Alright, If you’re like me (and by “me,” we mean “a nerd who spends too much time in front of their computer”), then you might have heard about Python’s `__match_args__` attribute. But what is it? And why should you care?
Well, let’s start with the basics.In Python, a pattern matching statement looks like this:
# This script is using pattern matching in Python to print different statements based on the value of x.
# The "match" statement is used to compare the value of x to different cases.
# The "_" case is used as a catch-all for any value of x that does not match the previous cases.
match x:
case 1: # This case checks if x is equal to 1.
print("One!")
case "hello": # This case checks if x is equal to the string "hello".
print("Hello there!")
case _: # This case will be executed if x does not match any of the previous cases.
print("Something else...")
This code checks the value of `x`, and if it’s equal to `1`, prints out “One!” If it’s equal to “hello,” it prints out “Hello there!” And if it doesn’t match either of those cases, it falls back to printing “Something else…”. Pretty straightforward, right?
But what if you have a class with multiple arguments that you want to pattern match against? That’s where `__match_args__` comes in. Let me explain.
Imagine we have this simple class:
# This is a simple class called Point with two arguments, x and y.
class Point:
# This is the constructor method that initializes the class with the given arguments.
def __init__(self, x, y):
# This line assigns the value of x to the class attribute self.x.
self.x = x
# This line assigns the value of y to the class attribute self.y.
self.y = y
Now let’s say you want to pattern match against a `Point`. You could do something like this:
# Here is the context before the script:
# Now let's say you want to pattern match against a `Point`. You could do something like this:
# Here is the script:
# The original script has incorrect syntax and does not provide any annotations or explanations.
# Import the necessary module for pattern matching
from dataclasses import dataclass
# Define the Point class with x and y coordinates
@dataclass
class Point:
x: int
y: int
# Create an instance of Point with coordinates (1, 2)
point = Point(1, 2)
# Use the match statement to pattern match against the Point instance
# The match statement checks if the given expression matches any of the specified cases
# In this case, it checks if the Point instance has coordinates (1, 2)
match point:
# If the Point instance has coordinates (1, 2), execute the following code
case Point(1, 2):
# Print a message indicating that the magic point has been found
print("Found the magic point!")
# If the Point instance does not have coordinates (1, 2), execute the following code
case _:
# Print a generic message indicating that something else has been found
print("Something else...")
# Output:
# Found the magic point!
But what if you don’t want to specify all of the arguments in your pattern match? That’s where `__match_args__` comes in. You can define a list or tuple of allowed positional arguments for your class, and then use those arguments in your pattern matches:
# The `Point` class represents a point in a 2D coordinate system
class Point:
# The `__init__` method is used to initialize the `Point` object with the given `x` and `y` coordinates
def __init__(self, x, y):
# The `self` parameter refers to the current instance of the `Point` class
self.x = x # Assign the `x` coordinate to the `x` attribute of the `Point` object
self.y = y # Assign the `y` coordinate to the `y` attribute of the `Point` object
# The `__match_args__` attribute is used to specify the allowed positional arguments for the `Point` class
__match_args__ = ("x", "y") # A tuple of allowed positional arguments, in this case, `x` and `y` coordinates
Now you can do this:
# Define the "Point" class
class Point:
def __init__(self, x):
self.x = x
# Create an instance of the "Point" class with a value of 1
point = Point(1)
# Use the "match" statement to check for a specific value in the "point" variable
match point:
# Use the "case" statement to define the pattern to be matched against the value
case Point(1):
# If the value matches the pattern, execute the following code block
print("Found the magic x-coordinate!")
# Use the "_" wildcard pattern to match any value
case _:
# If no match is found, execute the following code block
print("Something else...")
Pretty cool, right? But wait there’s more! If you have a class with keyword arguments (like `Point(x=1, y=2)`), then you can use those in your pattern matches as well. Just add them to the `__match_args__` list:
# The following script creates a class called Point with two attributes, x and y.
# The __init__ function is used to initialize the attributes when an instance of the class is created.
# The __match_args__ list is used to specify which attributes can be used in pattern matching.
class Point:
def __init__(self, x, y): # The __init__ function is used to initialize the attributes when an instance of the class is created.
self.x = x # The x attribute is set to the value passed in as an argument.
self.y = y # The y attribute is set to the value passed in as an argument.
__match_args__ = ("x", "y") # The __match_args__ list is used to specify which attributes can be used in pattern matching.
Now you can do this:
# This script is using the "match" statement to check for specific cases and execute corresponding code.
# The "match" statement is similar to "switch" statement in other programming languages.
# The "point" variable is being checked for different cases.
# The "case" statement is used to specify the different cases to be checked.
# The "Point(x=1)" case is checking if the x-coordinate of the point is equal to 1.
# The "print" function is used to display a message if the condition is met.
# The "_" case is used as a default case, in case none of the specified cases are met.
# The "print" function is used to display a message if none of the specified cases are met.
# The "match" statement is used to check for specific cases and execute corresponding code.
match point:
# The "case" statement is used to specify the different cases to be checked.
case Point(x=1):
# The "print" function is used to display a message if the condition is met.
print("Found the magic x-coordinate!")
# The "_" case is used as a default case, in case none of the specified cases are met.
case _:
# The "print" function is used to display a message if none of the specified cases are met.
print("Something else...")
And that’s it, Python’s `__match_args__` attribute is a powerful tool for pattern matching against classes with multiple arguments (or keyword arguments). It makes your code more concise and easier to read especially if you have complex constructors or initialization logic in your classes.
So go ahead, give it a try! And remember when in doubt, always use `__match_args__`. Unless you’re not sure what `__match_args__` is… then maybe don’t do that. But hey, at least now you know about it!