initial pushshift/rexport merge implementation

my/core/source.py

@@ -0,0 +1,26 @@
+from typing import Any, Iterator, TypeVar, Callable, Optional, Iterable
+from my.core.warnings import warn
+
+T = TypeVar("T")
+
+# this is probably more generic and results in less code, but is not mypy-friendly
+def import_source(factory: Callable[[], Any], default: Any) -> Any:
+    try:
+        res = factory()
+        return res
+    except ImportError: # presumable means the user hasn't installed the module
+        warn(f"Module {factory.__name__} could not be imported, or isn't configured propertly")
+        return default
+
+
+# For an example of this, see the reddit.all file
+def import_source_iter(factory: Callable[[], Iterator[T]], default: Optional[Iterable[T]] = None) -> Iterator[T]:
+    if default is None:
+        default = []
+    try:
+        res = factory()
+        yield from res
+    except ImportError: # presumable means the user hasn't installed the module
+        warn(f"Module {factory.__name__} could not be imported, or isn't configured propertly")
+        yield from default
+