diff --git a/GNUmakefile b/GNUmakefile
index 97fbf1e05f5..9492275a32d 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -236,6 +236,10 @@ ifndef PYTHON
 		ifeq (, $(shell command -v $(PYTHON)))
 			PYTHON:=python
 		endif
+	else
+		# Don't generate __pycache__ files in lib folder, they
+		# can interfere with updates.
+		PYTHON:=$(PYTHON) -B
 	endif
 endif