3.8. Unpacking Assignment

3.8.1. Rationale

>>> a = 1
>>> a, b = 1, 2
>>> a, b, c = 1, 2, 3
>>> a, b, c = (1, 2, 3)
>>> a, b, c = [1, 2, 3]
>>> a, b, c = {1, 2, 3}
>>> (a, b, c) = (1, 2, 3)
>>> (a, b, c) = [1, 2, 3]
>>> [a, b, c] = [1, 2, 3]
>>> [a, b, c] = (1, 2, 3)

3.8.2. Errors

>>> {a, b, c} = {1, 2, 3}
Traceback (most recent call last):
SyntaxError: can't assign to literal
>>> a, b, c = [1, 2, 3, 4]
Traceback (most recent call last):
ValueError: too many values to unpack (expected 3)
>>> a, b, c, d = [1, 2, 3]
Traceback (most recent call last):
ValueError: not enough values to unpack (expected 4, got 3)

3.8.3. Arbitrary Number of Arguments

Unpacking values at the right side:

>>> a, b, *c = [1, 2, 3, 4]
>>>
>>> a
1
>>> b
2
>>> c
[3, 4]

Unpacking values at the left side:

>>> *a, b, c = [1, 2, 3, 4]
>>>
>>> a
[1, 2]
>>> b
3
>>> c
4

Unpacking values from both sides at once:

>>> a, *b, c = [1, 2, 3, 4]
>>>
>>> a
1
>>> b
[2, 3]
>>> c
4

Unpacking from variable length:

>>> a, *b, c = [1, 2]
>>>
>>> a
1
>>> b
[]
>>> c
2

Cannot unpack from both sides at once:

>>> *a, b, *c = [1, 2, 3, 4]
Traceback (most recent call last):
SyntaxError: two starred expressions in assignment

Unpacking requires values for required arguments:

>>> a, *b, c = [1]
Traceback (most recent call last):
ValueError: not enough values to unpack (expected at least 2, got 1)

3.8.4. Nested

>>> a, (b, c) = [1, (2, 3)]
>>>
>>> a
1
>>> b
2
>>> c
3

3.8.5. Convention

>>> first, *middle, last = [1, 2, 3, 4]
>>>
>>> first
1
>>> middle
[2, 3]
>>> last
4
>>> first, second, *others = [1, 2, 3, 4]
>>>
>>> first
1
>>> second
2
>>> others
[3, 4]

3.8.6. Skipping Values

  • _ is regular variable name, not a special Python syntax

  • _ by convention is used for data we don't want to access in future

    >>> _ = 'Jan Twardowski'
    >>> print(_)
    Jan Twardowski
    
    >>> line = 'Jan,Twardowski,1'
    >>> firstname, lastname, _ = line.split(',')
    >>>
    >>> print(firstname)
    Jan
    >>> print(lastname)
    Twardowski
    
    >>> line = 'Jan,Twardowski,1,2,3,4,5'
    >>> firstname, lastname, *_ = line.split(',')
    >>>
    >>> print(firstname)
    Jan
    >>> print(lastname)
    Twardowski
    
    >>> a, _, c = 1, 2, 3
    >>>
    >>> print(a)
    1
    >>> print(c)
    3
    
    >>> _, b, _ = 1, 2, 3
    >>>
    >>> print(b)
    2
    
    >>> line = '4.9,3.1,1.5,0.1,setosa'
    >>> *_, label = line.split(',')
    >>>
    >>> print(label)
    setosa
    
    >>> line = 'twardowski:x:1001:1001:Jan Twardowski:/home/twardowski:/bin/bash'
    >>> username, _, _, _, fullname, *_ = line.split(':')
    >>>
    >>> print(username)
    twardowski
    >>> print(fullname)
    Jan Twardowski
    
    >>> line = 'twardowski:x:1001:1001:Jan Twardowski:/home/twardowski:/bin/bash'
    >>> username, *_, home, _ = line.split(':')
    >>>
    >>> print(username)
    twardowski
    >>> print(home)
    /home/twardowski
    
    >>> _, (interesting, _) = [1, (2, 3)]
    >>>
    >>> print(interesting)
    2
    

3.8.7. Use Cases

>>> import sys
>>>
>>> sys.version_info
sys.version_info(major=3, minor=9, micro=1, releaselevel='final', serial=0)
>>>
>>> major, minor, *_ = sys.version_info
>>> print(major, minor, sep='.')
3.9
>>> *features, label = (5.8, 2.7, 5.1, 1.9, 'virginica')
>>>
>>> features
[5.8, 2.7, 5.1, 1.9]
>>> label
'virginica'
>>> *features, label = (5.8, 2.7, 5.1, 1.9, 'virginica')
>>> avg = sum(features) / len(features)
>>>
>>> print(label, avg)
virginica 3.875
>>> line = 'ares3,watney,lewis,vogel,johanssen'
>>> mission, *crew = line.split(',')
>>>
>>> mission
'ares3'
>>> crew
['watney', 'lewis', 'vogel', 'johanssen']
>>> first, second, *others = range(10)
>>>
>>> first
0
>>> second
1
>>> others
[2, 3, 4, 5, 6, 7, 8, 9]

3.8.8. Assignments

Code 3.17. Solution
"""
* Assignment: Function Unpack Nested
* Complexity: easy
* Lines of code: 1 lines
* Time: 3 min

English:
    1. Use data from "Given" section (see below)
    2. Separate header and records
    3. Use asterisk `*` notation
    4. Compare result with "Tests" section (see below)

Polish:
    1. Użyj danych z sekcji "Given" (patrz poniżej)
    2. Odseparuj nagłówek od danych
    3. Skorzystaj z konstrukcji z gwiazdką `*`
    4. Porównaj wyniki z sekcją "Tests" (patrz poniżej)

Tests:
    >>> type(header)
    <class 'tuple'>
    >>> type(data)
    <class 'list'>
    >>> assert all(type(x) is str for x in header)
    >>> assert all(type(row) is tuple for row in data)
    >>> header
    ('Sepal length', 'Sepal width', 'Petal length', 'Petal width', 'Species')
    >>> data  # doctest: +NORMALIZE_WHITESPACE
    [(5.8, 2.7, 5.1, 1.9, 'virginica'),
     (5.1, 3.5, 1.4, 0.2, 'setosa'),
     (5.7, 2.8, 4.1, 1.3, 'versicolor'),
     (6.3, 2.9, 5.6, 1.8, 'virginica'),
     (6.4, 3.2, 4.5, 1.5, 'versicolor'),
     (4.7, 3.2, 1.3, 0.2, 'setosa')]
"""


# Given
DATA = [('Sepal length', 'Sepal width', 'Petal length', 'Petal width', 'Species'),
        (5.8, 2.7, 5.1, 1.9, 'virginica'),
        (5.1, 3.5, 1.4, 0.2, 'setosa'),
        (5.7, 2.8, 4.1, 1.3, 'versicolor'),
        (6.3, 2.9, 5.6, 1.8, 'virginica'),
        (6.4, 3.2, 4.5, 1.5, 'versicolor'),
        (4.7, 3.2, 1.3, 0.2, 'setosa')]


Code 3.18. Solution
"""
* Assignment: Function Unpack Split
* Complexity: easy
* Lines of code: 1 lines
* Time: 3 min

English:
    1. Use data from "Given" section (see below)
    2. Using `str.split()` split input data by white space
    3. Separate ip address and host names
    4. Use asterisk `*` notation
    5. Compare result with "Tests" section (see below)

Polish:
    1. Użyj danych z sekcji "Given" (patrz poniżej)
    2. Używając `str.split()` podziel dane wejściowe po białych znakach
    3. Odseparuj adres ip i nazw hostów
    4. Skorzystaj z notacji z gwiazdką `*`
    5. Porównaj wyniki z sekcją "Tests" (patrz poniżej)

Hints:
    * Use `str.split()` without any argument

Tests:
    >>> type(ip)
    <class 'str'>
    >>> type(hosts)
    <class 'list'>
    >>> assert all(type(host) is str for host in hosts)
    >>> ip
    '10.13.37.1'
    >>> hosts
    ['nasa.gov', 'esa.int', 'roscosmos.ru']
"""


# Given
DATA = '10.13.37.1      nasa.gov esa.int roscosmos.ru'