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Date: Fri, 14 Sep 2012 11:28:22 +0200 (CEST)
From: Jean-Michel Pichavant <jeanmichel@sequans.com>
To: python-list@python.org
In-Reply-To: <f7e2ec78-2c4a-402e-9a90-6cf0e7e55a54@c8g2000pbe.googlegroups.com>
Subject: Re: Decorators not worth the effort
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Cc: alex23 <wuwei23@gmail.com>
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----- Original Message -----
> On Sep 14, 3:54=C2=A0am, Jean-Michel Pichavant <jeanmic...@sequans.com>
> wrote:
> > I don't like decorators, I think they're not worth the mental
> > effort.
>=20
> Because passing a function to a function is a huge cognitive burden?
> --
> http://mail.python.org/mailman/listinfo/python-list
>=20

I was expecting that. Decorators are very popular so I kinda already know t=
hat the fault is mine. Now to the reason why I have troubles writing them, =
I don't know. Every time I did use decorators, I spent way too much time wr=
iting it (and debugging it).

I wrote the following one, used to decorate any function that access an equ=
ipment, it raises an exception when the timeout expires. The timeout is ada=
pted to the platform, ASIC of FPGA so people don't need to specify everytim=
e one timeout per platform.

In the end it would replace=20

def boot(self, timeout=3D15):
    if FPGA:
        self.sendCmd("bootMe", timeout=3Dtimeout*3)
    else:
        self.sendCmd("bootMe", timeout=3Dtimeout)

with

@timeout(15)
def boot(self, timeout=3DNone):
    self.sendCmd("bootMe", timeout)

I wrote a nice documentation with sphinx to explain this, how to use it, ho=
w it can improve code. After spending hours on the decorator + doc, feedbac=
k from my colleagues : What the F... !!=20

Decorators are very python specific (probably exists in any dynamic languag=
e though, I don't know), in some environment where people need to switch fr=
om C to python everyday, decorators add python magic that not everyone is f=
amiliar with. For example everyone in the team is able to understand and de=
bug the undecorated version of the above boot method. I'm the only one capa=
ble of reading the decorated version. And don't flame my colleagues, they'r=
e amazing people (just in case they're reading this :p) who are not python =
developers, more of users.

Hence my original "decorators are not worth the mental effort". Context spe=
cific I must admit.

Cheers,

JM

PS : Here's the decorator, just to give you an idea about how it looks. Sma=
ll piece of code, but took me more than 2 hours to write it. I removed some=
 sensible parts so I don't expect it to run.

class timeout(object):
        """Substitute the timeout keyword argument with the appropriate val=
ue"""
=09FACTORS =3D {
=09=09IcebergConfig().platform.ASIC : 1,
=09=09IcebergConfig().platform.FPGA : 3,
=09=09=09}

=09def __init__(self, asic, fpga=3DNone, palladium=3DNone):
=09=09self.default =3D asic
=09=09self.fpga =3D fpga
=09
=09def _getTimeout(self):
=09=09platform =3D config().platform
=09=09factor =3D self.FACTORS[platform.value]
=09=09timeout =3D {
=09=09=09=09platform.ASIC : self.default*factor,
=09=09=09=09platform.FPGA : self.fpga or self.default*factor,
=09=09=09=09}[platform.value]
=09=09return timeout

=09def __call__(self, func):
=09=09def decorated(*args, **kwargs):
=09=09=09names, _, _, defaults =3D  inspect.getargspec(func)
=09=09=09defaults =3D defaults or []
=09=09=09if 'timeout' not in names:
=09=09=09=09raise ValueError('A "timeout" keyword argument is required')
=09=09=09if 'timeout' not in kwargs: # means the timeout keyword arg is not=
 in the call
=09=09=09=09index =3D names.index('timeout')
=09=09=09=09argsLength =3D (len(names) - len(defaults))
=09=09=09=09if index < argsLength:
=09=09=09=09=09raise NotImplementedError('This decorator does not support n=
on keyword "timeout" argument')
=09=09=09=09if index > len(args)-1: # means the timeout has not be passed u=
sing a pos argument
=09=09=09=09=09timeoutDef =3D defaults[index-argsLength]
=09=09=09=09=09if timeoutDef is not None:
=09=09=09=09=09=09_log.warning("Decorating a function with a default timeou=
t value <> None")
=09=09=09=09=09kwargs['timeout'] =3D self._getTimeout()
=09=09=09else:
=09=09=09=09_log.warning('Timeout value specified during the call, please c=
heck "%s" @timeout decorator.' % func.__name__)
=09=09=09ret =3D func(*args, **kwargs)
=09=09=09return ret
=09=09return decorated