Negation in Python

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Question :

Negation in Python

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"])

Answer #1:

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"])

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.


blues_sounds_path = "/usr/share/sounds/blues"
if not os.path.exists(blues_sounds_path):
    except OSError:
        # Handle the case where the directory could not be created.
Answered By: David Mulder

Answer #2:

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.

Answered By: Karl Nicoll

Answer #3:

try instead:

if not os.path.exists(pathName):
    do this
Answered By: Cat Plus Plus

Answer #4:

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):
Answered By: mshell_lauren

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