10.1. While About
Iterate over sequences (iterables)
Repeat if until result is False
10.1.1. Syntax
Continue execution when argument is
TrueStops if argument is
False
Generic syntax:
while <condition>:
<do something>
Example:
>>>
... while True:
... pass
10.1.2. Getitem with Const
>>> data = ['a', 'b', 'c']
>>>
>>>
>>> print(data[0])
a
>>>
>>> print(data[1])
b
>>>
>>> print(data[2])
c
>>>
>>> print(data[3])
Traceback (most recent call last):
IndexError: list index out of range
10.1.3. Getitem with Variable
>>> data = ['a', 'b', 'c']
>>> i = 0 # i = 0
>>>
>>>
>>> print(data[i]) # data[0]
a
>>> i += 1
>>>
>>>
>>> print(data[i]) # data[1]
b
>>> i += 1
>>>
>>>
>>> print(data[i]) # data[2]
c
>>> i += 1
>>>
>>> print(data[i]) # data[3]
Traceback (most recent call last):
IndexError: list index out of range
10.1.4. Getitem with Boundary Checking
>>> data = ['a', 'b', 'c']
>>> until = len(data) # 3
>>> i = 0
>>>
>>> if i < until: # True
... print(data[i]) # data[0]
... i += 1 # i = 1
a
>>>
>>> if i < until: # True
... print(data[i]) # data[1]
... i += 1 # i = 2
b
>>>
>>> if i < until: # True
... print(data[i]) # data[2]
... i += 1 # i = 3
c
>>>
>>> if i < until: # False
... print(data[i]) # will not execute
... i += 1 # will not execute
10.1.5. While Loop
>>> data = ['a', 'b', 'c']
>>> until = len(data) # 3
>>> i = 0
>>>
>>> while i < until:
... print(data[i])
... i += 1
a
b
c
10.1.6. Sequence Iteration
Better idea for this is to use for loop. for loop supports Iterators. len() must write all numbers to memory, to calculate its length:
>>> data = ['a', 'b', 'c']
>>>
>>> i = 0
>>> while i < 3:
... print(i, data[i])
... i += 1
0 a
1 b
2 c
>>> data = ['a', 'b', 'c']
>>>
>>> i = 0
>>> until = 3
>>> while i < until:
... print(i, data[i])
... i += 1
0 a
1 b
2 c
>>> data = ['a', 'b', 'c']
>>>
>>> i = 0
>>> until = len(data)
>>> while i < until:
... print(i, data[i])
... i += 1
0 a
1 b
2 c
10.1.7. Convention
The longer the loop scope, the longer the variable name should be
Avoid one letters if scope is longer than one line
Use
ifor loop counter (it is traditional name in almost all languages)
10.1.8. Assignments
# %% 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
# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`
# %% About
# - Name: While About Range
# - Difficulty: easy
# - Lines: 5
# - Minutes: 3
# %% English
# 1. Generate `result: list[int]` with numbers from 0 to 5 (exclusive)
# 2. Do not use `range()`
# 3. Run doctests - all must succeed
# %% Polish
# 1. Wygeneruj `result: list[int]` z liczbami od 0 do 5 (rozłącznie)
# 2. Nie używaj `range()`
# 3. Uruchom doctesty - wszystkie muszą się powieść
# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'
>>> assert type(result) is list
>>> assert all(type(x) is int for x in result)
>>> result
[0, 1, 2, 3, 4]
"""
# List with numbers from 0 to 5 (exclusive)
# type: list[int]
result = ...
# %% 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
# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`
# %% About
# - Name: While About Count
# - Difficulty: easy
# - Lines: 10
# - Minutes: 8
# %% English
# 1. Count occurrences of each color in `DATA`
# 2. Do not use `list.count()`
# 3. Run doctests - all must succeed
# %% Polish
# 1. Zlicz wystąpienia każdego z kolorów w `DATA`
# 2. Nie używaj `list.count()`
# 3. Uruchom doctesty - wszystkie muszą się powieść
# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'
>>> assert red is not Ellipsis, \
'Assign your result to variable `red`'
>>> assert green is not Ellipsis, \
'Assign your result to variable `green`'
>>> assert blue is not Ellipsis, \
'Assign your result to variable `blue`'
>>> assert type(red) is int, \
'Variable `red` has invalid type, should be list'
>>> assert type(green) is int, \
'Variable `green` has invalid type, should be list'
>>> assert type(blue) is int, \
'Variable `blue` has invalid type, should be list'
>>> red
3
>>> green
2
>>> blue
2
"""
DATA = ['red', 'green', 'blue', 'red', 'green', 'red', 'blue']
red = 0
green = 0
blue = 0
# If variable DATA is:
# - 'red' then increment variable `red` by 1
# - 'green' then increment variable `green` by 1
# - 'blue' then increment variable `blue` by 1
...
# %% 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
# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`
# %% About
# - Name: While About To Float
# - Difficulty: easy
# - Lines: 5
# - Minutes: 5
# %% English
# 1. Create `result: list[float]`
# 2. Use `while` to iterate over `DATA`
# 3. Convert current elements of `DATA` to `float`
# 4. Converted value append to `result`
# 5. Run doctests - all must succeed
# %% Polish
# 1. Stwórz `result: list[float]`
# 2. Użyj `while` do iterowania po `DATA`
# 3. Przekonwertuj obecny element `DATA` do `float`
# 4. Przekonwertowaną wartość dodaj na koniec `result`
# 5. Uruchom doctesty - wszystkie muszą się powieść
# %% Hints
# - `Stop` or `Ctrl+C` kills infinite loop
# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'
>>> type(result)
<class 'list'>
>>> assert all(type(x) is float for x in result)
>>> result
[1.0, 2.0, 3.0]
"""
DATA = (1, 2, 3)
# Values from DATA converted to float
# type: list[float]
result = ...
# %% 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
# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`
# %% About
# - Name: While About To Str
# - Difficulty: easy
# - Lines: 4
# - Minutes: 5
# %% English
# 1. Create `result: str`
# 2. Use `while` to iterate over `DATA`
# 3. Add current element of `DATA` to `result`
# 4. Do not use `str.join()`
# 5. Run doctests - all must succeed
# %% Polish
# 1. Stwórz `result: str`
# 2. Użyj `while` do iterowania po `DATA`
# 3. Dodaj obecny element z `DATA` do `result`
# 4. Nie używaj `str.join()`
# 5. Uruchom doctesty - wszystkie muszą się powieść
# %% Hints
# - `Stop` or `Ctrl+C` kills infinite loop
# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'
>>> type(result)
<class 'str'>
>>> result
'hello'
"""
DATA = ['h', 'e', 'l', 'l', 'o']
# Joined DATA values
# type: str
result = ...
# %% 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
# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`
# %% About
# - Name: While About Remove PL Chars
# - Difficulty: medium
# - Lines: 5
# - Minutes: 5
# %% English
# 1. Use `while` to iterate over `DATA`
# 2. If letter is in `PL` then use conversion value as letter
# 3. Add letter to `result`
# 4. Run doctests - all must succeed
# %% Polish
# 1. Użyj `while` do iteracji po `DATA`
# 2. Jeżeli litera jest w `PL` to użyj skonwertowanej wartości jako litera
# 3. Dodaj literę do `result`
# 4. Uruchom doctesty - wszystkie muszą się powieść
# %% Hints
# - `Stop` or `Ctrl+C` kills infinite loop
# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'
>>> type(result)
<class 'str'>
>>> result
'zazolc gesla jazn'
"""
PL = {
'ą': 'a',
'ć': 'c',
'ę': 'e',
'ł': 'l',
'ń': 'n',
'ó': 'o',
'ś': 's',
'ż': 'z',
'ź': 'z',
}
DATA = 'zażółć gęślą jaźń'
# DATA with substituted PL diacritic chars to ASCII letters
# type: str
result = ...