Structural Pattern Matching in Python II

In this blog, which is a second of a three-part series, we continue our discussion and introduce value pattern, sequence pattern, and mapping pattern.

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Structural Pattern Matching in Python II

In the previous blog, we discussed the structured pattern matching feature in Python 3.10 where we explain literal patterns, capture patterns, wildcard patterns, AS patterns, OR patterns, and guard patterns.

Value Pattern matching

Any variable attribute of an object can be used as a value pattern.

Let us understand with an example

def value_patterns(subject):

    class Fuel():
        DIESEL = 'diesel'

    fuel = 'petrol'    

    match subject:
        case Fuel.DIESEL:
            print("What are diesel prices these days?")  
        case fuel:
            print(f"{fuel} won't do, my car runs on diesel.")

Here is the output in different cases of subject

    subject = "water" # Output: water won't do, my car runs on diesel.
    subject = "diesel" # Output: What are diesel prices these days?

For a more standardized way, we can use named tuples, data classes, and enums.

If a pre-defined variable is used, it will be considered as an irrefutable pattern and used as a capture pattern as shown in the above example

Sequence Pattern matching

Any object that inherits from collections. abc. The sequence is eligible to be matched as a sequence pattern except for str, bytes, byte array, and iterators

def sequence_patterns(subject):

    match subject:
        case 'a', *tail:
            print(f"The trailing values after a are {tail}")
        case ('H', *mid, 'd'):
            print(f"The subject starts with H and ends with d")

Here are different types of outputs

    subject = ['a',(2, 3)] # Output: The trailing values after a are [(2, 3)]
    subject = list("HelloWorld") # Output: The subject starts with H and ends with d

  • variable_name can be used to get an unknown number of elements.

The * can also be used in combination with wildcard _. Ex: case 'a', *_

Mapping Pattern matching

These patterns use single underscore _ to match a part of or whole subject.

subject = {'id': 1, 'name': 'Apple'} # Output: Apple is a fruit

match subject:
    case {'name': 'Apple'}:
        print("Apple is a fruit")
    case {'id': 1, 'name': 'Apple'}:
        print("We found an Apple with id 1")

While matching a mapping pattern not all parts(key-value pairs) of the subject need to be present in the pattern. As we can see in the above example the 1st case {'name': 'Apple'} was a successful match of the subject. To keep track of extra key-value pairs or items-pattern we can use ** with a free variable(capture pattern)

subject = {'name': 'Apple', 'colour': 'Red'} # Output: The other properties of Apple are {'colour': 'Red'}

match subject:
    case {'name': 'Apple', **extras}:
        print(f"The other properties of Apple are {extras}")

For a scenario where the subject should only include the pattern, We can make use of a guard.

subject = {'name': 'Apple', 'colour': 'Red'} # Output: I found a red apple

match subject:
    case {'name': 'Apple', **extras} if not extras:
        print("Apple is a fruit")
    case {'name': 'Apple', 'colour': 'Red', **extras} if not extras:
        print("I found a red apple")

get() method of an object(subject) is used for matching the mapping pattern. A get() method can only fetch a key if it already exists in the subject, Hence subjects with class defaultdict no new keys will be created if not found and won’t be a successful match.

That concludes part 2 of the structured pattern matching in the python series. In the next blog, we close the discussion with Class Pattern.

Hi, I am Harris Hujare. As a software developer, I design and implement scalable and maintainable systems. I have a passion for learning new technologies and creating software utilities making lives easier.

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