I fixed the error message according to Jelmer's comment. I am not able to use `breezy.global_state` to store this value because that object is not initialized when `config_dir()` is called for the first four times. The advantage of storing an attribute on the `config_dir` function within `config.py` is that it is guaranteed to be available when the function is called.

Here's the experiment I tried:

--- old/breezy/config.py        2017-06-15 01:07:35 +0000
+++ new/breezy/config.py        2017-07-03 18:35:48 +0000
@@ -1405,10 +1405,16 @@

     By default this is %APPDATA%/breezy on Windows, $XDG_CONFIG_HOME/breezy on
     Mac OS X and Linux. If the breezy config directory doesn't exist but
-    the bazaar one (see bazaar_config_dir()) does, use that instead.
+    the bazaar one (see bazaar_config_dir()) does, use that instead, with a
+    warning.

     TODO: Global option --config-dir to override this.
     """
+    # return memo-ized result if we have already been here
+    existing_dir = getattr(breezy.global_state, 'config_dir', None)
+    if existing_dir is not None:
+        return existing_dir
+
     base = osutils.path_from_environ('BRZ_HOME')
     if sys.platform == 'win32':
         if base is None:
@@ -1423,12 +1429,21 @@
             base = osutils.pathjoin(osutils._get_home_dir(), ".config")
     breezy_dir = osutils.pathjoin(base, 'breezy')
     if osutils.isdir(breezy_dir):
+        print "setting global_state.config_dir", breezy_dir
+        breezy.global_state.config_dir = breezy_dir
         return breezy_dir
     # If the breezy directory doesn't exist, but the bazaar one does, use that:
     bazaar_dir = bazaar_config_dir()
     if osutils.isdir(bazaar_dir):
-        trace.mutter(
-            "Using Bazaar configuration directory (%s)", bazaar_dir)
+        sys.stderr.write("Using fallback Bazaar configuration directory (%s) "
+                         "rather than (%s).\n" % (bazaar_dir, breezy_dir))
+        try:
+            print "setting global_state.config_dir", bazaar_dir
+            breezy.global_state.config_dir = bazaar_dir
+        except AttributeError:
+            print "failed to set global_state.config_dir"
+            import traceback; traceback.print_stack()
+            pass
         return bazaar_dir
     return breezy_dir


Gives the following output where it tries to set global_state.config_dir four times unsuccessfully:

$ ./brz --help
Using fallback Bazaar configuration directory (/Users/neil/.bazaar) rather than (/Users/neil/.config/breezy).
setting global_state.config_dir /Users/neil/.bazaar
failed to set global_state.config_dir
File "./brz", line 111, in <module>
    library_state = breezy.initialize()
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/__init__.py", line 228, in initialize
    ui_factory = breezy.ui.make_ui_for_terminal(stdin, stdout, stderr)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/__init__.py", line 528, in make_ui_for_terminal
    stdin = _wrap_in_stream(stdin)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/text.py", line 651, in _wrap_in_stream
    encoding = _get_stream_encoding(stream)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/text.py", line 634, in _get_stream_encoding
    encoding = config.GlobalStack().get('output_encoding')
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 3913, in __init__
    gstore = self.get_shared_store(GlobalStore())
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 3378, in __init__
    config_dir(), possible_transports=possible_transports)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 1445, in config_dir
    import traceback; traceback.print_stack()
Using fallback Bazaar configuration directory (/Users/neil/.bazaar) rather than (/Users/neil/.config/breezy).
setting global_state.config_dir /Users/neil/.bazaar
failed to set global_state.config_dir
  File "./brz", line 111, in <module>
    library_state = breezy.initialize()
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/__init__.py", line 228, in initialize
    ui_factory = breezy.ui.make_ui_for_terminal(stdin, stdout, stderr)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/__init__.py", line 529, in make_ui_for_terminal
    stdout = _wrap_out_stream(stdout)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/text.py", line 659, in _wrap_out_stream
    encoding = _get_stream_encoding(stream)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/text.py", line 634, in _get_stream_encoding
    encoding = config.GlobalStack().get('output_encoding')
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 3913, in __init__
    gstore = self.get_shared_store(GlobalStore())
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 3378, in __init__
    config_dir(), possible_transports=possible_transports)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 1445, in config_dir
    import traceback; traceback.print_stack()
Using fallback Bazaar configuration directory (/Users/neil/.bazaar) rather than (/Users/neil/.config/breezy).
setting global_state.config_dir /Users/neil/.bazaar
failed to set global_state.config_dir
  File "./brz", line 111, in <module>
    library_state = breezy.initialize()
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/__init__.py", line 228, in initialize
    ui_factory = breezy.ui.make_ui_for_terminal(stdin, stdout, stderr)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/__init__.py", line 530, in make_ui_for_terminal
    stderr = _wrap_out_stream(stderr)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/text.py", line 659, in _wrap_out_stream
    encoding = _get_stream_encoding(stream)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/text.py", line 634, in _get_stream_encoding
    encoding = config.GlobalStack().get('output_encoding')
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 3913, in __init__
    gstore = self.get_shared_store(GlobalStore())
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 3378, in __init__
    config_dir(), possible_transports=possible_transports)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 1445, in config_dir
    import traceback; traceback.print_stack()
Using fallback Bazaar configuration directory (/Users/neil/.bazaar) rather than (/Users/neil/.config/breezy).
setting global_state.config_dir /Users/neil/.bazaar
failed to set global_state.config_dir
  File "./brz", line 111, in <module>
    library_state = breezy.initialize()
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/__init__.py", line 228, in initialize
    ui_factory = breezy.ui.make_ui_for_terminal(stdin, stdout, stderr)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/__init__.py", line 531, in make_ui_for_terminal
    return TextUIFactory(stdin, stdout, stderr)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/text.py", line 177, in __init__
    self._progress_view = self.make_progress_view()
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/ui/text.py", line 287, in make_progress_view
    pb_type = config.GlobalStack().get('progress_bar')
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 3913, in __init__
    gstore = self.get_shared_store(GlobalStore())
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 3378, in __init__
    config_dir(), possible_transports=possible_transports)
  File "/Users/neil/proj/brz/warn-bazaar-directory/breezy/config.py", line 1445, in config_dir
    import traceback; traceback.print_stack()
Using fallback Bazaar configuration directory (/Users/neil/.bazaar) rather than (/Users/neil/.config/breezy).
setting global_state.config_dir /Users/neil/.bazaar
Breezy 3.0.0dev1 -- a free distributed version-control tool
[...]

Martin, can you advise on how to proceed with improving the function to do less work and also only print a single warning?