how to replace some methods with instrumented ones

[Peter Otten]

georgeryoung at gmail.com wrote:

> [python 2.7, linux]
> I have a python app. I cannot modify the file.  But I can import it and
> mess with it.  I need to perform brief tasks before and after some of the
> member functions.
> I'd like to do this in as clear and maintainable way as possible (no third
> party imports).  Here's what I have now(which works):
> 
> from newvbr import *  #get all the stuff from the newvbr app.
> 
> def perf_time(tag):  # I do a little more than this, but you get the idea.
>     print >>sys.stderr, datetime.datetime.now().isoformat() + tag
> 
> def perf(tag):
>     def decorator(fn):
>         def wrapper(*args):
>             perf_time('start %s' % tag)
>             ret = fn(*args)
>             perf_time('end %s' % tag)
>             return ret
>         return wrapper
>     return decorator
> 
> 
> I use this on various members of various classes in the newvbr app:
> 
> old_database_snapshot = VerticaSession.database_snapshot
> @perf('db_snap')
> def new_database_snapshot(self, name):
>     old_database_snapshot(self, name)
> VerticaSession.database_snapshot = new_database_snapshot
> 
> old_object_snapshot = VerticaSession.object_snapshot
> @perf('obj_snap')
> def new_object_snapshot(self, name):
>     old_object_snapshot(self, name)
> VerticaSession.object_snapshot = new_object_snapshot
> 
> main()  #do the usual stuff of the imported app, but with annotations.
> 
> 
> This is still pretty laborious.  How can I make something roughly like:
> 
>    super_tagger(VerticaSession.database_snapshot)
> 
> that does all the above?  I need to tag a bunch of function members like
> this.  I don't insist on using decorators, it just looked like that kind
> of problem.

Remember that

@foo
def bar(...):
   ...

is just syntactic sugar for

def bar(...): ...
bar = foo(bar)

In your case the latter form is more convenient, so instead of

> old_object_snapshot = VerticaSession.object_snapshot
> @perf('obj_snap')
> def new_object_snapshot(self, name):
>     old_object_snapshot(self, name)
> VerticaSession.object_snapshot = new_object_snapshot

use

VerticalSession.object_snapshot = perf("obj_snap")(VerticalSession.object_snapshot)

If you like you can write a helper function along these lines

def wrap_attr(obj, attrname, name=None):
    if name is None: 
        name = attrname
    setattr(obj, attrname
        perf(name)(getattr(obj, attrname)))

wrap_attr(VerticalSession, "object_snapshot")