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Pampy: Pattern Matching for Python

Pampy is pretty small (150 lines), reasonably fast, and often makes your code more readable and hence easier to reason about. There is also a JavaScript version, called Pampy.js.

You can write many patterns

Patterns are evaluated in the order they appear.

You can write Fibonacci

The operator _ means "any other case I didn't think of".

from pampy import match, _

def fibonacci(n):
    return match(n,
        1, 1,
        2, 1,
        _, lambda x: fibonacci(x-1) + fibonacci(x-2)
    )

You can write a Lisp calculator in 5 lines

from pampy import match, REST, _

def lisp(exp):
    return match(exp,
        int,                lambda x: x,
        callable,           lambda x: x,
        (callable, REST),   lambda f, rest: f(*map(lisp, rest)),
        tuple,              lambda t: list(map(lisp, t)),
    )

plus = lambda a, b: a + b
minus = lambda a, b: a - b
from functools import reduce

lisp((plus, 1, 2))                 	# => 3
lisp((plus, 1, (minus, 4, 2)))     	# => 3
lisp((reduce, plus, (range, 10)))       # => 45

You can match so many things!

match(x,
    3,              "this matches the number 3",

    int,            "matches any integer",

    (str, int),     lambda a, b: "a tuple (a, b) you can use in a function",

    [1, 2, _],      "any list of 3 elements that begins with [1, 2]",

    {'x': _},       "any dict with a key 'x' and any value associated",

    _,              "anything else"
)

You can match [HEAD, TAIL]

from pampy import match, HEAD, TAIL, _

x = [1, 2, 3]

match(x, [1, TAIL],     lambda t: t)            # => [2, 3]

match(x, [HEAD, TAIL],  lambda h, t: (h, t))    # => (1, [2, 3])

TAIL and REST actually mean the same thing.

You can nest lists and tuples

from pampy import match, _

x = [1, [2, 3], 4]

match(x, [1, [_, 3], _], lambda a, b: [1, [a, 3], b])           # => [1, [2, 3], 4]

You can nest dicts. And you can use _ as key!

pet = { 'type': 'dog', 'details': { 'age': 3 } }

match(pet, { 'details': { 'age': _ } }, lambda age: age)        # => 3

match(pet, { _ : { 'age': _ } },        lambda a, b: (a, b))    # => ('details', 3)

It feels like putting multiple _ inside dicts shouldn't work. Isn't ordering in dicts not guaranteed ? But it does because in Python 3.7, dict maintains insertion key order by default

You can match class hierarchies

class Pet:          pass
class Dog(Pet):     pass
class Cat(Pet):     pass
class Hamster(Pet): pass

def what_is(x):
    return match(x,
        Dog, 		'dog',
        Cat, 		'cat',
        Pet, 		'any other pet',
          _, 		'this is not a pet at all',
    )

what_is(Cat())      # => 'cat'
what_is(Dog())      # => 'dog'
what_is(Hamster())  # => 'any other pet'
what_is(Pet())      # => 'any other pet'
what_is(42)         # => 'this is not a pet at all'

Using Dataclasses

Pampy supports Python 3.7 dataclasses. You can pass the operator _ as arguments and it will match those fields.

@dataclass
class Pet:
    name: str
    age: int

pet = Pet('rover', 7)

match(pet, Pet('rover', _), lambda age: age)                    # => 7
match(pet, Pet(_, 7), lambda name: name)                        # => 'rover'
match(pet, Pet(_, _), lambda name, age: (name, age))            # => ('rover', 7)

Using typing

Pampy supports typing annotations.

class Pet:          pass
class Dog(Pet):     pass
class Cat(Pet):     pass
class Hamster(Pet): pass

timestamp = NewType("year", Union[int, float])

def annotated(a: Tuple[int, float], b: str, c: E) -> timestamp:
    pass

match((1, 2), Tuple[int, int], lambda a, b: (a, b))             # => (1, 2)
match(1, Union[str, int], lambda x: x)                          # => 1
match('a', Union[str, int], lambda x: x)                        # => 'a'
match('a', Optional[str], lambda x: x)                          # => 'a'
match(None, Optional[str], lambda x: x)                         # => None
match(Pet, Type[Pet], lambda x: x)                              # => Pet
match(Cat, Type[Pet], lambda x: x)                              # => Cat
match(Dog, Any, lambda x: x)                                    # => Dog
match(Dog, Type[Any], lambda x: x)                              # => Dog
match(15, timestamp, lambda x: x)                               # => 15
match(10.0, timestamp, lambda x: x)                             # => 10.0
match([1, 2, 3], List[int], lambda x: x)                        # => [1, 2, 3]
match({'a': 1, 'b': 2}, Dict[str, int], lambda x: x)            # => {'a': 1, 'b': 2}
match(annotated, 
    Callable[[Tuple[int, float], str, Pet], timestamp], lambda x: x
)                                                               # => annotated

For iterable generics actual type of value is guessed based on the first element.

match([1, 2, 3], List[int], lambda x: x)                        # => [1, 2, 3]
match([1, "b", "a"], List[int], lambda x: x)                    # => [1, "b", "a"]
match(["a", "b", "c"], List[int], lambda x: x)                  # raises MatchError
match(["a", "b", "c"], List[Union[str, int]], lambda x: x)      # ["a", "b", "c"]

match({"a": 1, "b": 2}, Dict[str, int], lambda x: x)            # {"a": 1, "b": 2}
match({"a": 1, "b": "dog"}, Dict[str, int], lambda x: x)        # {"a": 1, "b": "dog"}
match({"a": 1, 1: 2}, Dict[str, int], lambda x: x)              # {"a": 1, 1: 2}
match({2: 1, 1: 2}, Dict[str, int], lambda x: x)                # raises MatchError
match({2: 1, 1: 2}, Dict[Union[str, int], int], lambda x: x)    # {2: 1, 1: 2}

Iterable generics also match with any of their subtypes.

match([1, 2, 3], Iterable[int], lambda x: x)                     # => [1, 2, 3]
match({1, 2, 3}, Iterable[int], lambda x: x)                     # => {1, 2, 3}
match(range(10), Iterable[int], lambda x: x)                     # => range(10)

match([1, 2, 3], List[int], lambda x: x)                         # => [1, 2, 3]
match({1, 2, 3}, List[int], lambda x: x)                         # => raises MatchError
match(range(10), List[int], lambda x: x)                         # => raises MatchError

match([1, 2, 3], Set[int], lambda x: x)                          # => raises MatchError
match({1, 2, 3}, Set[int], lambda x: x)                          # => {1, 2, 3}
match(range(10), Set[int], lambda x: x)                          # => raises MatchError

For Callable any arg without annotation treated as Any.

def annotated(a: int, b: int) -> float:
    pass
    
def not_annotated(a, b):
    pass
    
def partially_annotated(a, b: float):
    pass

match(annotated, Callable[[int, int], float], lambda x: x)     # => annotated
match(not_annotated, Callable[[int, int], float], lambda x: x) # => raises MatchError
match(not_annotated, Callable[[Any, Any], Any], lambda x: x)   # => not_annotated
match(annotated, Callable[[Any, Any], Any], lambda x: x)       # => raises MatchError
match(partially_annotated, 
    Callable[[Any, float], Any], lambda x: x
)                                                              # => partially_annotated

TypeVar is not supported.

All the things you can match

As Pattern you can use any Python type, any class, or any Python value.

The operator _ and built-in types like int or str, extract variables that are passed to functions.

Types and Classes are matched via instanceof(value, pattern).

Iterable Patterns match recursively through all their elements. The same goes for dictionaries.

Using default

By default match() is strict. If no pattern matches, it raises a MatchError.

You can instead provide a fallback value using default to be used when nothing matches.

>>> match([1, 2], [1, 2, 3], "whatever")
MatchError: '_' not provided. This case is not handled: [1, 2]

>>> match([1, 2], [1, 2, 3], "whatever", default=False)
False

Using Regular Expressions

Pampy supports Python's Regex. You can pass a compiled regex as pattern, and Pampy is going to run pattern.search(), and then pass to the action function the result of .groups().

def what_is(pet):
    return match(pet,
        re.compile('(\w+)-(\w+)-cat$'),     lambda name, my: 'cat '+name,
        re.compile('(\w+)-(\w+)-dog$'),     lambda name, my: 'dog '+name,
        _,                                  "something else"
    )

what_is('fuffy-my-dog')     # => 'dog fuffy'
what_is('puffy-her-dog')    # => 'dog puffy'
what_is('carla-your-cat')   # => 'cat carla'
what_is('roger-my-hamster') # => 'something else'

Install for Python3

Pampy works in Python >= 3.6 Because dict matching can work only in the latest Pythons.

To install it:

$ pip install pampy

or $ pip3 install pampy

If you really must use Python2

Pampy is Python3-first, but you can use most of its features in Python2 via this backport by Manuel Barkhau:

pip install backports.pampy

from backports.pampy import match, HEAD, TAIL, _