2.1. Unpacking Assignment

2.1.1. Rationale

  • Scalar assignment

  • Vector assignment

>>> 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)
../../_images/unpacking-assignment,args,params.png

2.1.2. 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)

2.1.3. 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)

2.1.4. Nested

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

2.1.5. 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]

2.1.6. 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

2.1.7. Use Cases

>>> import sys
>>>
>>> sys.version_info
sys.version_info(major=3, minor=9, micro=7, 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
>>> DATA = [(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')]
>>>
>>>
>>> for *features, label in DATA:
...     avg = sum(features) / len(features)
...     print(label, avg)
virginica 3.875
setosa 2.55
versicolor 3.475
>>> 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]

2.1.8. Assignments

Code 2.17. Solution
"""
* Assignment: Unpacking Assignment List
* Complexity: easy
* Lines of code: 1 lines
* Time: 3 min

English:
    1. Using `str.split()` split input data by white space
    2. Separate ip address and host names
    3. Use asterisk `*` notation
    4. Run doctests - all must succeed

Polish:
    1. Używając `str.split()` podziel dane wejściowe po białych znakach
    2. Odseparuj adres ip i nazw hostów
    3. Skorzystaj z notacji z gwiazdką `*`
    4. Uruchom doctesty - wszystkie muszą się powieść

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

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> type(ip)
    <class 'str'>
    >>> type(hosts)
    <class 'list'>
    >>> assert all(type(host) is str for host in hosts)
    >>> '' not in hosts
    True
    >>> ip
    '10.13.37.1'
    >>> hosts
    ['nasa.gov', 'esa.int', 'roscosmos.ru']
"""

DATA = ['10.13.37.1', 'nasa.gov', 'esa.int', 'roscosmos.ru']

ip: str
hosts: list


Code 2.18. Solution
"""
* Assignment: Unpacking Assignment Func
* Complexity: easy
* Lines of code: 1 lines
* Time: 3 min

English:
    1. Using `str.split()` split input data by white space
    2. Separate ip address and host names
    3. Use asterisk `*` notation
    4. Run doctests - all must succeed

Polish:
    1. Używając `str.split()` podziel dane wejściowe po białych znakach
    2. Odseparuj adres ip i nazw hostów
    3. Skorzystaj z notacji z gwiazdką `*`
    4. Uruchom doctesty - wszystkie muszą się powieść

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

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> type(ip)
    <class 'str'>
    >>> type(hosts)
    <class 'list'>
    >>> assert all(type(host) is str for host in hosts)
    >>> '' not in hosts
    True
    >>> ip
    '10.13.37.1'
    >>> hosts
    ['nasa.gov', 'esa.int', 'roscosmos.ru']
"""

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

ip: str
hosts: list


Code 2.19. Solution
"""
* Assignment: Unpacking Assignment Nested
* Complexity: easy
* Lines of code: 1 lines
* Time: 3 min

English:
    1. Separate header and records
    2. Use asterisk `*` notation
    3. Run doctests - all must succeed

Polish:
    1. Odseparuj nagłówek od danych
    2. Skorzystaj z konstrukcji z gwiazdką `*`
    3. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> type(header)
    <class 'tuple'>
    >>> type(data)
    <class 'list'>
    >>> len(header) > 0
    True
    >>> len(data) > 0
    True
    >>> 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')]

"""

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')]

header: tuple
data: list