Traceback for import errors in Trac's loader

[Sebastian Krysmanski <sebastian@…>]

A Trac plugin I'm currently writing could not be loaded in Trac. Opening the Trac environment with trac-admin resulted in the following (not very good) error message:

> trac-admin testenv/
12:29:53 Trac[env] INFO: -------------------------------- environment startup [Trac 0.12.1] --------------------------------
12:29:53 Trac[loader] ERROR: Skipping "multitrac.cli = multitrac.cli": (can't import "ImportError: cannot import name MultiTracManager")
cannot import name MultiTracManager
Len:  0
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Trac [/srv/tracsandbox/testenv]> 

The problem was a nasty cyclic dependency problem. I've created a patch that also logs the traceback of ImportErrors so that they can be more easily "debugged". With this patch the output will look like this:

> trac-admin testenv/
13:04:20 Trac[env] INFO: -------------------------------- environment startup [Trac 0.12.1] --------------------------------
13:04:20 Trac[loader] ERROR: Skipping "multitrac.cli = multitrac.cli": import error 
Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/Trac-0.12.1-py2.6.egg/trac/loader.py", line 76, in _load_eggs
    entry.load(require=True)
  File "/usr/lib/python2.6/dist-packages/pkg_resources.py", line 1948, in load
    entry = __import__(self.module_name, globals(),globals(), ['__name__'])
  File "/mnt/hgfs/Linux-Share/MultiTrac/plugins/_Incubator/MultiTracAdminPlugin/multitrac/cli.py", line 6, in <module>
    from multitrac.api.manager_apis import MultiTracManager
  File "/mnt/hgfs/Linux-Share/MultiTrac/plugins/_Incubator/MultiTracAdminPlugin/multitrac/api/manager_apis.py", line 4, in <module>
    from multitrac.model.factory import IItemDescriptorFactory
  File "/mnt/hgfs/Linux-Share/MultiTrac/plugins/_Incubator/MultiTracAdminPlugin/multitrac/model/__init__.py", line 5, in <module>
    from repository import *
  File "/mnt/hgfs/Linux-Share/MultiTrac/plugins/_Incubator/MultiTracAdminPlugin/multitrac/model/repository.py", line 5, in <module>
    from multitrac.api.manager_apis import MultiTracManager
ImportError: cannot import name MultiTracManager
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Trac [/srv/tracsandbox/testenv]> 

[Sebastian Krysmanski <sebastian@…>]

Patch against Trac 0.12.1

[Remy Blank]

Looks good. I know it's an enhancement, but I'm still scheduling it for 0.12.2 because it's a small change and a big help (I have been bitten by the same kind of issue).

[Christian Boos]

If we really want backtraces in this situation, the following patch is enough:

trac/loader.py

@@ -54 +54 @@
                               item, ue)
             elif isinstance(e, UnknownExtra):
                 env.log.error('Skipping "%s": (unknown extra "%s")', item, ue)
-            elif isinstance(e, ImportError):
-                env.log.error('Skipping "%s": (can\'t import "%s")', item, ue)
             else:
                 env.log.error('Skipping "%s": %s)', item,
                               exception_to_unicode(e, traceback=True))

(remove the special case for ImportError).

But I'm not sure if this is really a good idea… most of the time, the import errors are self-explaining, from the name of the module that failed to be imported you can figure out the problem.

[Remy Blank]

Replying to cboos:

If we really want backtraces in this situation, the following patch is enough:

Indeed, that's probably even better.

But I'm not sure if this is really a good idea… most of the time, the import errors are self-explaining, from the name of the module that failed to be imported you can figure out the problem.

That's not my experience. The import error can be due to a module imported by the imported module, or any import along the chain of dependencies. Having the traceback allows identifying the real source of the exception.

[Sebastian Krysmanski <sebastian@…>]

I wasn't sure about this either. The current (old) implementation creates an error message like this:

(can't import "ImportError: cannot import name MultiTracManager")

I guess the implementation of exception_to_unicode() has been changed sometime in the past to include the exception type (like ImportError). So before this change the error message would have read:

(can't import "cannot import name MultiTracManager")

However, this error message is a little bit … strange, too. From the current code​ my guess would be that all error messages will look that "strange".

def _log_error(item, e):
	ue = exception_to_unicode(e)
	if isinstance(e, DistributionNotFound):
		env.log.debug('Skipping "%s": ("%s" not found)', item, ue)
	elif isinstance(e, VersionConflict):
		env.log.error('Skipping "%s": (version conflict "%s")',
					  item, ue)
	elif isinstance(e, UnknownExtra):
                env.log.error('Skipping "%s": (unknown extra "%s")', item, ue)
	elif isinstance(e, ImportError):
		env.log.error('Skipping "%s": (can\'t import "%s")', item, ue)
	else:
		env.log.error('Skipping "%s": %s)', item,
                              exception_to_unicode(e, traceback=True))

[Christian Boos]

Replying to rblank:

Replying to cboos: … from the name of the module that failed to be imported you can figure out the problem.

That's not my experience. The import error can be due to a module imported by the imported module, or any import along the chain of dependencies. Having the traceback allows identifying the real source of the exception.

Well, firing up a python interpreter and redoing that toplevel import will show you the backtrace, but maybe not in some cases… So in the end, please go ahead and do the change if you'd like to ;-)

[Remy Blank]

Patch applied in [10416].