Determining Whether a Directory is Writeable

Question

Although what Christophe suggested is a more Pythonic solution, the os module does have the os.access function to check access:

os.access('/path/to/folder', os.W_OK) # W_OK is for writing, R_OK for reading, etc.

It may seem strange to suggest this, but a common Python idiom is

It's easier to ask for forgiveness than for permission

Following that idiom, one might say:

Try writing to the directory in question, and catch the error if you don't have the permission to do so.

My solution using the tempfile module:

import tempfile
import errno

def isWritable(path):
    try:
        testfile = tempfile.TemporaryFile(dir = path)
        testfile.close()
    except OSError as e:
        if e.errno == errno.EACCES:  # 13
            return False
        e.filename = path
        raise
    return True

Update: After testing the code again on Windows I see that there is indeed an issue when using tempfile there, see issue22107: tempfile module misinterprets access denied error on Windows. In the case of a non-writable directory, the code hangs for several seconds and finally throws an IOError: [Errno 17] No usable temporary file name found. Maybe this is what user2171842 was observing? Unfortunately the issue is not resolved for now so to handle this, the error needs to be caught as well:

    except (OSError, IOError) as e:
        if e.errno == errno.EACCES or e.errno == errno.EEXIST:  # 13, 17

The delay is of course still present in these cases then.

Stumbled across this thread searching for examples for someone. First result on Google, congrats!

People talk about the Pythonic way of doing it in this thread, but no simple code examples? Here you go, for anyone else who stumbles in:

import sys

filepath = 'C:\\path\\to\\your\\file.txt'

try:
    filehandle = open( filepath, 'w' )
except IOError:
    sys.exit( 'Unable to write to file ' + filepath )

filehandle.write("I am writing this text to the file\n")

This attempts to open a filehandle for writing, and exits with an error if the file specified cannot be written to: This is far easier to read, and is a much better way of doing it rather than doing prechecks on the file path or the directory, as it avoids race conditions; cases where the file becomes unwriteable between the time you run the precheck, and when you actually attempt to write to the file.

If you only care about the file perms, os.access(path, os.W_OK) should do what you ask for. If you instead want to know whether you can write to the directory, open() a test file for writing (it shouldn't exist beforehand), catch and examine any IOError, and clean up the test file afterwards.

More generally, to avoid TOCTOU attacks (only a problem if your script runs with elevated privileges -- suid or cgi or so), you shouldn't really trust these ahead-of-time tests, but drop privs, do the open(), and expect the IOError.