[fontc] Code review and fixups

Lib/gftools/builder/fontc.py

@@ -1,7 +1,10 @@
 """functionality for running fontc via gftools
 
-This mostly exists so that we can keep as much of the fontc logic in one place,
-and not need to dirty up anything else.
+gftools has a few special flags that allow it to use fontc, an alternative
+font compiler (https://github.com/googlefonts/fontc).
+
+This module exists to keep the logic related to fontc in one place, and not
+dirty up everything else.
 """
 
 from argparse import Namespace
@@ -12,14 +15,13 @@
 
 
 class FontcArgs:
-    simple_output_path: Union[Path, None]
-    fontc_bin_path: Union[Path, None]
-    single_source: Union[str, None]
-
     # init with 'None' returns a default obj where everything is None
     def __init__(self, args: Union[Namespace, None]) -> None:
         if not args:
-            return None
+            self.simple_output_path = None
+            self.fontc_bin_path = None
+            self.single_source = None
+            return
         self.simple_output_path = abspath(args.experimental_simple_output)
         self.fontc_bin_path = abspath(args.experimental_fontc)
         self.single_source = args.experimental_single_source

Lib/gftools/builder/operations/fontc/__init__.py

@@ -2,21 +2,22 @@
 from typing import List
 from gftools.builder.operations import OperationBase
 
-FONTC_PATH = "FONTC_PATH_NOT_SET"
+_FONTC_PATH = None
 
 
 # should only be called once, from main, before doing anything else. This is a
 # relatively non-invasive way to smuggle this value into FontcOperationBase
 def set_global_fontc_path(path: Path):
-    global FONTC_PATH
-    FONTC_PATH = path
+    global _FONTC_PATH
+    assert _FONTC_PATH is None, "set_global_fontc_path should only be called once"
+    _FONTC_PATH = path
 
 
 class FontcOperationBase(OperationBase):
     @property
     def variables(self):
         vars = super().variables
-        vars["fontc_path"] = FONTC_PATH
+        vars["fontc_path"] = _FONTC_PATH
         args = vars.get("args")
         if args:
             vars["args"] = rewrite_fontmake_args_for_fontc(args)