Unix filename pattern matching in Python

Unix filename pattern matching in Python is accomplished using the fnmatch module. This module provides shell-style pattern matching capabilities to compare filenames against patterns and returns True or False based on matches.

Importing the fnmatch Module

First, import the fnmatch module from Python's standard library ?

import fnmatch

Unix Wildcard Patterns

The fnmatch module supports standard Unix wildcards ?

• * − Matches any number of characters (including none)
• ? − Matches exactly one character
• [seq] − Matches any character in the sequence
• [!seq] − Matches any character not in the sequence

To search for literal asterisk or question mark characters, use [*] or [?].

The fnmatch() Method

The fnmatch() method compares a filename against a pattern. It takes two arguments: the filename and the pattern ?

import fnmatch

# Test various patterns
patterns = [
    ('hello.txt', 'h*.txt'),
    ('test123.py', 'test???.py'),
    ('data.csv', '*.csv'),
    ('file.TXT', '*.txt')  # Case sensitive
]

for filename, pattern in patterns:
    result = fnmatch.fnmatch(filename, pattern)
    print(f'File: {filename:12} Pattern: {pattern:10} Match: {result}')

File: hello.txt     Pattern: h*.txt     Match: True
File: test123.py    Pattern: test???.py Match: True
File: data.csv      Pattern: *.csv      Match: True
File: file.TXT      Pattern: *.txt      Match: False

The filter() Method

The filter() method returns a list of filenames that match the pattern. It takes a list of names and a pattern as parameters ?

import fnmatch

files = ['test_file1.txt', 'test_file2.png', 'another_file.txt', 
         'test_file3.txt', 'abc.txt', 'TEST_FILE4.txt']

pattern = 'test_f*'
matched_files = fnmatch.filter(files, pattern)

print('All files:', files)
print('\nMatched files:', matched_files)

All files: ['test_file1.txt', 'test_file2.png', 'another_file.txt', 'test_file3.txt', 'abc.txt', 'TEST_FILE4.txt']

Matched files: ['test_file1.txt', 'test_file2.png', 'test_file3.txt']

The translate() Method

The translate() method converts shell-style patterns to regular expressions. This is useful when you need regex pattern objects ?

import fnmatch
import re

pattern = 'test_f*.txt'
regex_pattern = fnmatch.translate(pattern)
regex_object = re.compile(regex_pattern)

print('Original pattern:', pattern)
print('Regular expression:', regex_pattern)

# Test the regex
test_file = 'test_file_abcd123.txt'
match = regex_object.match(test_file)
print(f'Match result: {match}')

Original pattern: test_f*.txt
Regular expression: (?s:test_f.*\.txt)\Z
Match result: <re.Match object; span=(0, 21), match='test_file_abcd123.txt'>

Practical Example

Here's a complete example showing how to find specific file types in a directory ?

import fnmatch
import os

def find_files(directory, pattern):
    """Find files matching pattern in directory"""
    try:
        all_files = os.listdir(directory)
        matching_files = fnmatch.filter(all_files, pattern)
        return matching_files
    except FileNotFoundError:
        return []

# Example usage
patterns = ['*.txt', '*.py', 'test_*']
directory = './files'

for pattern in patterns:
    matches = find_files(directory, pattern)
    print(f'Files matching "{pattern}": {matches}')

Conclusion

The fnmatch module provides powerful Unix-style pattern matching for filenames. Use fnmatch() for single file checks, filter() for filtering lists, and translate() when you need regex objects for complex matching operations.