Structural Pattern Matching in Python

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October 16, 2024
Structural Pattern Matching in Python
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Structural pattern matching is a powerful control flow construct invented decades ago that’s traditionally used by compiled languages, especially within the functional programming paradigm.

Most mainstream programming languages have since adopted some form of pattern matching, which offers concise and readable syntax while promoting a declarative code style. Although Python was late to join the party, it introduced structural pattern matching in the 3.10 release.

In this tutorial, you’ll:

  • Master the syntax of the match statement and case clauses
  • Explore various types of patterns supported by Python
  • Learn about guards, unions, aliases, and name binding
  • Extract values from deeply nested hierarchical data structures
  • Customize pattern matching for user-defined classes
  • Identify and avoid common pitfalls in Python’s pattern matching

To get the most out of this tutorial, you should have a basic understanding of conditional statements, loops, functions, and classes in Python. Additionally, familiarity with Python’s built-in data structures, such as tuples, lists, and dictionaries, will be beneficial.

Take the Quiz: Test your knowledge with our interactive “Structural Pattern Matching” quiz. You’ll receive a score upon completion to help you track your learning progress:

Structural Pattern Matching in Python

Interactive Quiz

Structural Pattern Matching

In this quiz, you'll test your understanding of structural pattern matching in Python. This powerful control flow construct, introduced in Python 3.10, offers concise and readable syntax while promoting a declarative code style.

Getting to Know Structural Pattern Matching

Before taking advantage of structural pattern matching in your code, make sure that you’re running Python 3.10 or later, as you won’t be able to use it in earlier Python versions. Note that although the name structural pattern matching is often shortened to just pattern matching, the qualifier structural is crucial to understanding the use cases for this feature. In this section, you’ll get a high-level overview of structural pattern matching.

What Is Pattern Matching?

You can think of pattern matching as a form of syntactic sugar built on top of existing language constructs, including conditional statements and tuple unpacking. While you can absolutely live without pattern matching, it gives you new superpowers, making this feature more convenient than the conventional syntax in some situations.

Pattern matching often leads to more elegant, concise, and readable code written in a declarative style. To get a taste of it, take a quick look at the following example without trying to fully understand how it works just yet:

Python 
import json

def log(event):
    match json.loads(event):
        case {"keyboard": {"key": {"code": code}}}:
            print(f"Key pressed: {code}")
        case {"mouse": {"cursor": {"screen": [x, y]}}}:
            print(f"Mouse cursor: {x=}, {y=}")
        case _:
            print("Unknown event type")

The match statement takes a subject, which can be any valid Python expression, such as a string literal or a function call, and compares the resulting value to one or more patterns listed in the case clauses. The first pattern that matches the given subject will trigger the corresponding case block to run. You’ll learn more about the match statement and case clauses later in this tutorial.

At first glance, the syntax of structural pattern matching in Python looks a bit like the switch statement found in the C-family programming languages if you squint your eyes:

C 
void log_event(enum Event event) {
    switch (event) {
        case KEYBOARD:
            printf("Keyboard event\n");
            break;
        case MOUSE:
            printf("Mouse event\n");
            break;
        default:
            printf("Unknown event\n");
    }
}

This resemblance is deceptive, though. The classic switch statement controls the execution flow based on the exact value stored in a variable. It effectively works as a chained sequence of mutually exclusive if..elif... equality comparisons, but with a more succinct and readable syntax.

Although you can use pattern matching this way, you’d be missing out on its true power and flexibility. Structural pattern matching was designed to go beyond value comparisons. In particular, it combines conditional statements or branching based on a logical predicate with destructuring or object deconstruction, which is the inverse of object construction. You’ll see examples of destructuring in the next section.

The brief code snippet above merely scratches the surface of what you can achieve with pattern matching, but it already shows you its expressiveness, especially when you compare it with the traditional if...elif... statements and isinstance() checks. Here’s one of the many ways you can implement the equivalent logic using standard Python:

Python 
import json

def log(event):
    parsed_event = json.loads(event)
    if (
        "keyboard" in parsed_event and
        "key" in parsed_event["keyboard"] and
        "code" in parsed_event["keyboard"]["key"]
    ):
        code = parsed_event["keyboard"]["key"]["code"]
        print(f"Key pressed: {code}")
    elif (
        "mouse" in parsed_event and
        "cursor" in parsed_event["mouse"] and
        "screen" in parsed_event["mouse"]["cursor"]
    ):
        screen = parsed_event["mouse"]["cursor"]["screen"]
        if isinstance(screen, list) and len(screen) == 2:
            x, y = screen
            print(f"Mouse cursor: x={x}, y={y}")
        else:
            print("Unknown event type")
    else:
        print("Unknown event type")

This code is functionally identical to the previous version but is longer and has more indentation levels than before. Additionally, it looks more verbose and imperative in style, describing not only what to do but also how to perform the individual steps. Granted, you could try making it slightly shorter by using the Walrus operator and following the EAFP principle without explicit checks, but it’d remain somewhat convoluted.

It’s worth noting that structural pattern matching first emerged in compiled functional languages with static typing. The attempt to implement it in Python, which is a dynamic language, presented completely new and unique challenges. You can read more about them in the paper entitled Dynamic Pattern Matching with Python, which was co-authored by Guido van Rossum and published in the proceedings of the Dynamic Languages Symposium in 2020.

Now that you’ve seen the most basic form of pattern matching in Python, it’s time to unravel the meaning of a structural pattern.

What Is a Structural Pattern?

Read the full article at https://realpython.com/structural-pattern-matching/ »

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October 16, 2024 at 07:30PM
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