Check if calling script used "if __name__ == "__main__" (to comply with multiprocessing requirement)

Question

I wrote a package that is using multiprocessing.Pool inside one of its functions.

Due to this reason, it is mandatory (as specified in here under "Safe importing of main module") that the outermost calling function can be imported safely e.g. without starting a new process. This is usually achieved using the if __name__ == "__main__": statement as explicitly explained at the link above.

My understanding (but please correct me if I'm wrong) is that multiprocessing imports the outermost calling module. So, if this is not "import-safe", this will start a new process that will import again the outermost module and so on recursively, until everything crashes.

If the outermost module is not "import-safe" when the main function is launched it usually hangs without printing any warning, error, message, anything.

Since using if __name__ == "__main__": is not usually mandatory and the user is usually not always aware of all the modules used inside a package, I would like to check at the beginning of my function if the user complied with this requirement and, if not, raise a warning/error.

Is this possible? How can I do this?

To show this with an example, consider the following example.

Let's say I developed my_module.py and I share it online/in my company.

# my_module.py
from multiprocessing import Pool

def f(x):
    return x*x

def my_function(x_max):
    with Pool(5) as p:
        print(p.map(f, range(x_max)))

If a user (not me) writes his own script as:

# Script_of_a_good_user.py
from my_module import my_function

if __name__ == '__main__':
    my_function(10)

all is good and the output is printed as expected.

However, if a careless user writes his script as:

# Script_of_a_careless_user.py
from my_module import my_function

my_function(10)

then the process hangs, no output is produces, but no error message or warning is issued to the user.

Is there a way inside my_function, BEFORE opening Pool, to check if the user used the if __name__ == '__main__': condition in its script and, if not, raise an error saying it should do it?

NOTE: I think this behavior is only a problem on Windows machines where fork() is not available, as explained here.

Answer 1

You can use the traceback module to inspect the stack and find the information you're looking for. Parse the top frame, and look for the main shield in the code.

I assume this will fail when you're working with a .pyc file and don't have access to the source code, but I assume developers will test their code in the regular fashion first before doing any kind of packaging, so I think it's safe to assume your error message will get printed when needed.

Version with verbose messages:

import traceback
import re

def called_from_main_shield():
    print("Calling introspect")
    tb = traceback.extract_stack()
    print(traceback.format_stack())
    print(f"line={tb[0].line} lineno={tb[0].lineno} file={tb[0].filename}")
    try:
        with open(tb[0].filename, mode="rt") as f:
            found_main_shield = False
            for i, line in enumerate(f):
                if re.search(r"__name__.*['\"]__main__['\"]", line):
                    found_main_shield = True
                if i == tb[0].lineno:
                    print(f"found_main_shield={found_main_shield}")
                    return found_main_shield
    except:
        print("Coulnd't inspect stack, let's pretend the code is OK...")
        return True

print(called_from_main_shield())

if __name__ == "__main__":
    print(called_from_main_shield())

In the output, we see that the first called to called_from_main_shield returns False, while the second returns True:

$ python3 introspect.py
Calling introspect
['  File "introspect.py", line 24, in <module>\n    print(called_from_main_shield())\n', '  File "introspect.py", lin
e 7, in called_from_main_shield\n    print(traceback.format_stack())\n']
line=print(called_from_main_shield()) lineno=24 file=introspect.py
found_main_shield=False
False
Calling introspect
['  File "introspect.py", line 27, in <module>\n    print(called_from_main_shield())\n', '  File "introspect.py", lin
e 7, in called_from_main_shield\n    print(traceback.format_stack())\n']
line=print(called_from_main_shield()) lineno=27 file=introspect.py
found_main_shield=True
True

More concise version:

def called_from_main_shield():
    tb = traceback.extract_stack()
    try:
        with open(tb[0].filename, mode="rt") as f:
            found_main_shield = False
            for i, line in enumerate(f):
                if re.search(r"__name__.*['\"]__main__['\"]", line):
                    found_main_shield = True
                if i == tb[0].lineno:
                    return found_main_shield
    except:
        return True

Now, it's not super elegant to use re.search() like I did, but it should be reliable enough. Warning: since I defined this function in my main script, I had to make sure that line didn't match itself, which is why I used ['\"] to match the quotes instead of using a simpler RE like __name__.*__main__. Whatever you chose, just make sure it's flexible enough to match all legal variants of that code, which is what I aimed for.

Answer 2

I think the best you can do is to try execute the code and provide a hint if it fails. Something like this:

# my_module.py
import sys  # Use sys.stderr to print to the error stream.
from multiprocessing import Pool

def f(x):
    return x*x

def my_function(x_max):
    try:    
        with Pool(5) as p:
            print(p.map(f, range(x_max)))
    except RuntimeError as e:
        print("Whoops! Did you perhaps forget to put the code in `if __name__ == '__main__'`?", file=sys.stderr)
        raise e

This is of course not a 100% solution, as there might be several other reasons the code throws a RuntimeError.

---

If it doesn't raise a RuntimeError, an ugly solution would be to explicitly force the user to pass in the name of the module.

# my_module.py
from multiprocessing import Pool

def f(x):
    return x*x

def my_function(x_max, module):
    """`module` must be set to `__name__`, for example `my_function(10, __name__)`"""
    if module == '__main__':    
        with Pool(5) as p:
            print(p.map(f, range(x_max)))
    else:
        raise Exception("This can only be called from the main module.")

And call it as:

# Script_of_a_careless_user.py
from my_module import my_function
my_function(10, __name__)

This makes it very explicit to the user.