The Pattern Matching for Python you always dreamed of

# 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.

Pattern Example What it means Matched Example Arguments Passed to function NOT Matched Example
"hello" only the string "hello" matches "hello" nothing any other value
None only None None nothing any other value
int Any integer 42 42 any other value
float Any float number 2.35 2.35 any other value
str Any string "hello" "hello" any other value
tuple Any tuple (1, 2) (1, 2) any other value
list Any list [1, 2] [1, 2] any other value
MyClass Any instance of MyClass. And any object that extends MyClass. MyClass() that instance any other object
_ Any object (even None) that value
ANY The same as _ that value
(int, int) A tuple made of any two integers (1, 2) 1 and 2 (True, False)
[1, 2, _] A list that starts with 1, 2 and ends with any value [1, 2, 3] 3 [1, 2, 3, 4]
[1, 2, TAIL] A list that start with 1, 2 and ends with any sequence [1, 2, 3, 4] [3, 4] [1, 7, 7, 7]
{'type':'dog', age: _ } Any dict with type: "dog" and with an age {"type":"dog", "age": 3} 3 {"type":"cat", "age":2}
{'type':'dog', age: int } Any dict with type: "dog" and with an int age {"type":"dog", "age": 3} 3 {"type":"dog", "age":2.3}
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, _


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