Dynamic class loading in Python 2.6: RuntimeWarnin

问题:

I am working on a plugin system where plugin modules are loaded like this:

def load_plugins():
   plugins=glob.glob("plugins/*.py")
   instances=[]
   for p in plugins:
      try:
         name=p.split("/")[-1]
         name=name.split(".py")[0]
         log.debug("Possible plugin: %s", name)
         f, file, desc=imp.find_module(name, ["plugins"])
         plugin=imp.load_module('plugins.'+name, f, file, desc)
         getattr(plugin, "__init__")(log)
         instances=instances+plugin.get_instances()
      except Exception as e:
         log.info("Failed to load plugin: "+str(p))
         log.info("Error: %s " % (e))
         log.info(traceback.format_exc(e))
   return instances

The code works, but for each import statement in the plugin code i get a warning like this:

plugins/plugin.py:2: RuntimeWarning: Parent module 'plugins' not found while handling absolute import
  import os

No errors are reported for the main program code, and the plugins work.

Can somebody explain what the warning means and what I doing wrong. Do I need to create an empty plugins module separately and import it to keep python happy?

回答1:

If the directory plugins were a real package (contained __init__.py fine), you could easily use pkgutils to enumerate its plugin files and load them.

import pkgutil
# import our package
import plugins
list(pkgutil.iter_modules(plugins.__path__))

However, it can work without a plugin package anyway, try this:

import pkgutil
list(pkgutil.iter_modules(["plugins"]))

Also it is possible to make a package that only exists at runtime:

import types
import sys
plugins = types.ModuleType("plugins")
plugins.__path__ = ["plugins"]

sys.modules["plugins"] = plugins
import plugins.testplugin

However that hack that was mostly for fun!

回答2:

If the plugins directory does not have an __init__.py, it isn't a package, so when you create plugins.whatever, Python warns you that such a thing shouldn't really exist. (It couldn't be created by "import plugins.whatever" no matter what your path is.)

Also,

• Don't split on /, which is unportable. Use os.path.split.
• Don't use .split(".py") to get the name without the extension, which is buggy. Use os.path.splitext.
• Don't use getattr with a string literal. getattr(plugin, "__init__") is spelled plugin.__init__.
• I am confused why you are calling a module-level __init__ function. This doesn't seem right. Perhaps you want a "set_logger" function or better, to instantiate a class that takes a logger.
• Don't use L = L + some_other_list to extend a list, use the extend method, which has better performance and is more idiomatic.
• Don't squash unknown exceptions by except Exception. If you cannot plan to do something sane in response to an exception, your program cannot go on sanely.

回答3:

The problem here is with the dot ('.') in the module name:

imp.load_module('plugins.'+name, f, file, desc)

Don't include a '.' after 'plugins', or Python will think it's a module path.

回答4:

You can try adding below statement at the beginning of import statements.

from __future__ import absolute_import

回答5:

The Python imp documentation has been updated since this was answered. It now addresses this issue specifically in the find_module() method.

This function does not handle hierarchical module names (names containing dots). In order to find P.M, that is, submodule M of package P, use find_module() and load_module() to find and load package P, and then use find_module() with the path argument set to P.__path__. When P itself has a dotted name, apply this recipe recursively.

Note that P.__path__ is already a list when supplying it to find_module(). Also note what the find_module() documentation says about finding packages.

If the module is a package, file is None, pathname is the package path and the last item in the description tuple is PKG_DIRECTORY.

So from the OP's question, to import the plugin without the RuntimeError warnings, follow the directions in the updated Python documentation:

# first find and load the package assuming it is in
# the current working directory, '.'

f, file, desc = imp.find_module('plugins', ['.'])
pkg = imp.load_module('plugins', f, file, desc)

# then find the named plugin module using pkg.__path__
# and load the module using the dotted name

f, file, desc = imp.find_module(name, pkg.__path__)
plugin = imp.load_module('plugins.' + name, f, file, desc)