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# File I/O
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File system utilities for finding and organizing files with pattern matching and filtering capabilities.
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## Capabilities
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### File Finding
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Utilities for recursively finding files based on various criteria.
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```python { .api }
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def find_files(substring, path, recursive=True, check_ext=None, ignore_invisible=True):
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    """
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    Find files matching specified criteria.
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    Parameters:
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    - substring: str, substring that must be in filename
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    - path: str, directory path to search in
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    - recursive: bool, search subdirectories recursively
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    - check_ext: str, file extension to filter by (e.g., '.txt')
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    - ignore_invisible: bool, ignore hidden files (starting with '.')
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    Returns:
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    - matching_files: list, list of file paths matching criteria
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    """
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def find_filegroups(paths, substring='', extensions=None, ignore_invisible=True):
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    """
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    Group files by specified criteria from multiple paths.
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    Parameters:
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    - paths: list, list of directory paths to search
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    - substring: str, substring filter for filenames
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    - extensions: list, list of file extensions to include
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    - ignore_invisible: bool, ignore hidden files
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    Returns:
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    - file_groups: dict, dictionary grouping files by criteria
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    """
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```
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## Usage Examples
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```python
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from mlxtend.file_io import find_files, find_filegroups
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import os
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# Find all Python files in current directory
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python_files = find_files(
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    substring='',
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    path='.',
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    recursive=True,
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    check_ext='.py',
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    ignore_invisible=True
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)
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print(f"Found {len(python_files)} Python files")
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# Find files with specific substring
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config_files = find_files(
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    substring='config',
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    path='.',
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    recursive=True,
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    ignore_invisible=True
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)
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print(f"Found {len(config_files)} files containing 'config'")
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# Group files by extensions
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file_groups = find_filegroups(
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    paths=['.', './docs'],
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    extensions=['.py', '.md', '.txt'],
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    ignore_invisible=True
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)
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print("File groups by extension:")
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for ext, files in file_groups.items():
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    print(f"  {ext}: {len(files)} files")
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```