Python 3 has removed the comma separated except clause in favor for 'as'. The new syntax is supported since Python 2.6.

try:
   ...
except SomeException, exc:
   ...

becomes

try:
   ...
except SomeException as exc:
   ...

After #5120 has been committed I'm planning to use python-modernize to change all except clauses to the new syntax. The tool is based on lib2to3, which uses a Python grammar-aware parser. The migration is safe and won't introduce bugs.