Python Class Patterns and Attribute Matching

Use class patterns to match an object type and selected attributes.

What You Will Learn

Class patterns like Point(x=0, y=value) test whether the subject is an instance of Point and whether named attributes match. Positional class patterns depend on __match_args__, which dataclasses generate for ordinary init fields; keyword patterns are often clearer. Matching an attribute does not guarantee any application-level validation.

Step-by-Step Example

The example below makes the behavior visible with small input values that you can change safely.

from dataclasses import dataclass

@dataclass
class Point:
    x: int
    y: int

def describe(point):
    match point:
        case Point(x=0, y=value):
            return f"On the vertical axis at {value}"
        case Point(x=x, y=y):
            return f"Point at {x}, {y}"

print(describe(Point(0, 7)))
print(describe(Point(2, 3)))

How the Example Works

The imports provide the standard-library tools used in the demonstration: dataclasses.
The definitions Point, describe separate the main steps into named components; calls to those components drive the example.
The print() calls make the return values or state visible so you can compare them with the explanation and check your prediction.

Expected output from the example:

On the vertical axis at 7
Point at 2, 3

When You Would Use This

Pattern matching can simplify dispatching application commands or decoding structured messages from a trusted parser. It chooses a branch based on shape and selected values, but application-level validation is still necessary. Compare it with an if/elif chain if the problem only involves simple Boolean expressions.

Practice: Modify the Code

Add a case for points on the horizontal axis and compare pattern order.

Verify the changed output against the behavior described above, including the relevant invalid or boundary case.

For complete parameters and version details, consult the official Python documentation.

Common Beginner Mistakes

Writing case Point() as if it automatically tests all field values.
Assuming positional class patterns are supported without considering __match_args__.

Key Takeaways

Use class patterns to match an object type and selected attributes.
Class patterns like Point(x=0, y=value) test whether the subject is an instance of Point and whether named attributes match.
Apply this operation only after checking the assumptions and input conditions shown in the example.