Issue:

Having a patchdescr.tmpl causes stg new to fail with a Python exception.

$ stg new test
Error: Unhandled exception:
Traceback (most recent call last):
  File "/Users/tyler/miniforge3/envs/memfault/lib/python3.6/site-packages/stgit/main.py", line 188, in _main
    ret = command.func(parser, options, args)
  File "/Users/tyler/miniforge3/envs/memfault/lib/python3.6/site-packages/stgit/commands/new.py", line 87, in func
    edit=(not options.save_template and options.message is None),
  File "/Users/tyler/miniforge3/envs/memfault/lib/python3.6/site-packages/stgit/commands/common.py", line 471, in update_commit_data
    cd = cd.set_message(cd.message + tmpl)
TypeError: can't concat str to bytes

Current environment:

$ stg --version
Stacked GIT 0.23-45-gb4eb-dirty
git version 2.24.3 (Apple Git-128)
Python version 3.6.9 | packaged by conda-forge | (default, Mar  6 2020, 19:19:02)
[GCC Clang 9.0.1 ]

I've also re-produced this issue with brew install stgitwhich uses python@3.9, as well as on an older version of stgit 0.22.

Fix

The fix should just be to use cd.message_str instead of cd.message.

diff --git a/stgit/commands/common.py b/stgit/commands/common.py
index 1fd50ce..b39f7a3 100644
--- a/stgit/commands/common.py
+++ b/stgit/commands/common.py
@@ -466,7 +466,7 @@ def update_commit_data(cd, message=None, author=None, sign_str=None, edit=False)
     if edit:
         tmpl = templates.get_template('patchdescr.tmpl')
         if tmpl:
-            cd = cd.set_message(cd.message + tmpl)
+            cd = cd.set_message(cd.message_str + tmpl)
         cd = cd.set_message(edit_bytes(cd.message, '.stgit-new.txt'))
     return cd

After patching this, using a template stored in .git/patchdescr.tmpl works again.

I apologize in advance for not putting up a fix myself.