Dive Into Python

Example 6.16. 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 depends on your platform. Just as 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 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 one of the stupid little functions I always need 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 6.17. 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.
	You 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. You use the same technique to assign each of them to separate variables.

Example 6.18. 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 you'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 6.19. 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]    

	os.listdir(directory) returns a list of all the files and folders in directory.
	Iterating through the list with f, you 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.
	Iterating through the normalized list with f again, you use os.path.splitext(f) to split each filename into name and extension.
	For each file, you see if the extension is in the list of file extensions you care about (fileExtList, which was passed to the listDirectory function).
	For each file you care about, you 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 platform supported by Python.

There is one other way to get the contents of a directory. It's very powerful, and it uses the sort of wildcards that you may already be familiar with from working on the command line.

Example 6.20. Listing Directories with glob

>>> os.listdir("c:\\music\\_singles\\")               
['a_time_long_forgotten_con.mp3', 'hellraiser.mp3',
'kairo.mp3', 'long_way_home1.mp3', 'sidewinder.mp3',
'spinning.mp3']
>>> import glob
>>> glob.glob('c:\\music\\_singles\\*.mp3')           
['c:\\music\\_singles\\a_time_long_forgotten_con.mp3',
'c:\\music\\_singles\\hellraiser.mp3',
'c:\\music\\_singles\\kairo.mp3',
'c:\\music\\_singles\\long_way_home1.mp3',
'c:\\music\\_singles\\sidewinder.mp3',
'c:\\music\\_singles\\spinning.mp3']
>>> glob.glob('c:\\music\\_singles\\s*.mp3')          
['c:\\music\\_singles\\sidewinder.mp3',
'c:\\music\\_singles\\spinning.mp3']
>>> glob.glob('c:\\music\\*\\*.mp3')                  

	As you saw earlier, os.listdir simply takes a directory path and lists all files and directories in that directory.
	The glob module, on the other hand, takes a wildcard and returns the full path of all files and directories matching the wildcard. Here the wildcard is a directory path plus "*.mp3", which will match all .mp3 files. Note that each element of the returned list already includes the full path of the file.
	If you want to find all the files in a specific directory that start with "s" and end with ".mp3", you can do that too.
	Now consider this scenario: you have a music directory, with several subdirectories within it, with .mp3 files within each subdirectory. You can get a list of all of those with a single call to glob, by using two wildcards at once. One wildcard is the "*.mp3" (to match .mp3 files), and one wildcard is within the directory path itself, to match any subdirectory within c:\music. That's a crazy amount of power packed into one deceptively simple-looking function!