Custom function decorators with TurboGears 2
I am exposing some library functions using a TurboGears2 controller (see
web-api-with-turbogears2). It turns out that some functions return a dict,
some a list, some a string, and TurboGears 2 only allows JSON serialisation for
dicts.
A simple work-around for this is to wrap the function result into a dict,
something like this:
@expose("json")
@validate(validator_dispatcher, error_handler=api_validation_error)
def list_colours(self, filter=None, productID=None, maxResults=100, **kw):
# Call API
res = self.engine.list_colours(filter, productID, maxResults)
# Return result
return dict(r=res)
It would be nice, however, to have an
@webapi() decorator that
automatically wraps the function result with the dict:
def webapi(func):
def dict_wrap(*args, **kw):
return dict(r=func(*args, **kw))
return dict_wrap
# ...in the controller...
@expose("json")
@validate(validator_dispatcher, error_handler=api_validation_error)
@webapi
def list_colours(self, filter=None, productID=None, maxResults=100, **kw):
# Call API
res = self.engine.list_colours(filter, productID, maxResults)
# Return result
return res
This works, as long as @webapi appears
last in the list of decorators.
This is because if it appears last it will be the first to wrap the function,
and so it will not interfere with the
tg.decorators machinery.
Would it be possible to create a decorator that can be put anywhere among the
decorator list? Yes, it is possible but tricky, and it gives me the feeling
that it may break in any future version of TurboGears:
class webapi(object):
def __call__(self, func):
def dict_wrap(*args, **kw):
return dict(r=func(*args, **kw))
# Migrate the decoration attribute to our new function
if hasattr(func, 'decoration'):
dict_wrap.decoration = func.decoration
dict_wrap.decoration.controller = dict_wrap
delattr(func, 'decoration')
return dict_wrap
# ...in the controller...
@expose("json")
@validate(validator_dispatcher, error_handler=api_validation_error)
@webapi
def list_colours(self, filter=None, productID=None, maxResults=100, **kw):
# Call API
res = self.engine.list_colours(filter, productID, maxResults)
# Return result
return res
As a convenience, TurboGears 2 offers, in the
decorators module, a way to
build decorator "hooks":
class before_validate(_hook_decorator):
'''A list of callables to be run before validation is performed'''
hook_name = 'before_validate'
class before_call(_hook_decorator):
'''A list of callables to be run before the controller method is called'''
hook_name = 'before_call'
class before_render(_hook_decorator):
'''A list of callables to be run before the template is rendered'''
hook_name = 'before_render'
class after_render(_hook_decorator):
'''A list of callables to be run after the template is rendered.
Will be run before it is returned returned up the WSGI stack'''
hook_name = 'after_render'
The way these are invoked can be found in the
_perform_call function in
tg/controllers.py.
To show an example use of those hooks, let's add a some polygen wisdom to every
data structure we return:
class wisdom(decorators.before_render):
def __init__(self, grammar):
super(wisdom, self).__init__(self.add_wisdom)
self.grammar = grammar
def add_wisdom(self, remainder, params, output):
from subprocess import Popen, PIPE
output["wisdom"] = Popen(["polyrun", self.grammar], stdout=PIPE).communicate()[0]
# ...in the controller...
@wisdom("genius")
@expose("json")
@validate(validator_dispatcher, error_handler=api_validation_error)
def list_colours(self, filter=None, productID=None, maxResults=100, **kw):
# Call API
res = self.engine.list_colours(filter, productID, maxResults)
# Return result
return res
These hooks cannot however be used for what I need, that is, to wrap the result
inside a dict. The reason is because they are called in this way:
controller.decoration.run_hooks(
'before_render', remainder, params, output)
and not in this way:
output = controller.decoration.run_hooks(
'before_render', remainder, params, output)
So it is possible to modify the output (if it is a mutable structure) but not
to exchange it with something else.
Can we do even better? Sure we can. We can assimilate
@expose and
@validate
inside
@webapi to avoid repeating those same many decorator lines over and
over again:
class webapi(object):
def __init__(self, error_handler = None):
self.error_handler = error_handler
def __call__(self, func):
def dict_wrap(*args, **kw):
return dict(r=func(*args, **kw))
res = expose("json")(dict_wrap)
res = validate(validator_dispatcher, error_handler=self.error_handler)(res)
return res
# ...in the controller...
@expose("json")
def api_validation_error(self, **kw):
pylons.response.status = "400 Error"
return dict(e="validation error on input fields", form_errors=pylons.c.form_errors)
@webapi(error_handler=api_validation_error)
def list_colours(self, filter=None, productID=None, maxResults=100, **kw):
# Call API
res = self.engine.list_colours(filter, productID, maxResults)
# Return result
return res
This got rid of
@expose and
@validate, and provides
almost all the
default values that I need. Unfortunately I could not find out how to access
api_validation_error from the decorator so that I can pass it to the
validator, therefore I remain with the inconvenience of having to explicitly
pass it every time.
This is both a release announcement for the next installment of The
MirBSD Korn Shell, mksh R40b,
and a follow-up to Sune s
article about small tools of various degrees of usefulness.
I hope I don t need to say too much about the first part; mksh(1)
is packaged in a gazillion of operating environments (dear Planet
readers, that of course includes Debian, which occasionally
gets a development snapshot; I ll wait uploading R40c until that two
month fixed gcc bug will finally find its way into the
packages for armel and armhf. Ah, we re getting Arch Linux (after
years) to include mksh now. (Probably because they couldn t stand
the teasing that Arch Hurd included it one day after having
been told about its existence, wondering why it built without needing
patches on Hurd ) MSYS is a supposedly supported target now, people
are working on WinAPI and DJGPP in their spare time, and Cygwin and
Debian packagers have deprecated pdksh in favour of mksh (thanks!).
So, everything looking well on that front.
I ve started a
collection of shell snippets some time ago, where most of those
small things of mine ends up. Even stuff I write at work we re an
Open Source company and can generally publish under (currently) AGPLv3
or (if extending existing code) that code s licence. I chose git as
SCM in that FusionForge instance so that people would hopefully use it
and contribute to it without fear, as it s hosted on my current money
source s servers. (Can just clone it.) Feel free to register and ask
for membership, to extend it (only if your shell-fu is up to the task,
KNOPPIX-style scripts would be a bad style(9) example as the primary
goal of the project is to give good examples to people who learn shell
coding by looking at other peoples code).
Maybe you like my editor, too? At
OpenRheinRuhr, the Atari people sure liked it as it uses WordStar
like key combinations, standardised across a lot of platforms and
vendors (DR DOS Editor, Turbo Pascal, Borland C++ for Windows, )
ObPromise: a posting to raise the level of ferrophility
on the Planet aggregators this wlog reaches (got pix)
Last night I wanted to buy 
After recent switch of this blog to Django, I've also planned to switch rest of my website to use same engine. Most of the code is already written, however I've found some forgotten parts, which nobody used for really a long time.
As most of the things there is free software, I don't like idea removing it completely, however it is quite unlikely that somebody will want to use these ancient things. Especially when it quite lacks documentation and I really forgot about that (most of them being Turbo Pascal things for DOS, Delphi components for Windows and so on). Anyway it will probably live only on 
I'm a terrible hoarder. I hang onto old stuff because I think it might be
fun to have a look at again later, when I've got nothing to do. The problem
is, I never have nothing to do, or when I do, I never think to go through
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Today's example: the 3.5" floppy disk. I've got a disk holder thingy with
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time.
In the spirit of 
H&R Block is like a KFC for your chicken.
No, not really.
This has robbed me of several days of my life, so I want to bring Google juice this this problem.
IF you have a Pylons or TurboGears application or anything else that uses the fantastic EvalException WSGI middleware for web debugging of you web program and have the following symptom:
* on a crash the traceback page shows up, but it has no css style and no images
Site was down for almost a week, due to my 
) will come to the rescue.
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