Negation in Python | 易学教程

问题

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()

回答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"])
    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.

回答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.

回答3:

try instead:

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

回答4:

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

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

来源：https://stackoverflow.com/questions/6117733/negation-in-python

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python

negation