In Python, can I call the main() of an imported module?

Question

In Python I have a module myModule.py where I define a few functions and a main(), which takes a few command line arguments.

I usu

Answer 1

Assuming you are trying to pass the command line arguments as well.

import sys
import myModule


def main():
    # this will just pass all of the system arguments as is
    myModule.main(*sys.argv)

    # all the argv but the script name
    myModule.main(*sys.argv[1:])

Answer 2

It's just a function. Import it and call it:

import myModule

myModule.main()

If you need to parse arguments, you have two options:

• Parse them in main(), but pass in sys.argv as a parameter (all code below in the same module myModule):

  def main(args):
      # parse arguments using optparse or argparse or what have you
  
  if __name__ == '__main__':
      import sys
      main(sys.argv[1:])
  

  Now you can import and call myModule.main(['arg1', 'arg2', 'arg3']) from other another module.

• Have main() accept parameters that are already parsed (again all code in the myModule module):

  def main(foo, bar, baz='spam'):
      # run with already parsed arguments
  
  if __name__ == '__main__':
      import sys
      # parse sys.argv[1:] using optparse or argparse or what have you
      main(foovalue, barvalue, **dictofoptions)
  

  and import and call myModule.main(foovalue, barvalue, baz='ham') elsewhere and passing in python arguments as needed.

The trick here is to detect when your module is being used as a script; when you run a python file as the main script (python filename.py) no import statement is being used, so python calls that module "__main__". But if that same filename.py code is treated as a module (import filename), then python uses that as the module name instead. In both cases the variable __name__ is set, and testing against that tells you how your code was run.

Answer 3

It depends. If the main code is protected by an if as in:

if __name__ == '__main__':
    ...main code...

then no, you can't make Python execute that because you can't influence the automatic variable __name__.

But when all the code is in a function, then might be able to. Try

import myModule

myModule.main()

This works even when the module protects itself with a __all__.

from myModule import * might not make main visible to you, so you really need to import the module itself.

Answer 4

The answer I was searching for was answered here: How to use python argparse with args other than sys.argv?

If main.py and parse_args() is written in this way, then the parsing can be done nicely

# main.py
import argparse
def parse_args():
    parser = argparse.ArgumentParser(description="")
    parser.add_argument('--input', default='my_input.txt')
    return parser

def main(args):
    print(args.input)

if __name__ == "__main__":
    parser = parse_args()
    args = parser.parse_args()
    main(args)

Then you can call main() and parse arguments with parser.parse_args(['--input', 'foobar.txt']) to it in another python script:

# temp.py
from main import main, parse_args
parser = parse_args()
args = parser.parse_args([]) # note the square bracket
# to overwrite default, use parser.parse_args(['--input', 'foobar.txt'])
print(args) # Namespace(input='my_input.txt')
main(args)

Answer 5

I had the same need using argparse too. The thing is parse_args function of an argparse.ArgumentParser object instance implicitly takes its arguments by default from sys.args. The work around, following Martijn line, consists of making that explicit, so you can change the arguments you pass to parse_args as desire.

def main(args):
    # some stuff
    parser = argparse.ArgumentParser()
    # some other stuff
    parsed_args = parser.parse_args(args)
    # more stuff with the args

if __name__ == '__main__':
    import sys
    main(sys.argv[1:])

The key point is passing args to parse_args function. Later, to use the main, you just do as Martijn tell.

Answer 6

Martijen's answer makes sense, but it was missing something crucial that may seem obvious to others but was hard for me to figure out.

In the version where you use argparse, you need to have this line in the main body.

args = parser.parse_args(args)

Normally when you are using argparse just in a script you just write

args = parser.parse_args()

and parse_args find the arguments from the command line. But in this case the main function does not have access to the command line arguments, so you have to tell argparse what the arguments are.

Here is an example

import argparse
import sys

def x(x_center, y_center):
    print "X center:", x_center
    print "Y center:", y_center

def main(args):
    parser = argparse.ArgumentParser(description="Do something.")
    parser.add_argument("-x", "--xcenter", type=float, default= 2, required=False)
    parser.add_argument("-y", "--ycenter", type=float, default= 4, required=False)
    args = parser.parse_args(args)
    x(args.xcenter, args.ycenter)

if __name__ == '__main__':
    main(sys.argv[1:])

Assuming you named this mytest.py To run it you can either do any of these from the command line

python ./mytest.py -x 8
python ./mytest.py -x 8 -y 2
python ./mytest.py 

which returns respectively

X center: 8.0
Y center: 4

or

X center: 8.0
Y center: 2.0

or

X center: 2
Y center: 4

Or if you want to run from another python script you can do

import mytest
mytest.main(["-x","7","-y","6"]) 

which returns

X center: 7.0
Y center: 6.0