[Python-Dev] Fix import errors to have data

[Jim Fulton]

A common idiom to optionally support some module if it is
present is to use ImportError handlers:

   try:
       import foo
   except ImportError:
       # Configure for absense of foo
       ...
   else:
       # Configure for presense of foo
       ...

Unfortunately, this is a bug trap.  The module foo
might be present and yet it's import could fail with an import error.
This can happen if one of *its* imports fails.  Code like that
above will hide such bugs.

Unfortunately, it's hard to do this correctly, because ImportErrors
don't have the module name as data.  When the import error is raised,
it is raised with an error message rather than data.  This is because
most standard exception classes share a common __str__ that
simply prints their initialization arguments.

At present, to conditionally support a module, you have to
use code like:

   try:
       import foo
   except ImportError, v:
       if not str(v).endsswith(' foo'):
           raise
       # Configure for absense of foo
       ...
   else:
       # Configure for presense of foo
       ...

which is ugly and brittle.

I'd like to get this fixed.

I propose to:

   - Provide ImportError with an __init__ that takes a module name
     and sets a module_name attribute

   - Provide ImportError with an __str__ that produces the message
     we have now

   - Change standard code that raises import errors to provide just the
     module name.

With this change, one could write careful conditional
import code like this:

   try:
       import foo
   except ImportError, v:
       if v.module_name != 'foo':
           raise
       # Configure for absense of foo
       ...
   else:
       # Configure for presense of foo
       ...

which is much cleaner IMO.

Jim

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