A class pattern is similar to the above but matches attributes instead of keys. It looks like datetime.date(year=y, day=d). It matches instances of the given type, having at least the specified attributes, as long as the attributes match with the corresponding sub-patterns. It binds whatever the sub-patterns bind when matching with the values of the given attributes. An optional protocol also allows matching positional arguments.

6.9.1. Problem

>>> class User:
...     pass
>>>
>>> class Staff:
...     pass
>>>
>>> class Admin:
...     pass
>>>
>>>
>>> alice = User()
>>>
>>> if isinstance(alice, User):
...     pass
... elif isinstance(alice, Staff):
...     pass
... elif isinstance(alice, Admin):
...     pass

6.9.2. Solution

>>> class User:
...     pass
>>>
>>> class Staff:
...     pass
>>>
>>> class Admin:
...     pass
>>>
>>>
>>> alice = User()
>>>
>>> match alice:
...     case User():  pass
...     case Staff():  pass
...     case Admin():  pass

6.9.3. Match Args

Match Args with two attributes:

>>> class User:
...     __match_args__ = ('firstname', 'lastname')
...
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> alice = User('Alice', 'Apricot')
>>>
>>> match alice:
...     case User('Alice', 'Apricot'):  print('alice')
...     case User('Bob', 'Blackthorn'):  print('bob')
...     case User('Carol', 'Corn'):  print('carol')
...
alice

Match Args with one attribute:

>>> class User:
...     __match_args__ = ('firstname',)
...
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> alice = User('Alice', 'Apricot')
>>>
>>> match alice:
...     case User('Alice'):  print('alice')
...     case User('Bob'):  print('bob')
...     case User('Carol'):  print('carol')
...
alice

6.9.4. Does Not Create New Instances

>>> class User:
...     __match_args__ = ('firstname', 'lastname')
...
...     def __init__(self, firstname, lastname):
...         print(f'Creating User: {firstname} {lastname}')
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> alice = User('Alice', 'Apricot')
Creating User: Alice Apricot
>>>
>>> match alice:
...     case User('Alice', 'Apricot'):  print('alice')
...     case User('Bob', 'Blackthorn'):  print('bob')
...     case User('Carol', 'Corn'):  print('carol')
...
alice

6.9.5. Match Other Fields

>>> class User:
...     __match_args__ = ('email',)
...
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
...         self.email = f'{firstname.lower()}@example.com'
>>>
>>>
>>> alice = User('Alice', 'Apricot')
>>>
>>> match alice:
...     case User('alice@example.com'):  print('alice')
...     case User('bob@example.com'):  print('bob')
...     case User('carol@example.com'):  print('carol')
alice

6.9.6. Case Study

SetUp:

Using if:

>>> if isinstance(data, int):
...     pass
... elif isinstance(data, float):
...     pass
... elif isinstance(data, str):
...     pass

Using match:

>>> match data:
...     case int():  pass
...     case float():  pass
...     case str():  pass

6.9.7. Use Case - 1

>>> x = True
>>>
>>> match x:
...     case bool():    print('Is a bool')
...     case int():     print('Is an int')
...     case float():   print('Is a float')
...
Is a bool

6.9.8. Use Case - 2

>>> x = True
>>>
>>> match x:
...     case int():     print('Is an int')
...     case bool():    print('Is a bool')
...     case float():   print('Is a float')
...
Is an int

Is equivalent to:

>>> if isinstance(x, int):
...     print('Is an int')
... elif isinstance(x, bool):
...     print('Is a bool')
... elif isinstance(x, float):
...     print('Is a float')
...
Is an int

Because:

>>> isinstance(True, bool)
True
>>>
>>> isinstance(True, int)
True

Every bool inherits from int:

>>> bool.mro()
[<class 'bool'>, <class 'int'>, <class 'object'>]

6.9.9. Use Case - 1

>>> from datetime import datetime
>>>
>>> obj = datetime(1969, 7, 21, 2, 56, 15)
>>> obj
datetime.datetime(1969, 7, 21, 2, 56, 15)
>>>
>>> match obj:
...     case datetime():  print('Instance of a datetime class')
...     case date():      print('Instance of a date class')
...     case time():      print('Instance of a time class')
...     case timedelta(): print('Instance of a timedelta class')
...
Instance of a datetime class

6.9.10. Use Case - 2

>>> import json
>>> from datetime import date, time, datetime, timezone
>>>
>>>
>>> DATA = {'firstname': 'Alice',
...         'lastname': 'Apricot',
...         'birthdate': date(2000, 1, 1)}
>>>
>>>
>>> def encoder(value):
...     match value:
...         case datetime() | date() | time():
...             return value.isoformat()
...         case timedelta():
...             return value.total_seconds()
>>>
>>>
>>> json.dumps(DATA, default=encoder)
'{"firstname": "Alice", "lastname": "Apricot", "birthdate": "2000-01-01"}'

6.9.11. Use Case - 3

>>> import json
>>> from datetime import date, time, datetime, timezone
>>> from pprint import pprint
>>>
>>>
>>> DATA = {'mission': 'Ares 3',
...         'launch_date': datetime(2035, 6, 29),
...         'destination': 'Mars',
...         'destination_landing': datetime(2035, 11, 7),
...         'destination_location': 'Acidalia Planitia',
...         'crew': [{'name': 'Melissa Lewis', 'birthdate': date(1995, 7, 15)},
...                  {'name': 'Rick Martinez', 'birthdate': date(1996, 1, 21)},
...                  {'name': 'Alex Vogel', 'birthdate': date(1994, 11, 15)},
...                  {'name': 'Chris Beck', 'birthdate': date(1999, 8, 2)},
...                  {'name': 'Beth Johanssen', 'birthdate': date(2006, 5, 9)},
...                  {'name': 'Mark Watney', 'birthdate': date(1994, 10, 12)}]}

>>> json.dumps(DATA)
Traceback (most recent call last):
TypeError: Object of type datetime is not JSON serializable
when serializing dict item 'launch_date'

>>> def encoder(obj):
...     match obj:
...         case datetime() | date() | time():
...             return obj.isoformat()
...         case timedelta():
...             return obj.total_seconds()
>>>
>>> result = json.dumps(DATA, default=encoder, indent=2)
>>> print(result)
{
  "mission": "Ares 3",
  "launch_date": "2035-06-29T00:00:00",
  "destination": "Mars",
  "destination_landing": "2035-11-07T00:00:00",
  "destination_location": "Acidalia Planitia",
  "crew": [
    {
      "name": "Melissa Lewis",
      "birthdate": "1995-07-15"
    },
    {
      "name": "Rick Martinez",
      "birthdate": "1996-01-21"
    },
    {
      "name": "Alex Vogel",
      "birthdate": "1994-11-15"
    },
    {
      "name": "Chris Beck",
      "birthdate": "1999-08-02"
    },
    {
      "name": "Beth Johanssen",
      "birthdate": "2006-05-09"
    },
    {
      "name": "Mark Watney",
      "birthdate": "1994-10-12"
    }
  ]
}

6.9.12. Assignments