Pattern Matching

Rules

1. If the value is the constant, paragram.Any, then any message will match.
2. If the value is a type object or a class, then any message that isinstance() of the type or class will match.
3. If the value is callable(), then a message will match if the function/method returns True when called with the message.
4. If the value is a tuple, then a message will match only if it is a tuple of the same length (but see next point). Also, each value in the tuple is used as a pattern and the tuple as a whole will match only if every sub-pattern in the tuple matches its associated value in the message.
5. If the last item in a tuple is the constant paragram.etc, then it will match any remaining elements in the tuple, similar to an *args specification in a paramater list.
6. If the value is a list, then the same rule for tuples applies.
7. If the value is a dictionary, then a message will match only if it is a dictionary that contains all of the same keys as the pattern value. Also, each value in the dictionary is used as a pattern and the dictionary as a whole will match only if every pattern in the dictionary matches its associated value in the message.
8. Any other value is treated as a literal pattern, and matches any object that is equal to it.

Note that every message in paragram is a tuple, so in order to match any number of any objects, you should add a receiver for paragram.etc.

Examples

Let’s illustrate these rules by example.

In the table below, the first column contains a Python value that is used as a pattern. The second column contains Python values that match the pattern and the third column contains Python values that do not match the pattern.

Pattern	Matches	Non-Matches
Any	'text', 13.7, (1, '', lambda: true)
'land shark'	'land shark'	'dolphin', 42, []
13.7	13.7	'text', 13.6, {'A': 14}
int	13, 42, 0	'text', 13.7, []
str	'plumber', ''	42, 0.9, lambda: True
lambda x,y,z: x+y+z == 3	(1, 1, 1), (3, 1, -1)	(3,), (1, 2, 3)
(str, int)	('shark', 42), ('dolphin', 0)	['shark', 42], ('dolphin', 42, 0)
(str, int, etc)	('shark', 42), ('dolphin', 0, 1, 2, True)	['shark', 42], ('dolphin', 42, 0)
(str, 20, lambda x: x < 0)	('shark', 20, -54.76), ('dolphin', 20, -1)	('shark', 21, -6), (20, 20, -1), ('', 20)
['A', str, str]	['a', 'b', 'c', 'd'], ['a', 'b']	['C', 'B', 'A'], ['A']
[str, int]	['dolphin', 42, 0], ['shark']	[42, 0], ['dolphin', 42, 'shark']
[Any]	['dolphin', 42, 0.9], []	('dolphin', 42, 0.9), 'shark'
{'S': int, 19: str}	{'S': 3, 19: 'foo'}, {'S': -65, 19: 'bar', 'T': 'me'}	{'S': 'Charlie', 19: 'foo'}, {'S': 3}