Structural Pattern Matching in Python

Structural Pattern Matching
Haim Michael
September 20th
, 2022
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© 2021 Haim Michael All Rights Reserved 2
Introduction
 As of Python 3.10 we can use patterns matching. There are
three PEPs (Python Enhancement Proposals) describing this
feature:
PEP 634: The Specification
https://www.python.org/dev/peps/pep-0634
PEP 635: Motivation and Rationale
PEP 636: Patterns Matching Tutorial

Switch Statement on Steroids
 We can consider patterns matching as a switch statement on
steroids.

Matching Specific Values
 The simplest form of patterns matching in Python would be a
simple use case similar to switch statement.
command = "+"
match command:
case '+':
print('plus...')
case '-':
print('minus')
case '*':
print('multiply')
case '/':
print('divide')

The _ Wildcard
 We can use the _ character as a wild card. We shall place the
wild card as the last case possibility. If none of the other
cases matches, the wild card will.
command = "%"
match command:
case '+':
print('plus...')
case '-':
print('minus')
case '/':
print('divide')
case _:
print('soething else')

Matching Sequences
 We can match a sequence with possible sequences
composed of literals or variables.
operation = ("deposit",1200,123222)
operation = ("transfer",800,123222,101888)
match operation:
case operation,sum,account_id:
print("%s %f %d" % (operation,sum,account_id))
case operation, sum, from_id, to_id:
print("%s %f from %d to %d" % (operation, sum, from_id, to_id))

Matching Sequences
operation = ["playing","pacman"]
match operation:
case ["eat",food]:
print("eating %s is great" % (food,))
case ["watching",show]:
print("enjoy watching %s" % (show,))
case ["playing",game]:
print("playing %s is awesome" % (game,))
case _:
print("your operation cannot be recognized")

Matching Multiple Values
 We can use the packing capability similarly to the way we use
it in assignments.

Matching Multiple Values
operation = ("delete","Hello.py","Calcu.py","Demo.py")
match operation:
case "delete",*files:
print("the following files will be deleted:")
for file in files:
print(file)
case "makedir",*directories:
print("the following directories will be created:")
for directory in directories:
print(directory)
case _:
print("the requesed operation is not recognized")

Composing Patters
 We can compose new patterns composed from others.
Patterns that don't include other patterns can be any of the
following: capture patterns, literal patterns, and the wildcard
pattern _.

Composing Patters
operation = ("update_mark",("mosh",234234),103444,88)
match operation:
case "register_course",student,course_id:
print("registering to course")
print(student)
print(course_id)
case "update_mark",(student_name,student_id),course_id,mark:
print("updating mark in specific course")
print("student name: %s", student_name)
print("student id: %d" % (student_id,))
print("course id: %d" % (course_id,))
print("mark: %.2f" % (mark,))

Or Patterns
 We can use the | operator (AKA the or operator) in order to
create a case that includes more than one pattern. Matching
any of the patterns will be considered as matching the entire
case.

Or Patterns
command = ("del","temp.txt")
match command:
case ("delete",file) | ("remove",file) | ("del",file):
print("deleting file %s" % (file,))
case ("copy",file,newfile):
print("copying file %s to %s" % (file,newfile))

The as Pattern
 We can use the or operator for creating a sub pattern
composed of multiple possibilities. We can use the as pattern
for finding which is the exact possibility that match.

The as Pattern
data = ("del","temp.txt")
match data:
case (("delete" | "remove" | "del") as command, file):
print("command=%s" % (command,))

Adding Conditions
 We can add a condition to specific cases. The condition
includes the use of the if keyword followed by expression that
its value is of the type bool.

Adding Conditions
data = ("del","temp.txt")
files = ["readme.txt", "temp.txt", "index.txt"]
match data:
case (("delete" | "remove" | "del") as command, file) if file in files:
print("command=%s" % (command,))
case (("delete" | "remove" | "del") as command, file) if file not in files:
print("the file %s cannot be deleted" % file)

Matching Objects Types
 When having an object we need to examine its type and its
attributes we can avoid the use of isinstance() and we
can avoid checking its attributes.

class Dog:
def hau(self):
return "hau hau"
class Cat:
def __init__(self,predator):
self.predator = predator
def miaou(self):
return "miaou miaou"
class Cow:
def moo(self):
return "moo moo"
ob = Cat(True)

match ob:
case Dog():
print(ob.hau())
case Cat(predator=False):
print(ob.miaou())
case Cat(predator=True):
print("%s grrrrr" % ob.miaou())
case Cow():
print(ob.moo())

Matching Attributes By Position
 When using objects that were instantiated from a class
marked as dataclass we can describe the matched
attributes by position.

Matching Attributes By Position
from dataclasses import dataclass
from math import pow, sqrt
@dataclass
class Point:
x: int
y: int
@dataclass
class Line:
p1: Point
p2: Point
ob = Line(Point(3, 4), Point(6, 8))
match ob:
case Line(p1, p2):
print(sqrt(pow(p2.x - p1.x, 2) + pow(p2.y - p1.y, 2)))
case Point(x, y):
print(sqrt(pow(x, 2) + pow(y, 2)))

The __match_args__ Attribute
 When using objects that were not instantiated from a class
marked as dataclass and were not instantiated from the
tuple type we can still describe the matched attributes by
position if we add the __match_args__ attribute to the
class definition.
 The __match_args__ special attribute defines an explicit
order for the attributes.

from math import pow, sqrt
class Point:
__match_args__ = ("x", "y")
def __init__(self, x, y):
self.x = x
self.y = y
class Line:
__match_args__ = ("p1", "p2")
def __init__(self, p1, p2):
self.p1 = p1
self.p2 = p2
ob = Line(Point(3, 4), Point(6, 8))

match ob:
case Line(p1, p2):
print(sqrt(pow(p2.x - p1.x, 2) + pow(p2.y - p1.y, 2)))
case Point(x, y):
print(sqrt(pow(x, 2) + pow(y, 2)))

Enums Matching
 We can match agains possible values that were defined as
part of an Enum object.
from enum import Enum
class Color(Enum):
BLUE = 1
GREEN = 2
RED = 3
BLACK = 4
WHITE = 5
class Car:
__match_args__ = ("brand", "id", "color")
def __init__(self, brand, id, color):
self.brand = brand
self.id = id
self.color = color

Enums Matching
ob = Car("Mazda 6", 2342343, Color.GREEN)
match ob:
case Car(car_brand, car_id, Color.WHITE):
print("white cars are simpler to handle")
case Car(car_brand, car_id, Color.BLACK):
print("black cars aborb the heat")
case Car(car_brand, car_id, _):
print("colorful cars are more fun")

Mappings Matching
 We can match an expression against a dict object. The
matching will be based on the keys.

Mappings Matching
 We can match an expression against a dict object. The
matching will be based on the keys.
command = {
'action': 'getbalance',
'account': {
'accountid': 204714,
'owners_ids': (24537564, 51634545)
}}

Mappings Matching
match command:
case {'action': 'getbalance', 'account': account}:
print("getting balance")
print("account id: %s" % (account['accountid'],))
print("account owners are:")
print(account['owners_ids'])
case {'action': 'deposit', 'sum': sum, 'account': account}:
print("deposit %f" % sum)
print("account id: %s" % (account,))

Mappings Matching
 We can use the dict packing capability when matching a dict
object with additional key value pairs that don't match any of
the pattern's parts.
command = {
'account': {
'owners_ids': (24537564, 51634545)
},
'date': {'day':12,'month':1,'year':1980},
'time': {'hour': 14, 'minutes': 20}
}

Mappings Matching
match command:
case {'action': 'getbalance', 'account': account, **ob}:
print("getting balance")
print(ob)
case {'action': 'deposit', 'sum': sum, 'account': account, **ob}:
print("deposit %f" % sum)
print("account id: %s" % (account,))
print(ob)

Matching Builtin Types
 We can use builtin types for validating the types of specific
parts in the expression we check.
data = {
'sum':800,
'account': {
'owners_ids': (24537564, 51634545)
}
}

Matching Builtin Types
match data:
case {'action': str() as action, 'account': account}:
print(action)
case _:
print(data)

Questions & Answers
Thanks for Your Time!
Haim Michael
haim.michael@lifemichael.com
+972+3+3726013 ext:700
life
michael

Structural Pattern Matching in Python

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