How to handle subprocess.run() error/exception

Question

How do I handle a subprocess.run() error in Python? For example, I want to run cd + UserInput with subprocess.run(). What if the user types in a directory name which does not exist? How do I handle this type of error?

Answer 1

As @match has mentioned, you can't run cd as a subprocess, because cd isn't a program, it's a shell built-in command.

But if you're asking about any subprocess failures, besides cd:

try:
    subprocess.run(command_that_might_not_exist)  # like ['abcd']
except Exception:
    # handle the error

result = subprocess.run(command_that_might_fail)  # like ['ls', 'abcd/']
if result.returncode != 0:
    # handle the error

Answer 2

There is no way running cd in a subprocess is useful. The subprocess will change its own directory and then immediately exit, leaving no observable change in the parent process or anywhere else.

For the same reason, there is no binary command named cd on most systems; the cd command is a shell built-in.

Generally, if you run subprocess.run() without the check=True keyword argument, any error within the subprocess will simply be ignored. So if /bin/cd or a similar command existed, you could run

# purely theoretical, and utterly useless
subprocess.run(['cd', UserInput])

and simply not know whether it did anything or not.

If you do supply check=True, the exception you need to trap is CalledProcessError:

try:
    # pointless code as such; see explanation above
    subprocess.run(['cd', UserInput], check=True)
except subprocess.CalledProcessError:
    print('Directory name %s misspelled, or you lack the permissions' % UserInput)

But even more fundamentally, allowing users to prod the system by running arbitrary unchecked input in a subprocess is a horrible idea. (Allowing users to run arbitrary shell script with shell=True is a monumentally, catastrophically horrible idea, so let's not even go there. Maybe see Actual meaning of shell=True in subprocess)

A somewhat more secure approach is to run the subprocess with a cwd= keyword argument.

# also vaguely pointless
subprocess.run(['true'], cwd=UserInput)

In this case, you can expect a regular FileNotFoundError if the directory does not exist, or a PermissionError if you lack the privileges.

You should probably still add check=True and be prepared to handle any resulting exception, unless you specifically don't care whether the subprocess succeeded. (There are actually cases where this makes sense, like when you grep for something but are fine with it not finding any matches, which raises an error if you use check=True.)

Perhaps see also Running Bash commands in Python

Answer 3

You might use a try and expect block to handle exceptions.