4.12. Match Walrus

A walrus pattern looks like d := datetime(year=2020, month=m). It matches only if its sub-pattern also matches. It binds whatever the sub-pattern match does, and also binds the named variable to the entire object.

>>> match get_shape():  
...     case Line(start := Point(x, y), end) if start == end:
...         print(f'Zero length line at {x}, {y}')

>>> match group_shapes():  
...     case [], [point := Point(x, y), *other]:
...         print(f'Got {point} in the second group')
...         process_coordinates(x, y)
...         ...

4.12.1. SetUp

>>> import re

4.12.2. Assignment Expression

  • A.K.A. Walrus Operator

  • re.match() returns re.Match | None

>>> email = 'mwatney@nasa.gov'
>>> result = re.match('^[a-z]+@nasa.gov$', email)
>>>
>>> print(result.group())
mwatney@nasa.gov

>>> email = 'mwatney@esa.int'  # invalid domain
>>> result = re.match('^[a-z]+@nasa.gov$', email)
>>>
>>> print(result.group())
Traceback (most recent call last):
AttributeError: 'NoneType' object has no attribute 'group'

>>> email = 'mwatney@esa.int'  # invalid domain
>>> result = re.match('^[a-z]+@nasa.gov$', email)
>>>
>>> if result:
...     print(result.group())
... else:
...     print('Invalid domain')
Invalid domain

>>> email = 'mwatney@esa.int'  # invalid domain
>>>
>>> if result := re.match('^[a-z]+@nasa.gov$', email):
...     print(result.group())
... else:
...     print('Invalid domain')
Invalid domain