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)

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+)-cat$')	Any string that matches that regular expression expr	"my-fuffy-cat"	"my" and "puffy"	"fuffy-dog"
Pet(name=_, age=7)	Any Pet dataclass with age == 7	Pet('rover', 7)	['rover']	Pet('rover', 8)

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 patter.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 to 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, _