kopia lustrzana https://github.com/animator/learn-python
Ashita Prasad
GitHub
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d2d6db0111
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- [Protocols](protocols.md)
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- [Exception Handling in Python](exception-handling.md)
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- [Generators](generators.md)
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- [Match Case Statement](match-case.md)
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- [Closures](closures.md)
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- [Filter](filter-function.md)
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- [Reduce](reduce-function.md)
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# Match Case Statements
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## Introduction
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Match and case statements are introduced in Python 3.10 for structural pattern matching of patterns with associated actions. It offers more readible and
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cleaniness to the code as opposed to the traditional `if-else` statements. They also have destructuring, pattern matching and checks for specific properties in
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addition to the traditional `switch-case` statements in other languages, which makes them more versatile.
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## Syntax
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```
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match <statement>:
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case <pattern_1>:
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<do_task_1>
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case <pattern_2>:
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<do_task_2>
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case _:
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<do_task_wildcard>
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```
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A match statement takes a statement which compares it to the various cases and their patterns. If any of the pattern is matched successively, the task is performed accordingly. If an exact match is not confirmed, the last case, a wildcard `_`, if provided, will be used as the matching case.
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## Pattern Matching
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As discussed earlier, match case statements use pattern matching where the patterns consist of sequences, mappings, primitive data types as well as class instances. The structural pattern matching uses declarative approach and it nexplicitly states the conditions for the patterns to match with the data.
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### Patterns with a Literal
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#### Generic Case
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`sample text` is passed as a literal in the `match` block. There are two cases and a wildcard case mentioned.
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```python
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match 'sample text':
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case 'sample text':
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print('sample text')
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case 'sample':
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print('sample')
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case _:
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print('None found')
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```
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The `sample text` case is satisfied as it matches with the literal `sample text` described in the `match` block.
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O/P:
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```
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sample text
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```
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#### Using OR
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Taking another example, `|` can be used as OR to include multiple patterns in a single case statement where the multiple patterns all lead to a similar task.
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The below code snippets can be used interchangebly and generate the similar output. The latter is more consive and readible.
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```python
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match 'e':
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case 'a':
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print('vowel')
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case 'e':
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print('vowel')
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case 'i':
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print('vowel')
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case 'o':
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print('vowel')
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case 'u':
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print('vowel')
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case _:
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print('consonant')
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```
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```python
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match 'e':
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case 'a' | 'e' | 'i' | 'o' | 'u':
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print('vowel')
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case _:
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print('consonant')
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```
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O/P:
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```
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vowel
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```
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#### Without wildcard
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When in a `match` block, there is no wildcard case present there are be two cases of match being present or not. If the match doesn't exist, the behaviour is a no-op.
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```python
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match 'c':
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case 'a' | 'e' | 'i' | 'o' | 'u':
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print('vowel')
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```
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The output will be blank as a no-op occurs.
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### Patterns with a Literal and a Variable
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Pattern matching can be done by unpacking the assignments and also bind variables with it.
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```python
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def get_names(names: str) -> None:
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match names:
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case ('Bob', y):
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print(f'Hello {y}')
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case (x, 'John'):
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print(f'Hello {x}')
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case (x, y):
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print(f'Hello {x} and {y}')
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case _:
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print('Invalid')
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```
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Here, the `names` is a tuple that contains two names. The `match` block unpacks the tuple and binds `x` and `y` based on the patterns. A wildcard case prints `Invalid` if the condition is not satisfied.
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O/P:
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In this example, the above code snippet with the parameter `names` as below and the respective output.
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```
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>>> get_names(('Bob', 'Max'))
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Hello Max
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>>> get_names(('Rob', 'John'))
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Hello Rob
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>>> get_names(('Rob', 'Max'))
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Hello Rob and Max
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>>> get_names(('Rob', 'Max', 'Bob'))
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Invalid
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```
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### Patterns with Classes
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Class structures can be used in `match` block for pattern matching. The class members can also be binded with a variable to perform certain operations. For the class structure:
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```python
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class Person:
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def __init__(self, name, age):
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self.name = name
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self.age = age
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```
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The match case example illustrates the generic working as well as the binding of variables with the class members.
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```python
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def get_class(cls: Person) -> None:
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match cls:
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case Person(name='Bob', age=18):
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print('Hello Bob with age 18')
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case Person(name='Max', age=y):
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print(f'Age is {y}')
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case Person(name=x, age=18):
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print(f'Name is {x}')
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case Person(name=x, age=y):
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print(f'Name and age is {x} and {y}')
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case _:
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print('Invalid')
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```
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O/P:
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```
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>>> get_class(Person('Bob', 18))
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Hello Bob with age 18
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>>> get_class(Person('Max', 21))
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Age is 21
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>>> get_class(Person('Rob', 18))
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Name is Rob
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>>> get_class(Person('Rob', 21))
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Name and age is Rob and 21
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```
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Now, if a new class is introduced in the above code snippet like below.
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```python
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class Pet:
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def __init__(self, name, animal):
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self.name = name
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self.animal = animal
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```
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The patterns will not match the cases and will trigger the wildcard case for the original code snippet above with `get_class` function.
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```
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>>> get_class(Pet('Tommy', 'Dog'))
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Invalid
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```
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### Nested Patterns
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The patterns can be nested via various means. It can include the mix of the patterns mentioned earlier or can be symmetrical across. A basic of the nested pattern of a list with Patterns with a Literal and Variable is taken. Classes and Iterables can laso be included.
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```python
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def get_points(points: list) -> None:
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match points:
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case []:
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print('Empty')
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case [x]:
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print(f'One point {x}')
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case [x, y]:
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print(f'Two points {x} and {y}')
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case _:
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print('More than two points')
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```
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O/P:
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```
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>>> get_points([])
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Empty
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>>> get_points([1])
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One point 1
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>>> get_points([1, 2])
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Two points 1 and 2
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>>> get_points([1, 2, 3])
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More than two points
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```
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### Complex Patterns
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Complex patterns are also supported in the pattern matching sequence. The complex does not mean complex numbers but rather the structure which makes the readibility to seem complex.
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#### Wildcard
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The wildcard used till now are in the form of `case _` where the wildcard case is used if no match is found. Furthermore, the wildcard `_` can also be used as a placeholder in complex patterns.
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```python
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def wildcard(value: tuple) -> None:
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match value:
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case ('Bob', age, 'Mechanic'):
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print(f'Bob is mechanic of age {age}')
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case ('Bob', age, _):
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print(f'Bob is not a mechanic of age {age}')
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```
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O/P:
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The value in the above snippet is a tuple with `(Name, Age, Job)`. If the job is Mechanic and the name is Bob, the first case is triggered. But if the job is different and not a mechanic, then the other case is triggered with the wildcard.
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```
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>>> wildcard(('Bob', 18, 'Mechanic'))
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Bob is mechanic of age 18
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>>> wildcard(('Bob', 21, 'Engineer'))
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Bob is not a mechanic of age 21
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```
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#### Guard
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A `guard` is when an `if` is added to a pattern. The evaluation depends on the truth value of the guard.
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`nums` is the tuple which contains two integers. A guard is the first case where it checks whether the first number is greater or equal to the second number in the tuple. If it is false, then it moves to the second case, where it concludes that the first number is smaller than the second number.
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```python
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def guard(nums: tuple) -> None:
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match nums:
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case (x, y) if x >= y:
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print(f'{x} is greater or equal than {y}')
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case (x, y):
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print(f'{x} is smaller than {y}')
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case _:
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print('Invalid')
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```
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O/P:
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```
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>>> guard((1, 2))
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1 is smaller than 2
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>>> guard((2, 1))
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2 is greater or equal than 1
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>>> guard((1, 1))
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1 is greater or equal than 1
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```
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## Summary
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The match case statements provide an elegant and readible format to perform operations on pattern matching as compared to `if-else` statements. They are also more versatile as they provide additional functionalities on the pattern matching operations like unpacking, class matching, iterables and iterators. It can also use positional arguments for checking the patterns. They provide a powerful and concise way to handle multiple conditions and perform pattern matching
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## Further Reading
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This article provides a brief introduction to the match case statements and the overview on the pattern matching operations. To know more, the below articles can be used for in-depth understanding of the topic.
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- [PEP 634 – Structural Pattern Matching: Specification](https://peps.python.org/pep-0634/)
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- [PEP 636 – Structural Pattern Matching: Tutorial](https://peps.python.org/pep-0636/)
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