Python – Using importlib to Dynamically Import Modules with Relative Imports

pythonpython-3.x

I am trying to figure out how to programmatically execute a module that contains relative imports.

psuedo code

spec = importlib.util.spec_from_file_location(name, path)
mod = importlib.util.module_from_spec(spec)
spec.loader.exec_module(mod)

Where name is the class, and path is the absolute path to the .py

When the module that is being loaded contains relative imports, the call to the exec_module throws the following exception:

attempted relative import with no known parent package

Is there a way to programmatically execute a python module that itself contains relative imports? If so, how?

Best Answer

Your code works fine for me, as is.

One possible issue is what the value of the name you're using is. In order for relative imports to work, you need to fully specify the module name (e.g. name = "package1.package2.mymodule").

For example:

runimport.py

import importlib
import os

name = "testpack.inside" # NOT "inside"

spec = importlib.util.spec_from_file_location(name, 
    os.path.join(os.path.dirname(__file__), 'testpack/inside.py'))
mod = importlib.util.module_from_spec(spec)
spec.loader.exec_module(mod)

testpack/init.py

# empty

testpack/inside.py

from . import otherinside
print('I got', otherinside.data)

testpack/otherinside.py

data = 'other inside'

Now, python3 runimport.py prints "I got other inside". If you replace the name with "inside", it throws the error you describe.

Related Solutions

Python 3.5+ – Dynamically Import Module by File Path

The easiest solution I could come up with is to temporarily modify sys.path in the function doing the import:

from contextlib import contextmanager

@contextmanager
def add_to_path(p):
    import sys
    old_path = sys.path
    sys.path = sys.path[:]
    sys.path.insert(0, p)
    try:
        yield
    finally:
        sys.path = old_path

def path_import(absolute_path):
   '''implementation taken from https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly'''
   with add_to_path(os.path.dirname(absolute_path)):
       spec = importlib.util.spec_from_file_location(absolute_path, absolute_path)
       module = importlib.util.module_from_spec(spec)
       spec.loader.exec_module(module)
       return module

This should not cause any problems unless you do imports in another thread concurrently. Otherwise, since sys.path is restored to its previous state, there should be no unwanted side effects.

Edit:

I realize that my answer is somewhat unsatisfactory but, digging into the code reveals that, the line spec.loader.exec_module(module) basically results in exec(spec.loader.get_code(module.__name__),module.__dict__) getting called. Here spec.loader.get_code(module.__name__) is simply the code contained in lib.py.

Thus a better answer to the question would have to find a way to make the import statement behave differently by simply injecting one or more global variables through the second argument of the exec-statement. However, "whatever you do to make the import machinery look in that file's folder, it'll have to linger beyond the duration of the initial import, since functions from that file might perform further imports when you call them", as stated by @user2357112 in the question comments.

Unfortunately the only way to change the behavior of the import statement seems to be to change sys.path or in a package __path__. module.__dict__ already contains __path__ so that doesn't seem to work which leaves sys.path (Or trying to figure out why exec does not treat the code as a package even though it has __path__ and __package__ ... - But I don't know where to start - Maybe it has something to do with having no __init__.py file).

Furthermore this issue does not seem to be specific to importlib but rather a general problem with sibling imports.

Edit2: If you don't want the module to end up in sys.modules the following should work (Note that any modules added to sys.modules during the import are removed):

from contextlib import contextmanager

@contextmanager
def add_to_path(p):
    import sys
    old_path = sys.path
    old_modules = sys.modules
    sys.modules = old_modules.copy()
    sys.path = sys.path[:]
    sys.path.insert(0, p)
    try:
        yield
    finally:
        sys.path = old_path
        sys.modules = old_modules