[Python-ideas] Decorators that execute once rather than every import (was: codec and decorator hacks)

[Steven D'Aprano]

On Thu, Aug 21, 2014 at 11:42:59PM -0700, Gregory P. Smith wrote:
> On Wed, Aug 20, 2014 at 7:25 AM, Donald Stufft <donald at stufft.io> wrote:
> 
> >
> > mypy does have a codec that will ignore annotations on 2.x.
> >
> 
> Note that all codec hacks and decorator hacks have a down side other than
> being hacks: They significantly slow down program import and start time. At
> least the results of the codec hack are cached when a .pyc is generated.
> Decorator executions are not.

Decorator executions are no different from any other function call. They 
occur when you call them. If you only call the decorator once, at the 
top level of the module, they don't get called again when you import the 
module again. That's no different from any other code at the top-level:


[steve at ando ~]$ cat test_deco.py
def decorate(func):
    print("calling an expensive decorator...")
    func.__name__ = "decorated_%s" % func.__name__
    return func

@decorate
def spam(n):
    return "spam"*n

print("%s: %s" % (spam.__name__, spam(3)))

[steve at ando ~]$ python3.3
Python 3.3.0rc3 (default, Sep 27 2012, 18:44:58)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-52)] on linux
Type "help", "copyright", "credits" or "license" for more information.
py> import test_deco as d
calling an expensive decorator...
decorated_spam: spamspamspam
py> import test_deco  # importing again doesn't re-call the decorator
py> test_deco.spam.__name__
'decorated_spam'


> hmm.. could a flavor of decorator that is cacheable in .pyc's be created so
> the cost is only paid once rather than for each time the module is imported?

I'm not sure that you're trying to solve a real problem. Can you give 
an example?



-- 
Steven