Creating different classes dynamically?

Dave Angel d at
Mon Dec 10 01:53:30 CET 2012

On 12/09/2012 07:35 PM, Victor Hooi wrote:
> Hi,
> I have a directory tree with various XML configuration files.
> I then have separate classes for each type, which all inherit from a base. E.g.
>     class AnimalConfigurationParser:
>         ...
>     class DogConfigurationParser(AnimalConfigurationParser):
>         ...
>     class CatConfigurationParser(AnimalConfigurationParser):
>         ....
> I can identify the type of configuration file from the root XML tag.
> I'd like to walk through the directory tree, and create different objects based on the type of configuration file:
>     for root, dirs, files in os.walk('./'):
>         for file in files:
>             if file.startswith('ml') and file.endswith('.xml') and 'entity' not in file:
>                 with open(os.path.join(root, file), 'r') as f:
>                     try:
>                         tree = etree.parse(f)
>                         root = tree.getroot()
>                         print(
>                         print(root.tag)
>                         # Do something to create the appropriate type of parser
>                     except xml.parsers.expat.ExpatError as e:
>                         print('Unable to parse file {0} - {1}'.format(, e.message))
> I have a dict with the root tags - I was thinking of mapping these directly to the functions - however, I'm not sure if that's the right way to do it? Is there a more Pythonic way of doing this?
>     root_tags = {
>                    'DogRootTag': DogConfigurationParser(),
>                     'CatRootTag': CatConfigurationParser(),
>     }
> Cheers,
> Victor

Your subject line says you want to create the classes dynamically, but
that's not what your code implies.  if you just want to decide which
class to INSTANTIATE dynamically, that's easily done, and you have it
almost right.  In your dict you should leave off those parentheses.

Then the parser creation looks something like:
               parser_instance = root_tags[root.tag] (arg1, arg2)

where the arg1, arg2 are whatever arguments the __init__ of these
classes expects.




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