Pattern Matching ​

When using if ... elif ... elif ... else ... to make conditional judgments, the code can become lengthy and less readable. If you need to match several cases for a variable, you can use the match statement.

For example, if a student's score can only be A, B, or C, using the if statement would look like this:

score = 'B'
if score == 'A':
    print('score is A.')
elif score == 'B':
    print('score is B.')
elif score == 'C':
    print('score is C.')
else:
    print('invalid score.')

The same logic can be rewritten with the match statement:

score = 'B'

match score:
    case 'A':
        print('score is A.')
    case 'B':
        print('score is B.')
    case 'C':
        print('score is C.')
    case _:  # The underscore (_) matches any other case
        print('score is ???.')

When using the match statement, we match cases sequentially with case xxx, and we can add a final case _ to represent "any other value." This approach is more readable than using if ... elif ... else ....

Complex Matching ​

The match statement can match more than just single values; it can match multiple values, ranges, and bind matched values to variables:

age = 15

match age:
    case x if x < 10:
        print(f'< 10 years old: {x}')
    case 10:
        print('10 years old.')
    case 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18:
        print('11~18 years old.')
    case 19:
        print('19 years old.')
    case _:
        print('not sure.')

In this example:

• The first case x if x < 10 matches when age < 10, and assigns the value to the variable x.
• The second case 10 matches only the value 10.
• The third case 11 | 12 | ... | 18 matches multiple values using the | separator.
• The match statement demonstrates the flexibility of case matching.

Matching Lists ​

The match statement can also match lists, making it very powerful.

Suppose a user has input a command stored as args = ['gcc', 'hello.c']. The following code demonstrates how to use match to parse this list:

args = ['gcc', 'hello.c', 'world.c']
# args = ['clean']
# args = ['gcc']

match args:
    # If only 'gcc' appears, report an error:
    case ['gcc']:
        print('gcc: missing source file(s).')
    # If 'gcc' appears and at least one file is specified:
    case ['gcc', file1, *files]:
        print('gcc compile: ' + file1 + ', ' + ', '.join(files))
    # If only 'clean' appears:
    case ['clean']:
        print('clean')
    case _:
        print('invalid command.')

• The first case ['gcc'] matches when the list contains only the string 'gcc', indicating a missing file name.
• The second case ['gcc', file1, *files] matches when the first element is 'gcc', the second element is assigned to the variable file1, and the remaining elements are assigned to *files. This indicates at least one file is specified.
• The third case ['clean'] matches when the list contains only the string 'clean'.
• The final case _ matches any other cases.

The match statement's rules are very flexible, allowing for concise and clear code.