Davrot at 16:07, 17 October 2025

2025-10-17T16:07:41Z

← Older revision		Revision as of 16:07, 17 October 2025
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	~~== The goal ==~~
	There is a new flow control in town. A switch case replacement, called match case.		There is a new flow control in town. A switch case replacement, called match case.

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	== Unpacking parameters ==		== Unpacking parameters ==
	~~{: .topic-optional}~~ This is an optional topic!		'''This is an optional topic!'''



	Different from a normal switch we can deal with several input parameters at the same time and even use wildcards:<syntaxhighlight lang="python">def what_to_do(input):		Different from a normal switch we can deal with several input parameters at the same time and even use wildcards:<syntaxhighlight lang="python">def what_to_do(input):
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	=== OR ===		=== OR ===
	~~{: .topic-optional}~~ This is an optional topic!		'''This is an optional topic!'''



	We can combine patter by \| (or):<syntaxhighlight lang="python">def what_to_do(input):		We can combine patter by \| (or):<syntaxhighlight lang="python">def what_to_do(input):
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	=== If guards ===		=== If guards ===
	~~{: .topic-optional}~~ This is an optional topic!		'''This is an optional topic!'''



	We can distinguish between cases by using '''if guard''':<syntaxhighlight lang="python">def what_to_do(input):		We can distinguish between cases by using '''if guard''':<syntaxhighlight lang="python">def what_to_do(input):
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	=== Data classes ===		=== Data classes ===
	~~{: .topic-optional}~~ This is an optional topic!		'''This is an optional topic!'''



	We can combine it nicely with data classes:<syntaxhighlight lang="python">from dataclasses import dataclass		We can combine it nicely with data classes:<syntaxhighlight lang="python">from dataclasses import dataclass
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	=== As ===		=== As ===
	~~{: .topic-optional}~~ This is an optional topic!		'''This is an optional topic!'''



	We can use '''as''' to name a variable in a case clause (if you have a sequence i.e. tuple or list of elements then you can use '''as''' on the individual elements): <syntaxhighlight lang="python">from dataclasses import dataclass		We can use '''as''' to name a variable in a case clause (if you have a sequence i.e. tuple or list of elements then you can use '''as''' on the individual elements): <syntaxhighlight lang="python">from dataclasses import dataclass
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	=== List ===		=== List ===
	~~{: .topic-optional}~~ This is an optional topic!		'''This is an optional topic!'''



	This is how we can deal with lists that have different length:<syntaxhighlight lang="python">def what_to_do(input):		This is how we can deal with lists that have different length:<syntaxhighlight lang="python">def what_to_do(input):
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	=== Dictionaries ===		=== Dictionaries ===
	~~{: .topic-optional}~~ This is an optional topic!		'''This is an optional topic!'''



	The whole match apparatus works also with dictionaries. You can even distinguish between what kind of variable is stored behind a key (e.g. int vs str): <syntaxhighlight lang="python">def what_to_do(input):		The whole match apparatus works also with dictionaries. You can even distinguish between what kind of variable is stored behind a key (e.g. int vs str): <syntaxhighlight lang="python">def what_to_do(input):

Davrot: Created page with " == The goal == There is a new flow control in town. A switch case replacement, called match case. Questions to [mailto:davrot@uni-bremen.de David Rotermund] == The [https://docs.python.org/3/reference/compound_stmts.html#the-match-statement match] statement == The match statement is used for pattern matching. Syntax:match_stmt ::= 'match' subject_expr ":" NEWLINE INDENT case_block+ DEDENT subject_expr ::= star_named_expression "," st..."

2025-10-15T11:02:29Z

Created page with " == The goal == There is a new flow control in town. A switch case replacement, called match case. Questions to [mailto:davrot@uni-bremen.de David Rotermund] == The [https://docs.python.org/3/reference/compound_stmts.html#the-match-statement match] statement == The match statement is used for pattern matching. Syntax:<syntaxhighlight lang="python">match_stmt ::= 'match' subject_expr ":" NEWLINE INDENT case_block+ DEDENT subject_expr ::= star_named_expression "," st..."

New page

== The goal ==
There is a new flow control in town. A switch case replacement, called match case.

Questions to [mailto:davrot@uni-bremen.de David Rotermund]

== The [https://docs.python.org/3/reference/compound_stmts.html#the-match-statement match] statement ==
The match statement is used for pattern matching. Syntax:<syntaxhighlight lang="python">match_stmt ::= 'match' subject_expr ":" NEWLINE INDENT case_block+ DEDENT
subject_expr ::= star_named_expression "," star_named_expressions?
| named_expression
case_block ::= 'case' patterns [guard] ":" block</syntaxhighlight>

== Normal “switch” case ==
<syntaxhighlight lang="python">for i in range(0, 4):

match (i):
case 0:
print("This is a 0")

case 0:
print("This is a 0 too.")

case 1:
print("This is a 1.")

case 2:
print("This is a 2.")

case _:
print(f"I don't know what to do with a {i}!")</syntaxhighlight>Output:<syntaxhighlight lang="python">This is a 0
This is a 1.
This is a 2.
I don't know what to do with a 3!</syntaxhighlight>_ is the default. And after every case state has an in-build '''break;''' (If you don’t know what this means: A. You are not a C/C++ programmer. B. You can ignore this information. :-) ).

== Unpacking parameters ==
{: .topic-optional} This is an optional topic!

Different from a normal switch we can deal with several input parameters at the same time and even use wildcards:<syntaxhighlight lang="python">def what_to_do(input):
match (input):
case (0, 0):
print("This is (0,0) - A")

case (0, 1):
print("This is (0,1) - B")

case (x, 0):
print(f"This is a {x} - C")

case (x, y):
print(f"This is a {x} and {y} - D")

case _:
print(f"I don't know what to do with {input}- E!")
print("")

second_parameter: int = 0
for i in range(0, 2):
input = (i, second_parameter)
print(f"Input: {input}")
what_to_do(input)

second_parameter = 1
for i in range(0, 2):
input = (i, second_parameter)
print(f"Input: {input}")
what_to_do(input)

input = "Hello!"
print(f"Input: {input}")
what_to_do(input)</syntaxhighlight>Output:<syntaxhighlight lang="python">Input: (0, 0)
This is (0,0) - A

Input: (1, 0)
This is a 1 - C

Input: (0, 1)
This is (0,1) - B

Input: (1, 1)
This is a 1 and 1 - D

Input: Hello!
I don't know what to do with Hello!- E!</syntaxhighlight>

=== OR ===
{: .topic-optional} This is an optional topic!

We can combine patter by | (or):<syntaxhighlight lang="python">def what_to_do(input):
match (input):
case (0, 0) | (0, 1):
print("This is (0,0) or (0,1) - A")

case (x, 0):
print(f"This is a {x} - C")

case (x, y):
print(f"This is a {x} and {y} - D")

case _:
print(f"I don't know what to do with {input}- E!")
print("")

second_parameter: int = 0
for i in range(0, 2):
input = (i, second_parameter)
print(f"Input: {input}")
what_to_do(input)

second_parameter = 1
for i in range(0, 2):
input = (i, second_parameter)
print(f"Input: {input}")
what_to_do(input)</syntaxhighlight>Output:<syntaxhighlight lang="python">Input: (0, 0)
This is (0,0) or (0,1) - A

Input: (1, 0)
This is a 1 - C

Input: (0, 1)
This is (0,0) or (0,1) - A

Input: (1, 1)
This is a 1 and 1 - D</syntaxhighlight>You can also use the '''or''' on parts of the pattern (output is the same as before):<syntaxhighlight lang="python">def what_to_do(input):
match (input):
case (0, (0 | 1)):
print("This is (0,0) or (0,1) - A")

case (x, 0):
print(f"This is a {x} - C")

case (x, y):
print(f"This is a {x} and {y} - D")

case _:
print(f"I don't know what to do with {input}- E!")
print("")

second_parameter: int = 0
for i in range(0, 2):
input = (i, second_parameter)
print(f"Input: {input}")
what_to_do(input)

second_parameter = 1
for i in range(0, 2):
input = (i, second_parameter)
print(f"Input: {input}")
what_to_do(input)</syntaxhighlight>

=== If guards ===
{: .topic-optional} This is an optional topic!

We can distinguish between cases by using '''if guard''':<syntaxhighlight lang="python">def what_to_do(input):
match (input):
case (x, y) if x == y:
print(f"Both are {x} - A ")
case (x, y):
print(f"This is a {x} and {y} - B ")

print("")

second_parameter: int = 0
for i in range(0, 2):
input = (i, second_parameter)
print(f"Input: {input}")
what_to_do(input)

second_parameter = 1
for i in range(0, 2):
input = (i, second_parameter)
print(f"Input: {input}")
what_to_do(input)</syntaxhighlight>Output:<syntaxhighlight lang="python">Input: (0, 0)
Both are 0 - A

Input: (1, 0)
This is a 1 and 0 - B

Input: (0, 1)
This is a 0 and 1 - B

Input: (1, 1)
Both are 1 - A </syntaxhighlight>

=== Data classes ===
{: .topic-optional} This is an optional topic!

We can combine it nicely with data classes:<syntaxhighlight lang="python">from dataclasses import dataclass

@dataclass
class SomeDataStructure:
x: int
y: int

def what_to_do(input):
print(input)
match (input):
case SomeDataStructure(x=0, y=0):
print("case A")
case SomeDataStructure(x=1, y=0):
print("case B")
case SomeDataStructure():
print("case C")
case _:
print("What?")

print("")

data_0_0 = SomeDataStructure(0, 0)
what_to_do(data_0_0)
data_0_1 = SomeDataStructure(0, 1)
what_to_do(data_0_1)
data_1_0 = SomeDataStructure(1, 0)
what_to_do(data_1_0)
data_1_1 = SomeDataStructure(1, 1)
what_to_do(data_1_1)
data_broken = "I am broken!"
what_to_do(data_broken)</syntaxhighlight>Output:<syntaxhighlight lang="python">SomeDataStructure(x=0, y=0)
case A

SomeDataStructure(x=0, y=1)
case C

SomeDataStructure(x=1, y=0)
case B

SomeDataStructure(x=1, y=1)
case C

I am broken!
What?</syntaxhighlight>

=== As ===
{: .topic-optional} This is an optional topic!

We can use '''as''' to name a variable in a case clause (if you have a sequence i.e. tuple or list of elements then you can use '''as''' on the individual elements): <syntaxhighlight lang="python">from dataclasses import dataclass

@dataclass
class SomeDataStructure:
x: int
y: int

def what_to_do(input):
print(input)
match (input):
case SomeDataStructure(x=0, y=0):
print("case A")
case SomeDataStructure(x=1, y=0):
print("case B")
case SomeDataStructure() as someinput:
print(f"case C : {someinput.x} {someinput.y}")
case _:
print("What?")

print("")

data_0_0 = SomeDataStructure(0, 0)
what_to_do(data_0_0)
data_0_1 = SomeDataStructure(0, 1)
what_to_do(data_0_1)
data_1_0 = SomeDataStructure(1, 0)
what_to_do(data_1_0)
data_1_1 = SomeDataStructure(1, 1)
what_to_do(data_1_1)
data_broken = "I am broken!"
what_to_do(data_broken)</syntaxhighlight>Output:<syntaxhighlight lang="python">SomeDataStructure(x=0, y=0)
case A

SomeDataStructure(x=0, y=1)
case C : 0 1

SomeDataStructure(x=1, y=0)
case B

SomeDataStructure(x=1, y=1)
case C : 1 1

I am broken!
What?</syntaxhighlight>

=== List ===
{: .topic-optional} This is an optional topic!

This is how we can deal with lists that have different length:<syntaxhighlight lang="python">def what_to_do(input):
print(input)
match (input):
case []:
print("case A")
case [0]:
print("case B")
case [x]:
print(f"case C {x}")
case [0, 0]:
print("case D")
case _:
print("What?")

print("")

what_to_do([])
what_to_do([0])
what_to_do([1])
what_to_do([0, 0])
what_to_do([0, 1])
what_to_do("Hello")</syntaxhighlight>Output:<syntaxhighlight lang="python">[]
case A

[0]
case B

[1]
case C 1

[0, 0]
case D

[0, 1]
What?

Hello
What?</syntaxhighlight>

=== Dictionaries ===
{: .topic-optional} This is an optional topic!

The whole match apparatus works also with dictionaries. You can even distinguish between what kind of variable is stored behind a key (e.g. int vs str): <syntaxhighlight lang="python">def what_to_do(input):
print(input)
match (input):
case {"x": str() as x}:
print(f"{x} - str")
case {"x": int() as x}:
print(f"{x} - int")
case {"y": value}:
print(f"{value} - A")
case {"a": value_a, "b": value_b}:
print(f"a={value_a} b={value_b} - B")
print("")

dictionary_a = {"x": str(1)}
what_to_do(dictionary_a)
dictionary_b = {"x": int(1)}
what_to_do(dictionary_b)
dictionary_c = {"x": int(1), "y": 5}
what_to_do(dictionary_c)
dictionary_d = {"y": 5}
what_to_do(dictionary_d)
dictionary_e = {"a": 6, "b": 7}
what_to_do(dictionary_e)
dictionary_f = {"b": 8, "a": 10}
what_to_do(dictionary_f)</syntaxhighlight>Output:<syntaxhighlight lang="python">{'x': '1'}
1 - str

{'x': 1}
1 - int

{'x': 1, 'y': 5}
1 - int

{'y': 5}
5 - A

{'a': 6, 'b': 7}
a=6 b=7 - B

{'b': 8, 'a': 10}
a=10 b=8 - B</syntaxhighlight>

== Reference ==

* [https://peps.python.org/pep-0636/ PEP 636 – Structural Pattern Matching: Tutorial]

Flow Control: match case - Revision history

Davrot at 16:07, 17 October 2025