2.1. Unpack Assignment

  • a = 1 - Assignment

  • a, b = 1, 2 - Unpacking assignment

  • a = b = 1 - Chained assignment

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

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

Assignment:

>>> a = 1

Unpacking assignment:

>>> a, b = 1, 2

Chained assignment:

>>> a = b = 1

Chained unpacking assignment:

>>> a, b = c, d = 1, 2

2.1.1. Example

>>> def get_user_details(username):
...     return 'Mark', 'Watney', 'mwatney@nasa.gov'
>>>
>>>
>>> firstname, lastname, email = get_user_details('mwatney')
>>>
>>> firstname
'Mark'
>>>
>>> lastname
'Watney'
>>>
>>> email
'mwatney@nasa.gov'

2.1.2. Assignment

  • Scalar Assignment

  • identifier = object

  • a = 1

  • a = 1, 2

>>> a = 1
>>>
>>> print(f'{a=}')
a=1
>>> a = 1, 2
>>>
>>> print(f'{a=}')
a=(1, 2)

2.1.3. Unpacking Assignment

  • iterable[identifier] = iterable[object]

  • a, b = 1, 2

  • a, b, c = 1, 2, 3

  • Sequence -> tuple, list

  • Iterable -> tuple, list, set, frozenset, dict, ...

  • Length at right and left side must be the same

>>> a, b = 1, 2
>>>
>>> print(f'{a=}, {b=}')
a=1, b=2
>>> a, = 1,
>>> a, b = 1, 2
>>> a, b, c = 1, 2, 3
>>> a, b, c, d = 1, 2, 3, 4
>>> a, b, c, d, e = 1, 2, 3, 4, 5
>>> a, b, c = 1, 2
Traceback (most recent call last):
ValueError: not enough values to unpack (expected 3, got 2)
>>> a, b, c = 1, 2, 3, 4
Traceback (most recent call last):
ValueError: too many values to unpack (expected 3)

2.1.4. Chained Assignment

  • identifier1 = identifier2 = object

  • a = b = 1

  • a = b = c = 1

>>> a = b = 1
>>>
>>> print(f'{a=}, {b=}')
a=1, b=1
>>> a = b = 1
>>> a = b = c = 1
>>> a = b = c = d = 1
>>> a = b = c = d = e = 1

2.1.5. Chained Unpacking Assignment

  • iterable[identifier] = iterable[identifier] = iterable[object]

>>> a, b = c, d = 1, 2
>>>
>>> print(f'{a=}, {b=}, {c=}, {d=}')
a=1, b=2, c=1, d=2
>>> a = b, c = 1, 2
>>>
>>> print(f'{a=}, {b=}, {c=}')
a=(1, 2), b=1, c=2

2.1.6. Brackets

Brackets does not define tuple, commas do:

>>> a = 1, 2, 3
>>> b = (1, 2, 3)
>>>
>>> a == b
True
>>> a = (1, 2)
>>> type(a)
<class 'tuple'>
>>>
>>> a = 1, 2
>>> type(a)
<class 'tuple'>
>>> 1+2 * 3
7
>>>
>>> (1+2) * 3
9
>>>
>>> (1+2,) * 3
(3, 3, 3)

Right-Side Brackets:

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

Left-Side Brackets:

>>> (a, b, c) = 1, 2, 3
>>> [a, b, c] = 1, 2, 3
>>> {a, b, c} = 1, 2, 3
Traceback (most recent call last):
SyntaxError: cannot assign to set display here. Maybe you meant '==' instead of '='?

Warning:

>>> (a) = 1
>>> (a,) = 1,

Errors:

>>> (a,) = 1
Traceback (most recent call last):
TypeError: cannot unpack non-iterable int object
>>> [a] = 1
Traceback (most recent call last):
TypeError: cannot unpack non-iterable int object

2.1.7. Unpacking

>>> def get_user():
...     return 'Mark', 'Watney', 'mwatney@nasa.gov'
>>> get_user()
('Mark', 'Watney', 'mwatney@nasa.gov')
>>> user = get_user()
>>>
>>> print(user)
('Mark', 'Watney', 'mwatney@nasa.gov')
>>> firstname, lastname, email = get_user()
>>>
>>> print(firstname)
Mark
>>>
>>> print(lastname)
Watney
>>>
>>> print(email)
mwatney@nasa.gov

2.1.8. Nested

>>> def get_user():
...     return 'Mark', 'Watney', ('mwatney@nasa.gov', 'mwatney@gmail.com')
>>> firstname, lastname, emails = get_user()
>>>
>>> print(emails)
('mwatney@nasa.gov', 'mwatney@gmail.com')
>>> firstname, lastname, (email_work, email_home) = get_user()
>>>
>>> print(email_work)
mwatney@nasa.gov
>>>
>>> print(email_home)
mwatney@gmail.com
>>> firstname, lastname, email_work, email_home = get_user()
Traceback (most recent call last):
ValueError: not enough values to unpack (expected 4, got 3)

2.1.9. 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

>>> _ = 'Mark'
>>> print(_)
Mark
>>> def get_user():
...     return 'Mark', 'Watney', 'mwatney@nasa.gov'
>>>
>>>
>>> firstname, lastname, email = get_user()
>>> firstname, lastname, _ = get_user()
>>> firstname, _, _ = get_user()
>>> _, lastname, _ = get_user()
>>> _, _, email = get_user()

2.1.10. Use Case - 0x01

>>> a, b, c = range(0, 3)
>>> a, b, c, d, e = range(0, 5)
>>> a, b, c, d, e, f, g, h, i, j = range(0, 10)

2.1.11. Use Case - 0x02

>>> import sys
>>>
>>> major, minor, patch, *_ = sys.version_info
>>>
>>>
>>> print(major)
3
>>>
>>> print(minor)
11
>>>
>>> print(patch)
0

2.1.12. Use Case - 0x03

  • Line from /etc/passwd

>>> line = 'watney:x:1000:1000:Mark Watney:/home/watney:/bin/bash'
>>> username, _, uid, _, _, _, _ = line.split(':')
>>>
>>> print(f'{username=}, {uid=}')
username='watney', uid='1000'

2.1.13. Use Case - 0x04

  • Important

>>> _, 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))

2.1.14. Use Case - 0x05

>>> row = (5.8, 2.7, 5.1, 1.9, 'virginica')
>>> sl = row[0]
>>> sw = row[1]
>>> pl = row[2]
>>> pw = row[3]
>>> species = row[4]
>>>
>>> print(f'{sl=}, {sw=}, {pl=}, {pw=}, {species=}')
sl=5.8, sw=2.7, pl=5.1, pw=1.9, species='virginica'
>>> sl, sw, pl, pw, species = row
>>>
>>> print(f'{sl=}, {sw=}, {pl=}, {pw=}, {species=}')
sl=5.8, sw=2.7, pl=5.1, pw=1.9, species='virginica'

2.1.15. Use Case - 0x06

>>> 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 row in DATA:
...     sl = row[0]
...     sw = row[1]
...     pl = row[2]
...     pw = row[3]
...     species = row[4]
...     print(f'{sl=}, {sw=}, {pl=}, {pw=}, {species=}')
sl=5.8, sw=2.7, pl=5.1, pw=1.9, species='virginica'
sl=5.1, sw=3.5, pl=1.4, pw=0.2, species='setosa'
sl=5.7, sw=2.8, pl=4.1, pw=1.3, species='versicolor'
>>> for sl, sw, pl, pw, species in DATA:
...     print(f'{sl=}, {sw=}, {pl=}, {pw=}, {species=}')
sl=5.8, sw=2.7, pl=5.1, pw=1.9, species='virginica'
sl=5.1, sw=3.5, pl=1.4, pw=0.2, species='setosa'
sl=5.7, sw=2.8, pl=4.1, pw=1.3, species='versicolor'

2.1.16. Recap

  • Four types of assignments: Scalar, Unpacking, Chained, Chained Unpacking Assignment

  • For unpacking assignment, lengths at both sides must be the same

  • Both left and right expression side brackets are optional

  • Unpacking nested sequences

  • Skipping values is done by using _

Assignment:

>>> a = 1

Unpacking assignment:

>>> a, b = 1, 2

Chained assignment:

>>> a = b = 1

Chained unpacking assignment:

>>> a, b = c, d = 1, 2

Unpacking nested:

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

Skipping:

>>> _, (important, _) = 1, (2, 3)

2.1.17. Assignments

Code 2.11. Solution
"""
* Assignment: Unpack Assignment List
* Required: yes
* Complexity: easy
* Lines of code: 1 lines
* Time: 3 min

English:
    1. Separate ip address and host name
    2. Run doctests - all must succeed

Polish:
    1. Odseparuj adres ip i nazwę hosta
    2. Uruchom doctesty - wszystkie muszą się powieść

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

    >>> assert ip is not Ellipsis, \
    'Assign your result to variable `ip`'
    >>> assert host is not Ellipsis, \
    'Assign your result to variable `host`'
    >>> assert type(ip) is str, \
    'Variable `ip` has invalid type, should be str'
    >>> assert type(host) is str, \
    'Variable `hosts` has invalid type, should be str'

    >>> ip
    '10.13.37.1'

    >>> host
    'nasa.gov'
"""

DATA = ['10.13.37.1', 'nasa.gov']

# String with IP address: 10.13.37.1
# type: str
ip = ...

# String with host name: nasa.gov
# type: str
host = ...

Code 2.12. Solution
"""
* Assignment: Unpack Assignment Split
* Required: yes
* Complexity: easy
* Lines of code: 1 lines
* Time: 3 min

English:
    1. Split input data
    2. Separate ip address and host name
    3. Run doctests - all must succeed

Polish:
    1. Podziel dane wejściowe
    2. Odseparuj adres ip i nazwę hosta
    3. Uruchom doctesty - wszystkie muszą się powieść

Hints:
    * `str.split()`

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

    >>> assert ip is not Ellipsis, \
    'Assign your result to variable `ip`'
    >>> assert host is not Ellipsis, \
    'Assign your result to variable `host`'
    >>> assert type(ip) is str, \
    'Variable `ip` has invalid type, should be str'
    >>> assert type(host) is str, \
    'Variable `hosts` has invalid type, should be str'

    >>> ip
    '10.13.37.1'

    >>> host
    'nasa.gov'
"""

DATA = '10.13.37.1 nasa.gov'

# String with IP address: 10.13.37.1
# type: str
ip = ...

# String with host name: nasa.gov
# type: str
host = ...