Finding and loading subclasses dynamically. issubclass(x, base_plugin.Plugin) fails.

[Peter Otten]

Osmo Maatta wrote:

> Hello,
> 
> Sub class test fails.
> ======================
> I have a program that needs to load plugin-classes during runtime.
> 
> The program has these subdirectories (modules).
> 
> $ tree
> .
> `-- test.py
> |
> |-- plugins
> |   |-- base_plugin.py
> |   |-- base_plugin.pyc
> |   |-- __init__.py
> |   `-- oca
> |       |-- __init__.py
> |       |-- open_clipart.py
> 
> 
> The plugins (sub directory) contains one or more plugin modules, in this
> test-case there is only one; oca/open_clipart.py.
> 
> The plugins/base_plugin.py (contains class Plugin()) is a base class of
> all plugins.
> 
> I want to list and load all plugin-classes (that inherit from
> base_plugin.Plugin). I have a Python code that successfully browses the
> plugins, but the the test issubclass(x, base_plugin.Plugin) fails.
> 
> Please see this Python code:
> http://futuredesktop.com/tmp/test6.tar.gz
> 
> Why the issubclass(entry, cls) test fails?

base_plugin.py is imported twice, once as plugins.base_plugin and a second 
time as base_plugin. You can see that when you add a bit more debug code 
into your test.py:


--- a/test.py   Sun Aug 29 12:57:25 2010 +0200
+++ b/test.py   Sun Aug 29 12:58:25 2010 +0200
@@ -24,6 +24,8 @@
                 #print "entry's class<%s> -- cls's class=<%s>" % 
(obj1.get_name(), obj2.get_name(), )

                 print "Check %s against %s" % (entry, cls, )
+                print entry.__bases__
+                print cls

                 if issubclass(entry, cls):
                     print "Found a subclass: " + key

$ python test.py
Importing plugins.base_plugin
Importing plugins.oca.open_clipart
Check <class 'plugins.oca.open_clipart.OpenClipArt'> against <class 
'plugins.base_plugin.Plugin'>
(<class 'base_plugin.Plugin'>,)
<class 'plugins.base_plugin.Plugin'>
Got subclasses= []

The solution is to remove all sys.path gymnastics and to adapt your import 
statements accordingly:

$ hg diff
diff -r 9fe6129ba8fc plugins/oca/open_clipart.py
--- a/plugins/oca/open_clipart.py       Sun Aug 29 12:51:51 2010 +0200
+++ b/plugins/oca/open_clipart.py       Sun Aug 29 13:02:55 2010 +0200
@@ -2,9 +2,7 @@
 import os
 import threading, thread

-sys.path.insert(0, '..')
-import base_plugin
-#sys.path.insert(0, '../..')
+from .. import base_plugin

 # ------------------------------------------
 # class OpenClipArt
diff -r 9fe6129ba8fc test.py
--- a/test.py   Sun Aug 29 12:51:51 2010 +0200
+++ b/test.py   Sun Aug 29 13:02:55 2010 +0200
@@ -1,7 +1,6 @@
 import os, sys
 import inspect

-sys.path.insert(0, "plugins")

 # Thanks to http://www.luckydonkey.com/2008/01/02/python-style-plugins-
made-easy/
 def find_subclasses(path, cls):


$ python test.py
Importing plugins.base_plugin
Importing plugins.oca.open_clipart
Check <class 'plugins.oca.open_clipart.OpenClipArt'> against <class 
'plugins.base_plugin.Plugin'>
Found a subclass: OpenClipArt
Got subclasses= [<class 'plugins.oca.open_clipart.OpenClipArt'>]

You should also remove the __init__.py from the folder containing test.py 
which is just begging for the same problem when you import your plugins as 
test.plugins.whatever.

Peter