bpo-31536: Avoid wholesale rebuild after `make regen-all` (#3678)

* bpo-31536: Avoid wholesale rebuild after `make regen-all`

* Add NEWS

1 files changed, 28 insertions, 0 deletions

diff --git a/Tools/scripts/update_file.py b/Tools/scripts/update_file.py
new file mode 100644
index 0000000..224585c
--- /dev/null
+++ b/Tools/scripts/update_file.py
@@ -0,0 +1,28 @@
+"""
+A script that replaces an old file with a new one, only if the contents
+actually changed.  If not, the new file is simply deleted.
+
+This avoids wholesale rebuilds when a code (re)generation phase does not
+actually change the in-tree generated code.
+"""
+
+import os
+import sys
+
+
+def main(old_path, new_path):
+    with open(old_path, 'rb') as f:
+        old_contents = f.read()
+    with open(new_path, 'rb') as f:
+        new_contents = f.read()
+    if old_contents != new_contents:
+        os.replace(new_path, old_path)
+    else:
+        os.unlink(new_path)
+
+
+if __name__ == '__main__':
+    if len(sys.argv) != 3:
+        print("Usage: %s <path to be updated> <path with new contents>" % (sys.argv[0],))
+        sys.exit(1)
+    main(sys.argv[1], sys.argv[2])