmp/myfile1.txt'), PosixPath('/tmp/myfile2.txt'), PosixPath('/tmp/myfile3.txt')]

10.6.5. Use Case - 4

>>> from pathlib import Path
>>>
>>>
>>> mydir = Path('/tmp')
>>>
>>> files = chain(
...     mydir.rglob('*.txt'),
...     mydir.rglob('*.csv'),
...     mydir.rglob('*.json'),
... )
>>>
>>> for file in sorted(files):
...     print(file.name)
...
myfile1.csv
myfile1.json
myfile1.txt
myfile2.csv
myfile2.json
myfile2.txt
myfile2.json
myfile3.txt

10.6.6. Assignments

# %% About
# - Name: Iterator Chain Generators
# - 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. Chain `DATA1` and `DATA2` into a single sequence
# 2. Define variable `result: list[str]` with the solution
# 2. Run doctests - all must succeed

# %% Polish
# 1. Połącz (`chain`) `data1()` i `data1()` w jedną sekwencję
# 2. Zdefiniuj zmienną `result: list[str]` z rozwiązaniem
# 2. Uruchom doctesty - wszystkie muszą się powieść

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

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

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

>>> assert 'result' in globals(), \
'Variable `result` is not defined; assign result of your program to it.'

>>> assert result is not Ellipsis, \
'Variable `result` has an invalid value; assign result of your program to it.'

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

>>> list(result)
['Alice', 'Bob', 'Carol', 'Dave', 'Eve', 'Mallory']
"""

# %% 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
from typing import Iterator
from itertools import chain

# %% Types
result: Iterator[str]

# %% Data
def data1():
    yield 'Alice'
    yield 'Bob'
    yield 'Carol'

def data2():
    yield 'Dave'
    yield 'Eve'
    yield 'Mallory'


# %% Result
result = ...

# %% About
# - Name: Iterator Chain Sequence
# - 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. Chain `DATA1` and `DATA2` into a single sequence
# 2. Define variable `result: list[str]` with the solution
# 2. Run doctests - all must succeed

# %% Polish
# 1. Połącz (`chain`) `DATA1` i `DATA2` w jedną sekwencję
# 2. Zdefiniuj zmienną `result: list[str]` z rozwiązaniem
# 2. Uruchom doctesty - wszystkie muszą się powieść

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

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

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

>>> assert 'result' in globals(), \
'Variable `result` is not defined; assign result of your program to it.'

>>> assert result is not Ellipsis, \
'Variable `result` has an invalid value; assign result of your program to it.'

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

>>> list(result)
['Alice', 'Bob', 'Carol', 'Dave', 'Eve', 'Mallory']
"""

# %% 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
from typing import Iterator
from itertools import chain

# %% Types
result: Iterator[str]

# %% Data
DATA1 = ['Alice', 'Bob', 'Carol']
DATA2 = ['Dave', 'Eve', 'Mallory']

# %% Result
result = ...