properly deprecate the current behavior of the `default_format` option (#1419)

* ignore magnitude formats for units

* convert non-truthy to empty string

* match longer flags first

i.e. match `cfunits` before `cf`. There might be a better way to do
this, though.

* add a test to check for the deprecation

* consistently split between mspec and uspec

which allows emitting a DeprecationWarning

* correct the explanation of where the default format is coming from

* use `reverse=True` instead of sorting with `-len(x)`

Co-authored-by: Jules Chéron <43635101+jules-ch@users.noreply.github.com>

* make sure the warning is raised correctly

1 files changed, 8 insertions, 2 deletions

diff --git a/pint/formatting.py b/pint/formatting.py
index a04205d..5a458db 100644
--- a/pint/formatting.py
+++ b/pint/formatting.py
@@ -455,14 +455,20 @@ def siunitx_format_unit(units, registry):
 def extract_custom_flags(spec):
     import re
 
-    flag_re = re.compile("(" + "|".join(list(_FORMATTERS.keys()) + ["~"]) + ")")
+    if not spec:
+        return ""
+
+    # sort by length, with longer items first
+    known_flags = sorted(_FORMATTERS.keys(), key=len, reverse=True)
+
+    flag_re = re.compile("(" + "|".join(known_flags + ["~"]) + ")")
     custom_flags = flag_re.findall(spec)
 
     return "".join(custom_flags)
 
 
 def remove_custom_flags(spec):
-    for flag in list(_FORMATTERS.keys()) + ["~"]:
+    for flag in sorted(_FORMATTERS.keys(), key=len, reverse=True) + ["~"]:
         if flag:
             spec = spec.replace(flag, "")
     return spec