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## 3.14. The `os` module

The `os` module has lots of useful functions for manipulating files and processes, and `os.path` has functions for manipulating file and directory paths.

### Example 3.35. Constructing pathnames

    >>> import os
    >>> os.path.join("c:\\music\\ap\\", "mahadeva.mp3")
    'c:\\music\\ap\\mahadeva.mp3'
    >>> os.path.join("c:\\music\\ap", "mahadeva.mp3")
    'c:\\music\\ap\\mahadeva.mp3'
    >>> os.path.expanduser("~")
    'c:\\Documents and Settings\\mpilgrim\\My Documents'
    >>> os.path.join(os.path.expanduser("~"), "Python")
    'c:\\Documents and Settings\\mpilgrim\\My Documents\\Python'

| `os.path` is a reference to a module; which module it is depends on what platform you’re running on. Just like `getpass` encapsulates differences between platforms by setting `getpass` to a platform-specific function, `os` encapsulates differences between platforms by setting `path` to a platform-specific module.
| The `join` function of `os.path` constructs a pathname out of one or more partial pathnames. In this simple case, it simply concatenates strings. (Note that dealing with pathnames on Windows is annoying because the backslash character must be escaped.)
| In this slightly less trivial case, `join` will add an extra backslash to the pathname before joining it to the filename. I was overjoyed when I discovered this, since `addSlashIfNecessary` is always one of the stupid little functions I have to write when building up my toolbox in a new language. Do not write this stupid little function in Python; smart people have already taken care of it for you.
| `expanduser` will expand a pathname that uses `~` to represent the current user’s home directory. This works on any platform where users have a home directory, like Windows, UNIX, and Mac OS X; it has no effect on Mac OS.
| Combining these techniques, you can easily construct pathnames for directories and files under the user’s home directory.

### Example 3.36. Splitting pathnames

    >>> os.path.split("c:\\music\\ap\\mahadeva.mp3")
    ('c:\\music\\ap', 'mahadeva.mp3')
    >>> (filepath, filename) = os.path.split("c:\\music\\ap\\mahadeva.mp3")
    >>> filepath
    'c:\\music\\ap'
    >>> filename
    'mahadeva.mp3'
    >>> (shortname, extension) = os.path.splitext(filename)
    >>> shortname
    'mahadeva'
    >>> extension
    '.mp3'

| The `split` function splits a full pathname and returns a tuple containing the path and filename. Remember when I said you could use multi-variable assignment to return multiple values from a function? Well, `split` is such a function.
| We assign the return value of the `split` function into a tuple of two variables. Each variable receives the value of the corresponding element of the returned tuple.
| The first variable, `filepath`, receives the value of the first element of the tuple returned from `split`, the file path.
| The second variable, `filename`, receives the value of the second element of the tuple returned from `split`, the filename.
| `os.path` also contains a function `splitext`, which splits a filename and returns a tuple containing the filename and the file extension. We use the same technique to assign each of them to separate variables.

### Example 3.37. Listing directories

    >>> os.listdir("c:\\music\\_singles\\")
    ['a_time_long_forgotten_con.mp3', 'hellraiser.mp3', 'kairo.mp3',
    'long_way_home1.mp3', 'sidewinder.mp3', 'spinning.mp3']
    >>> dirname = "c:\\"
    >>> os.listdir(dirname)
    ['AUTOEXEC.BAT', 'boot.ini', 'CONFIG.SYS', 'cygwin', 'docbook',
    'Documents and Settings', 'Incoming', 'Inetpub', 'IO.SYS', 'MSDOS.SYS', 'Music',
    'NTDETECT.COM', 'ntldr', 'pagefile.sys', 'Program Files', 'Python20', 'RECYCLER',
    'System Volume Information', 'TEMP', 'WINNT']
    >>> [f for f in os.listdir(dirname) if os.path.isfile(os.path.join(dirname, f))]
    ['AUTOEXEC.BAT', 'boot.ini', 'CONFIG.SYS', 'IO.SYS', 'MSDOS.SYS',
    'NTDETECT.COM', 'ntldr', 'pagefile.sys']
    >>> [f for f in os.listdir(dirname) if os.path.isdir(os.path.join(dirname, f))]
    ['cygwin', 'docbook', 'Documents and Settings', 'Incoming',
    'Inetpub', 'Music', 'Program Files', 'Python20', 'RECYCLER',
    'System Volume Information', 'TEMP', 'WINNT']

| The `listdir` function takes a pathname and returns a list of the contents of the directory.
| `listdir` returns both files and folders, with no indication of which is which.
| You can use list filtering and the `isfile` function of the `os.path` module to separate the files from the folders. `isfile` takes a pathname and returns 1 if the path represents a file, and 0 otherwise. Here we’re using ``os.path`.`join`` to ensure a full pathname, but `isfile` also works with a partial path, relative to the current working directory. You can use `os.getcwd()` to get the current working directory.
| `os.path` also has a `isdir` function which returns 1 if the path represents a directory, and 0 otherwise. You can use this to get a list of the subdirectories within a directory.

### Example 3.38. Listing directories in `fileinfo.py`

    def listDirectory(directory, fileExtList):
        "get list of file info objects for files of particular extensions"
        fileList = [os.path.normcase(f) for f in os.listdir(directory)]
        fileList = [os.path.join(directory, f) for f in fileList \
                    if os.path.splitext(f)[1] in fileExtList]

These two lines of code combine everything we’ve learned so far about the `os` module, and then some.

1. `os.listdir(directory)` returns a list of all the files and folders in `directory`.
2. Iterating through the list with `f`, we use `os.path.normcase(f)` to normalize the case according to operating system defaults. `normcase` is a useful little function that compensates for case-insensitive operating systems that think that `mahadeva.mp3` and `mahadeva.MP3` are the same file. For instance, on Windows and Mac OS, `normcase` will convert the entire filename to lowercase; on UNIX-compatible systems, it will return the filename unchanged.
3. Iterating through the normalized list with `f` again, we use `os.path.splitext(f)` to split each filename into name and extension.
4. For each file, we see if the extension is in the list of file extensions we care about (`fileExtList`, which was passed to the `listDirectory` function).
5. For each file we care about, we use `os.path.join(directory, f)` to construct the full pathname of the file, and return a list of the full pathnames.

Whenever possible, you should use the functions in `os` and `os.path` for file, directory, and path manipulations. These modules are wrappers for platform-specific modules, so functions like `os.path.split` work on UNIX, Windows, Mac OS, and any other supported Python platform.

### Further reading

- Python Knowledge Base answers questions about the `os` module.
- Python Library Reference documents the `os` module and the `os.path` module.