How to do a Decorator Here?

[Gregory Piñero]

Need some decorator help.

I have a class.  And I want to add behavior to one of this class's
methods to be run before the class runs the actual method.  Is this
what decorators are for?

So the class I want to work with is string.Template

Let's say I have this:
from string import Template
a=Template("$var1 is a test")

def preprocess(var1):
   #Real code here will be more complicated, just an example
    var1=var1.upper()

a.substitute(var1="greg")

So how can I have preprocess run before substitute is run?  I want the
user to be able to call a.substitute and have preprocess run
automatically.

Or is this not what decorators do?  I'm trying to avoid subclassing if I can.

Thanks in advance for the help.

-Greg