[libvirt] [sandbox PATCH 03/11] Image: Add Hooking Mechanism

[Daniel P. Berrange]

On Thu, Jul 23, 2015 at 03:57:29PM +0000, Eren Yagdiran wrote:
> Any custom source provider can be added to virt-sandbox-image as a source
> ---
>  virt-sandbox-image/sources/Source.py     |  8 ++++++++
>  virt-sandbox-image/virt-sandbox-image.py | 30 +++++++++++++++++++++++++++++-
>  2 files changed, 37 insertions(+), 1 deletion(-)
>  create mode 100644 virt-sandbox-image/sources/Source.py
> 
> diff --git a/virt-sandbox-image/sources/Source.py b/virt-sandbox-image/sources/Source.py
> new file mode 100644
> index 0000000..cfc75d3
> --- /dev/null
> +++ b/virt-sandbox-image/sources/Source.py
> @@ -0,0 +1,8 @@
> +#!/usr/bin/python
> +
> +from abc import ABCMeta, abstractmethod
> +
> +class Source():
> +    __metaclass__ = ABCMeta
> +    def __init__(self):
> +        pass
> diff --git a/virt-sandbox-image/virt-sandbox-image.py b/virt-sandbox-image/virt-sandbox-image.py
> index 324e568..99ed46e 100644
> --- a/virt-sandbox-image/virt-sandbox-image.py
> +++ b/virt-sandbox-image/virt-sandbox-image.py
> @@ -1,5 +1,5 @@
>  #!/usr/bin/python -Es
> -#
> +# -*- coding: utf-8 -*-
>  # Authors: Daniel P. Berrange <berrange at redhat.com>
>  #          Eren Yagdiran <erenyagdiran at gmail.com>
>  #
> @@ -38,6 +38,34 @@ default_template_dir = "/var/lib/libvirt/templates"
>  debug = True
>  verbose = True
>  
> +sys.dont_write_bytecode = True
> +
> +
> +##Hook mechanism starts##
> +import __builtin__
> +from sources.Source import Source
> +__builtin__.hookHolder = {}
> +def add_hook(driverName,clazz):
> +    holder = __builtin__.hookHolder
> +    if not issubclass(clazz,Source):
> +        raise Exception("Loading %s failed. Make sure it is a subclass Of %s" %(clazz,Source))
> +    holder[driverName] = clazz
> +
> +def init_from_name(name):
> +    holder = __builtin__.hookHolder
> +    return holder.get(name,None)
> +
> +__builtin__.add_hook = add_hook
> +__builtin__.init_from_name = init_from_name
> +from sources import *
> +
> +def dynamic_source_loader(name):
> +    obj = init_from_name(name)
> +    if obj == None:
> +        raise IOError
> +    return obj()
> +##Hook mechanism ends

I think this can be a hell of alot simpler if we just define
a fixed convention for module and class naming.
eg something like this:

  import importlib

  def dynamic_source_loader(name)
      modname = "sources." + name
      mod = importlib.import_module(modname)
      classname = name[0].upper() + name[1:] + "Source"
      classimpl = mod.getattr(classname)
      return classimpl()



Regards,
Daniel
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