2.1. Unpack Assignment

  • a = 1 - Assignment

  • a, b = 1, 2 - Unpacking assignment

  • a = b = 1 - Multi 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

Multi assignment:

>>> a = b = 1

Multi unpacking assignment:

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

2.1.1. 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)

2.1.2. Unpacking Assignment

  • iterable[identifier] = iterable[object]

  • a, b = 1, 2

  • a, b, c = 1, 2, 3

  • Vector Assignment

  • Sequence Assignment

  • Iterable Assignment

  • Length at right and left side must be the same

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

>>> 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

2.1.3. Multi 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

2.1.4. Multi 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, d = 1, 2
>>>
>>> print(f'{a=}, {c=}')
a=1, c=1
>>>
>>> c = 0
>>> print(f'{a=}, {c=}')
a=1, c=0

2.1.5. Right-Side Brackets

Scalar assignments:

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

Unpacking assignments:

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

Rationale:

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

2.1.6. Left-Side Brackets

>>> (a) = (1)
>>>
>>> print(a)
1

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

2.1.7. Errors

>>> a, b, c = 1, 2, 3, 4
Traceback (most recent call last):
ValueError: too many values to unpack (expected 3)

>>> 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}
Traceback (most recent call last):
SyntaxError: cannot assign to set display here. Maybe you meant '==' instead of '='?

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

2.1.8. Unpacking

>>> data = [1, 2, 3]
>>> a, b, c = data
>>>
>>> print(f'{a=}, {b=}, {c=}')
a=1, b=2, c=3

>>> line = 'Mark,Watney,40'
>>> firstname, lastname, age = line.split(',')
>>>
>>> print(f'{firstname=}, {lastname=}, {age=}')
firstname='Mark', lastname='Watney', age='40'

>>> data = ['Mark', 'Watney', ('mwatney@nasa.gov', 'mwatney@gmail.com')]
>>> firstname, lastname, emails = data
>>>
>>> print(f'{firstname=}\n{lastname=}\n{emails=}')
firstname='Mark'
lastname='Watney'
emails=('mwatney@nasa.gov', 'mwatney@gmail.com')

2.1.9. Nested

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

>>> data = ['Mark', 'Watney', ('mwatney@nasa.gov', 'mwatney@gmail.com')]
>>> firstname, lastname, (email_work, email_home) = data
>>>
>>> print(f'{firstname=}\n{lastname=}\n{email_work=}\n{email_home=}')
firstname='Mark'
lastname='Watney'
email_work='mwatney@nasa.gov'
email_home='mwatney@gmail.com'

2.1.10. 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 Watney'
>>>
>>> print(_)
Mark Watney

>>> line = 'Mark,Watney,40'
>>> firstname, lastname, _ = line.split(',')
>>>
>>> print(f'{firstname=}, {lastname=}')
firstname='Mark', lastname='Watney'

>>> line = 'Mark,Watney,40,185,75.5'
>>> firstname, lastname, _, _, _ = line.split(',')
>>>
>>> print(f'{firstname=}, {lastname=}')
firstname='Mark', lastname='Watney'

2.1.11. Use Case - 0x01

>>> a, b, c = range(0, 3)
>>> a, b, c, d, e = range(0, 5)

2.1.12. Use Case - 0x02

  • Skip

>>> a, b, _ = 1, 2, 3
>>> a, _, _ = 1, 2, 3
>>> a, _, c = 1, 2, 3
>>> _, b, _ = 1, 2, 3
>>> _, _, c = 1, 2, 3

2.1.13. Use Case - 0x03

  • Passwd

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

2.1.14. 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))

2.1.15. Recap

  • Four types of assignments: Scalar, Vector, Multi

  • 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 _

Scalar assignment:

>>> a = 1

Vector assignment:

>>> a, b = 1, 2

Multi assignment:

>>> a = b = 1

Unpacking nested:

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

Skipping:

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

2.1.16. 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 = ...