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Better Config Support for Python (and More!)

From: andrew cooke <andrew@...>

Date: Fri, 1 Apr 2011 08:55:37 -0300

In http://www.acooke.org/cute/WhydoesntP0.html I was complaining that Python
doesn't have good support for configuration files.  After writing that, I
realised that it would be easy to fix.

So I've written a library, called pytyp, that I'll probably release this
weekend.  It supports simple configuration files, but it's actually much more
general - it provides encoding/decoding (pickling/unpickling) of Python
classes as dictionaries.  You get nice config files when you combine that with
JSON or YAML.

For example:

   >>> from pytyp import load
   >>> 
   >>> config = '''
   ... users:
   ... - name: bob
   ...   email: bob@...
   ... - name: andrew
   ...   email: andrew@...
   ... permission: 
   ...   resource: foo.txt
   ...   rw: r
   ... '''
   >>> 
   >>> class User():
   ...     
   ...     def __init__(self, name, email):
   ...         self.name = name
   ...         self.email = email
   ... 
   >>> 
   >>> class Permission():
   ...     
   ...     def __init__(self, resource, rw):
   ...         self.resource = resource
   ...         self.rw = rw
   ... 
   >>> 
   >>> class Config():
   ...     
   ...     def __init__(self, users:[User], permission:Permission):
   ...         self.users = users
   ...         self.permission = permission
   ... 
   >>> c = load(Config, config)
   >>> c.users[0].name
   'bob'
   >>> c.users[1].email
   'andrew@...'

See how we get real Python classes from a simple text file?! :o)

And we can go in the other direction too:

   >>> from pytyp import JSONEncoder
   >>> JSONEncoder().encode(c)
   '{"users": [{"name": "bob", "email": "bob@..."}, {"name": "andrew",
   >>> "email": "andrew@..."}], "permission": {"rw": "r", "resource":
   >>> "foo.txt"}}'

(I still need to integrate the pretty printing etc).  So you get YAML <-> JSON
translation for free!

Most of this is pretty basic: to serialise classes I just look for attributes
with the same names as variables in the constructor.  But unpacking requires an
extra twist: I use type annotations.  There's no information in the config
file above to say that the users should be modelled as User instances.  That
information is given in the Config constructor:

   def __init__(self, users:[User], permission:Permission): 

where [User] means, quite simply, a list of Users (and permission is a single
Permission instance).  The process is bootstrapped by providing the initial
class Config to the load() method:

   c = load(Config, config)

Andrew

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