Best Pythonic Approach to Annotation/Metadata?

[Sparky]

Hello Python community!

I am building a JSON-RPC web application that uses quite a few models.
I would like to return JSON encoded object information and I need a
system to indicate which properties should be returned when the object
is translated to a JSON encoded string. Unfortunately, this
application runs on top of Google's App Engine and, thus, private
attributes are not an option. Also, a preliminary search leads me to
believe that there are no real established ways to annotate variables.
Ideally I want to do something like:

def to_JSON(self):
returnDict = {}
for member in filter(someMethod, inspect.getmembers(self)):
returnDict[member[0]] = member[1]
return json.dumps(returnDict)

I recognize that two solutions would be to 1) include a prefix like
"public_" before variable names or 2) have a list/tuple of attributes
that should be transmitted, simply using the "in" operator. However,
both these options seem like a pretty ungraceful way to do it. Does
anyone else have an idea? Are there any established methods to apply
metadata / annotations to variables in Python or do you believe one of
the above is a good "pythonic" way to solve this problem? I am using
2.6.

Thanks,
Sam

 

[John Krukoff]

On Thu, 2010-07-15 at 12:37 -0700, Sparky wrote:

the above is a good "pythonic" way to solve this problem? I am using
2.6.

Hopefully a helpful correction, but if you're running on google app
engine, you're using python 2.5 on the google side irrespective of what
you're running for development.

 

[Sparky]

On Thu, 2010-07-15 at 12:37 -0700, Sparky wrote:



Hopefully a helpful correction, but if you're running on google app
engine, you're using python 2.5 on the google side irrespective of what
you're running for development.

Sorry about that and thanks for pointing out my mistake there.

Sam

 

[Chris Rebert]

Hello Python community!

I am building a JSON-RPC web application that uses quite a few models.
I would like to return JSON encoded object information and I need a
system to indicate which properties should be returned when the object
is translated to a JSON encoded string. Unfortunately, this
application runs on top of Google's App Engine and, thus, private
attributes are not an option. Also, a preliminary search leads me to
believe that there are no real established ways to annotate variables.
Ideally I want to do something like:

def to_JSON(self):
       returnDict = {}
       for member in filter(someMethod, inspect.getmembers(self)):
               returnDict[member[0]] = member[1]
       return json.dumps(returnDict)

I recognize that two solutions would be to 1) include a prefix like
"public_" before variable names or 2) have a list/tuple of attributes
that should be transmitted, simply using the "in" operator. However,
both these options seem like a pretty ungraceful way to do it. Does
anyone else have an idea? Are there any established methods to apply
metadata / annotations to variables in Python or do you believe one of
the above is a good "pythonic" way to solve this problem?

Those are about as Pythonic as you're going to get; I for one find the
prefix (or similar) solution rather neat (Good luck pulling it off in
a less dynamic language!), but option (2) is probably better in your
particular case.
Remember that at class-definition-time, Python has no idea what
instance variables an object will have, so nothing exists for you to
annotate; hence we're left with solutions of the forms you've given.
You /could/ do something involving decorators and property()s, but
that would be more verbose, more unnecessarily complicated, and less
Pythonic.

Cheers,
Chris