Enable lazy pipeline mutations via recipe functions

Author: moradology

pangeo_forge_runner/commands/bake.py

@@ -261,7 +261,7 @@ def start(self):
 
                 # if pangeo-forge-recipes is <=0.9, we have to specify a requirements.txt
                 # file even if it isn't present, as the image used otherwise will not have pangeo-forge-recipes
-                if isinstance(recipe, PTransform):
+                if isinstance(recipe, PTransform) or callable(recipe):
                     requirements_path = feedstock.feedstock_dir / "requirements.txt"
                     if requirements_path.exists():
                         extra_options["requirements_file"] = str(requirements_path)
@@ -283,7 +283,9 @@ def start(self):
                 # Chain our recipe to the pipeline. This mutates the `pipeline` object!
                 # We expect `recipe` to either be a beam PTransform, or an object with a 'to_beam'
                 # method that returns a transform.
-                if isinstance(recipe, PTransform):
+                if callable(recipe):
+                    recipe(pipeline)
+                elif isinstance(recipe, PTransform):
                     # This means we are in pangeo-forge-recipes >=0.9
                     pipeline | recipe
                 elif hasattr(recipe, "to_beam"):