2.1. Syntax Underscore
_is used to skip valuesIt is a regular variable name, not a special Python syntax
By convention it is used for data we don't want to access in future
It can be used multiple times in the same statement
2.1.1. Syntax
Underscore (
_) is a regular variable nameIt's not a special Python syntax
By convention it is used for data we don't want to access in future
>>> _ = 'Alice'
>>> print(_)
Alice
2.1.2. Single Underscore
>>> def get_user_from_database(user_id):
... return 'alice', 'secret', 'alice@example.com', 'alice@example.edu'
>>>
>>>
>>> username, password, email_priv, email_work = get_user_from_database(1)
>>> username, _, email_priv, email_work = get_user_from_database(1)
2.1.3. Multiple Underscores
>>> def get_user_from_database(user_id):
... return 'alice', 'secret', 'alice@example.com', 'alice@example.edu'
>>>
>>>
>>> username, password, email_priv, email_work = get_user_from_database(1)
>>> username, password, _, _ = get_user_from_database(1)
>>> username, _, _, email = get_user_from_database(1)
>>> username, _, email, _ = get_user_from_database(1)
2.1.4. For Loop
>>> for _ in range(0,3):
... print('hello')
...
hello
hello
hello
2.1.5. Recap
_is used to skip valuesIt is a regular variable name, not a special Python syntax
By convention it is used for data we don't want to access in future
It can be used multiple times in the same statement
2.1.6. Use Case - 1
>>> line = 'alice:x:1000:1000:Alice:/home/alice:/bin/bash'
>>> username, _, uid, _, _, home, _ = line.split(':')
>>>
>>> print(f'{username=}, {uid=}, {home=}')
username='alice', uid='1000', home='/home/alice'
2.1.7. Use Case - 2
Skip
>>> a, b, _ = 'red', 'green', 'blue'
>>> a, _, _ = 'red', 'green', 'blue'
>>> a, _, c = 'red', 'green', 'blue'
>>> _, b, _ = 'red', 'green', 'blue'
>>> _, _, c = 'red', 'green', 'blue'
2.1.8. Use Case - 3
>>> _, important, _ = 1, 2, 3
>>>
>>> print(important)
2
>>> _, (important, _) = [1, (2, 3)]
>>>
>>> print(important)
2
>>> _, _, important = (True, [1, 2, 3, 4], 5)
>>>
>>> print(important)
5
>>> _, _, important = (True, [1, 2, 3, 4], (5, True))
>>>
>>> print(important)
(5, True)
>>>
>>> _, _, (important, _) = (True, [1, 2, 3, 4], (5, True))
>>>
>>> print(important)
5
Python understands this as:
>>> _ = (True, [1, 2, 3, 4], (5, True))
>>>
>>> a,b,c = (object, object, object)
>>> a,b,(c,d) = (object, object, (object,object))
2.1.9. Assignments
# %% About
# - Name: Syntax Underscore One
# - 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. Unpack 3 variables from tuple `data`
# 2. Assign first to `firstname`, second to `lastname`, third to `_`
# 3. Run doctests - all must succeed
# %% Polish
# 1. Rozpakuj 3 zmienne z krotki `data`
# 2. Pierwszą przypisz do `firstname`, drugą do `lastname`, trzecią do `_`
# 3. Uruchom doctesty - wszystkie muszą się powieść
# %% Example
# >>> firstname
# 'Alice'
#
# >>> lastname
# 'Apricot'
# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'
>>> assert firstname is not Ellipsis, \
'Variable `firstname` has an invalid value; assign result of your program to it.'
>>> assert type(firstname) is str, \
'Variable `firstname` has an invalid type; expected: `str`.'
>>> firstname
'Alice'
>>> assert lastname is not Ellipsis, \
'Variable `lastname` has an invalid value; assign result of your program to it.'
>>> assert type(lastname) is str, \
'Variable `lastname` has an invalid type; expected: `str`.'
>>> lastname
'Apricot'
>>> assert _ is not Ellipsis, \
'Variable `_` has an invalid value; assign result of your program to it.'
>>> assert type(_) is int, \
'Variable `_` has an invalid type; expected: `int`.'
>>> _
30
"""
# %% 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
firstname: str
lastname: str
_: int
# %% Data
DATA = ('Alice', 'Apricot', 30)
# %% Result
# %% About
# - Name: Syntax Underscore Many
# - 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. Unpack 3 variables from tuple `data`
# 2. Assign first to `_`, second to `_`, third to `age`
# 3. Run doctests - all must succeed
# %% Polish
# 1. Rozpakuj 3 zmienne z krotki `data`
# 2. Pierwszą przypisz do `_`, drugą do `_`, trzecią do `age`
# 3. Uruchom doctesty - wszystkie muszą się powieść
# %% Example
# >>> age
# 30
# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'
>>> assert age is not Ellipsis, \
'Variable `age` has an invalid value; assign result of your program to it.'
>>> assert type(age) is int, \
'Variable `age` has an invalid type; expected: `int`.'
>>> age
30
>>> assert _ is not Ellipsis, \
'Variable `_` has an invalid value; assign result of your program to it.'
>>> assert type(_) is str, \
'Variable `_` has an invalid type; expected: `str`.'
>>> _
'Apricot'
"""
# %% 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
_: str
age: int
# %% Data
DATA = ('Alice', 'Apricot', 30)
# %% Result