Extern

Extern is an opinionated version of Python's subprocess, making it just that little bit more convenient to run shell commands from within Python code. Extern is Python-3 only, and is no longer compatible with Python-2, as of version 0.4.0.

It is reasonably straightforward:

>>> import extern
>>> extern.run("echo it works") #=> returns 'it works\n'
>>> extern.run("echo 1 2 5 |cat") #=> returns '1 2 5\n'

When a command that fails is run e.g.

>>> extern.run("cat /not_a_file")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "build/bdist.linux-x86_64/egg/extern/__init__.py", line 29, in run
extern.ExternCalledProcessError: Command cat /not_a_file returned non-zero exit status 1.
STDERR was: cat: /not_a_file: No such file or directory
STDOUT was: 

an exception is raised just like subprocess. However, the error message generated includes STDERR and STDOUT, which is more convenient for debugging. For reference, the subprocess equivalent:

>>> subprocess.check_output(['bash','-c','cat /not_a_file'])
cat: /not_a_file: No such file or directory
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/subprocess.py", line 573, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command '['bash', '-c', 'cat /not_a_file']' returned non-zero exit status 1

The useful thing is that Extern collects STDERR and only reports it when there is a non-zero exit status, discarding it otherwise.

IMPORTANT: use of this library with untrusted strings presents a security risk in the same way as little bobby tables, and shell=True in subprocess.

###Multiple commands run simultaneously

>>> extern.run_many(['echo once','echo twice','echo thrice'])
#=> ['once\n', 'twice\n', 'thrice\n']

Progress can also be observed:

>>> extern.run_many(['echo once','echo twice','echo thrice'], progress_stream=sys.stderr)
Finished processing 3 of 3 (100.00%) items.
#=> ['once\n', 'twice\n', 'thrice\n']

STDIN can be provided to run():

extern.run('cat',stdin='dog') #=> 'dog'

###Which There is also a which function, useful for determing where (and if) a program exists on the command line:

>>> import extern
>>> extern.which('cat') #=> '/bin/cat'
>>> extern.which('dog') #=> None

Installation

You can also install it directly from the Python Package Index with this command:

sudo pip install extern

Licence

See file LICENCE.txt in this folder

Contribute

Extern is an open-source software. Everyone is welcome to contribute !