Negation in Python

186
votes

I'm trying to create a directory if the path doesn't exist, but the ! (not) operator doesn't work. I'm not sure how to negate in Python... What's the correct way to do this?

if (!os.path.exists("/usr/share/sounds/blues")):
        proc = subprocess.Popen(["mkdir", "/usr/share/sounds/blues"])
        proc.wait()
4
pythonnegation
By the way, why not use Python's os.mkdir()? - Neil
I wasn't aware of the os.mkdir() function, although I figured there was something like that. - David Mulder
It's better here to just try: os.mkdir(path) and handle the error. If you check first ('look before you leap') someone else can create or delete that folder after your check (but before you create it), and there could still be an error. The check doesn't guarantee anything at the time of creation. This idea is sometimes called 'easier to ask forgiveness than permission'. Even better (but even more specific to this problem), you can do os.makedirs(path, exist_ok=True) to create the path and ignore a FileExistsError. - speedstyle

4 Answers

255
votes

The negation operator in Python is not. Therefore just replace your ! with not.

For your example, do this:

if not os.path.exists("/usr/share/sounds/blues") :
    proc = subprocess.Popen(["mkdir", "/usr/share/sounds/blues"])
    proc.wait()

For your specific example (as Neil said in the comments), you don't have to use the subprocess module, you can simply use os.mkdir() to get the result you need, with added exception handling goodness.

Example:

blues_sounds_path = "/usr/share/sounds/blues"
if not os.path.exists(blues_sounds_path):
    try:
        os.mkdir(blues_sounds_path)
    except OSError:
        # Handle the case where the directory could not be created.
37
votes

Python prefers English keywords to punctuation. Use not x, i.e. not os.path.exists(...). The same thing goes for && and || which are and and or in Python.

15
votes

try instead:

if not os.path.exists(pathName):
    do this
2
votes

Combining the input from everyone else (use not, no parens, use os.mkdir) you'd get...

special_path_for_john = "/usr/share/sounds/blues"
if not os.path.exists(special_path_for_john):
    os.mkdir(special_path_for_john)