fix: use importlib.import_module instead of __import__ (#745)

`import_module` is newer, and provides a couple of advantages over a bare `__import__`:
> The import_module() function acts as a simplifying wrapper around importlib.__import__(). This means all semantics of the function are derived from importlib.__import__(). The most important difference between these two functions is that import_module() returns the specified package or module (e.g. pkg.mod), while __import__() returns the top-level package or module (e.g. pkg).

It's unclear if this will actually fix any problems in the real world, but there's no recent to use `__import__` at this point AFAICT.

Author: bhearsum

src/taskgraph/config.py

@@ -148,7 +148,9 @@ def register(self):
         sys.path.insert(0, self.root_dir)
         register_path = self["taskgraph"].get("register")
         if register_path:
-            find_object(register_path)(self)
+            register = find_object(register_path)
+            assert callable(register)
+            register(self)
 
     @property
     def vcs_root(self):

src/taskgraph/decision.py

@@ -263,9 +263,9 @@ def get_decision_parameters(graph_config, options):
         parameters["optimize_target_tasks"] = options["optimize_target_tasks"]
 
     if "decision-parameters" in graph_config["taskgraph"]:
-        find_object(graph_config["taskgraph"]["decision-parameters"])(
-            graph_config, parameters
-        )
+        decision_params = find_object(graph_config["taskgraph"]["decision-parameters"])
+        assert callable(decision_params)
+        decision_params(graph_config, parameters)
 
     if options.get("try_task_config_file"):
         task_config_file = os.path.abspath(options.get("try_task_config_file"))

src/taskgraph/generator.py

@@ -37,12 +37,14 @@ class Kind:
     config: Dict
     graph_config: GraphConfig
 
-    def _get_loader(self):
+    def _get_loader(self) -> Callable:
         try:
-            loader = self.config["loader"]
+            loader_path = self.config["loader"]
         except KeyError:
-            loader = "taskgraph.loader.default:loader"
-        return find_object(loader)
+            loader_path = "taskgraph.loader.default:loader"
+        loader = find_object(loader_path)
+        assert callable(loader)
+        return loader
 
     def load_tasks(self, parameters, loaded_tasks, write_artifacts):
         loader = self._get_loader()

src/taskgraph/util/python_path.py

@@ -2,11 +2,12 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+import importlib
 import inspect
 import os
 
 
-def find_object(path):
+def find_object(path: str):
     """
     Find a Python object given a path of the form <modulepath>:<objectpath>.
     Conceptually equivalent to
@@ -19,11 +20,10 @@ def find_object(modulepath, objectpath):
         raise ValueError(f'python path {path!r} does not have the form "module:object"')
 
     modulepath, objectpath = path.split(":")
-    obj = __import__(modulepath)
-    for a in modulepath.split(".")[1:]:
-        obj = getattr(obj, a)
+    obj = importlib.import_module(modulepath)
     for a in objectpath.split("."):
         obj = getattr(obj, a)
+
     return obj