4.18 Creating Directories Including Necessary Parent Directories

Credit: Trent Mick, Alex Martelli

4.18.1 Problem

You want a way to make a directory that is more convenient than Python's standard os.mkdir.

4.18.2 Solution

A good make-directory function should, first of all, make the necessary parent directories, which os.makedirs does quite nicely. We also want our function to complete silently if the directory already exists but to fail if the needed directory exists as a plain file. To get that behavior, we need to write some code:

import os, errno
def mkdirs(newdir, mode=0777):
    try: os.makedirs(newdir, mode)
    except OSError, err:
        # Reraise the error unless it's about an already existing directory 
        if err.errno != errno.EEXIST or not os.path.isdir(newdir): 
            raise

4.18.3 Discussion

Python's standard os.mkdir works much like the underlying mkdir system call (i.e., in a pretty spare and rigorous way). For example, it raises an exception when the directory you're trying to make already exists. You almost always have to handle that exception, because it's not generally an error if the directory already exists as a directory, while it is indeed an error if a file of that name is in the way. Further, all the parent directories of the one you're trying to make must already exist, as os.mkdir itself only makes the leaf directory out of the whole path.

There used to be a time when mkdir, as used in Unix shell scripts, worked the same way, but we're spoiled now. For example, the --parents switch in the GNU version of mkdir implicitly creates all intermediate directories, and gives no error if the target directory already exists as a directory. Well, why not have the same convenience in Python? This recipe shows it takes very little to achieve this�the little function mkdirs can easily become part of your standard bag of tricks. Of course, Python's standard os.makedirs is doing most of the job. However, mkdirs adds the important convenience of not propagating an exception when the requested directory already exists and is indeed a directory. However, if the requested directory exists as a file or if the operating system diagnoses any other kind of trouble, function mkdirs does explicitly re-raise the exception, to ensure it propagates further.

4.18.4 See Also

Documentation for the os module in the Library Reference.