send DeprecationWarning if FONTTOOLS_GPOS_COMPACT_MODE env var is set

and factor out shared code to read the env var

Lib/fontTools/otlLib/builder.py

@@ -12,9 +12,8 @@ from fontTools.ttLib.tables.otBase import (
 from fontTools.ttLib.tables import otBase
 from fontTools.feaLib.ast import STATNameStatement
 from fontTools.otlLib.optimize.gpos import (
+    _compression_level_from_env,
     compact_lookup,
-    GPOS_COMPACT_MODE_DEFAULT,
-    GPOS_COMPACT_MODE_ENV_KEY,
 )
 from fontTools.otlLib.error import OpenTypeLibError
 from functools import reduce
@@ -1415,13 +1414,11 @@ class PairPosBuilder(LookupBuilder):
         # This is a good moment to do it because the compaction should create
         # smaller subtables, which may prevent overflows from happening.
         # Keep reading the value from the ENV until ufo2ft switches to the config system
-        env_level = os.environ.get(GPOS_COMPACT_MODE_ENV_KEY, GPOS_COMPACT_MODE_DEFAULT)
+        level = self.font.cfg.get(
-        if len(env_level) == 1 and env_level in "0123456789":
+            "fontTools.otlLib.optimize.gpos:COMPRESSION_LEVEL",
-            env_level = int(env_level)
+            default=_compression_level_from_env(),
-        else:
+        )
-            raise ValueError(f"Bad {GPOS_COMPACT_MODE_ENV_KEY}={env_level}")
+        if level != 0:
-        level = self.font.cfg.get("fontTools.otlLib.optimize.gpos:COMPRESSION_LEVEL", env_level)
-        if level and level != 0:
             log.info("Compacting GPOS...")
             compact_lookup(self.font, level, lookup)
 

Lib/fontTools/otlLib/optimize/gpos.py

@@ -1,4 +1,5 @@
 import logging
+import os
 from collections import defaultdict, namedtuple
 from functools import reduce
 from itertools import chain
@@ -14,13 +15,30 @@ log = logging.getLogger(__name__)
 
 COMPRESSION_LEVEL = OPTIONS[f"{__name__}:COMPRESSION_LEVEL"]
 
-
 # Kept because ufo2ft depends on it, to be removed once ufo2ft uses the config instead
 # https://github.com/fonttools/fonttools/issues/2592
 GPOS_COMPACT_MODE_ENV_KEY = "FONTTOOLS_GPOS_COMPACT_MODE"
 GPOS_COMPACT_MODE_DEFAULT = str(COMPRESSION_LEVEL.default)
 
 
+def _compression_level_from_env() -> int:
+    env_level = GPOS_COMPACT_MODE_DEFAULT
+    if GPOS_COMPACT_MODE_ENV_KEY in os.environ:
+        import warnings
+
+        warnings.warn(
+            f"'{GPOS_COMPACT_MODE_ENV_KEY}' environment variable is deprecated. "
+            "Please set the 'fontTools.otlLib.optimize.gpos:COMPRESSION_LEVEL' option "
+            "in TTFont.cfg.",
+            DeprecationWarning,
+        )
+
+        env_level = os.environ[GPOS_COMPACT_MODE_ENV_KEY]
+    if len(env_level) == 1 and env_level in "0123456789":
+        return int(env_level)
+    raise ValueError(f"Bad {GPOS_COMPACT_MODE_ENV_KEY}={env_level}")
+
+
 def compact(font: TTFont, level: int) -> TTFont:
     # Ideal plan:
     #  1. Find lookups of Lookup Type 2: Pair Adjustment Positioning Subtable

Lib/fontTools/varLib/merger.py

@@ -16,9 +16,8 @@ from fontTools.varLib.varStore import VarStoreInstancer
 from functools import reduce
 from fontTools.otlLib.builder import buildSinglePos
 from fontTools.otlLib.optimize.gpos import (
-    compact_pair_pos,
+	_compression_level_from_env,
-    GPOS_COMPACT_MODE_DEFAULT,
+	compact_pair_pos,
-    GPOS_COMPACT_MODE_ENV_KEY,
 )
 
 log = logging.getLogger("fontTools.varLib.merger")
@@ -851,13 +850,11 @@ def merge(merger, self, lst):
 		# This is a good moment to do it because the compaction should create
 		# smaller subtables, which may prevent overflows from happening.
 		# Keep reading the value from the ENV until ufo2ft switches to the config system
-		env_level = os.environ.get(GPOS_COMPACT_MODE_ENV_KEY, GPOS_COMPACT_MODE_DEFAULT)
+		level = merger.font.cfg.get(
-		if len(env_level) == 1 and env_level in "0123456789":
+			"fontTools.otlLib.optimize.gpos:COMPRESSION_LEVEL",
-			env_level = int(env_level)
+			default=_compression_level_from_env(),
-		else:
+        )
-			raise ValueError(f"Bad {GPOS_COMPACT_MODE_ENV_KEY}={env_level}")
+		if level != 0:
-		level = merger.font.cfg.get("fontTools.otlLib.optimize.gpos:COMPRESSION_LEVEL", env_level)
-		if level and level != 0:
 			log.info("Compacting GPOS...")
 			self.SubTable = compact_pair_pos(merger.font, level, self.SubTable)
 			self.SubTableCount = len(self.SubTable)