#+TITLE: Importing a file as module

I want to import a file (maybe generated at run-time, maybe even the
name only known at run-time) that does not exist in =sys.path=. In a
couple of previous jobs that has happened with Python bindings
generated from protocol descriptions. In the past I've done that by
temporarily adding the parent to =sys.path= then running
=importlib.import_module=, while being annoyed there is no function
=import_file(name: str, file_path: Path) -> ModuleType=. It turns out
that [[https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly][importlib.util]] has all the bits necessary, it's just a matter of
putting them together:

#+BEGIN_SRC python
  import importlib.util
  import os
  import sys
  from types import ModuleType

  def import_file(module_name: str, file_path: os.PathLike) -> ModuleType:
      spec = importlib.util.spec_from_file_location(module_name, file_path)
      # spec: ModuleSpec(name='bar', loader=<_frozen_importlib_external.SourceFileLoader object at 0x7f30b1d10310>, origin='/tmp/tmp.WpcbUEkm32/foo.py')
      # if spec_from_file_location fails, it returns None rather than raise an exception
      if spec is None:
	  raise ModuleNotFoundError(f"Can't import {module_name} from {file_path}")
      if spec.loader is None:
	  raise ModuleNotFoundError(f"Can't import {module_name} from {file_path}")
      module = importlib.util.module_from_spec(spec)
      sys.modules[module_name] = module
      spec.loader.exec_module(module)
      return module
#+END_SRC

I suppose it is optional to add the module to =sys.modules=. If we do,
that will allow future =import module_name= to find the same module
(which might even be useful for the purpose of injecting different
code into a 3rd party library...). If we don't, then we get a new copy
of the module every time we import it again. Eg. if
=/tmp/tmp.WpcbUEkm32/foo.py= defined a global:

#+BEGIN_SRC python
MODULE_GOBAL = 1

def myname():
    return __name__
#+END_SRC

And I try to imort it twice under the same name, same path, the module
globals are disconnected:

#+BEGIN_SRC python
  foo1 = import_file("foo", "/tmp/tmp.WpcbUEkm32/foo.py")
  foo2 = import_file("foo", "/tmp/tmp.WpcbUEkm32/foo.py")

  foo1.myname()
  # 'foo'
  foo2.myname()
  # 'foo'

  foo1.MODULE_GOBAL += 1
  print(foo1.MODULE_GOBAL)
  # 2, as expected
  print(foo2.MODULE_GOBAL)
  # 1, unmodified
#+END_SRC