ishtar2020 wrote: > Hi everyone > > I'm sure this question is kinda stupid and has been answered a few > times before... but I need your help! > > I'm writing a small application where the user can analyze some text > based on a set of changing conditions , and right now I'm stuck on a > point where I'd like to automatically generate new classes that operate > based on those user-defined conditions. > > Is there a way in python to define a class in runtime? For instance, > can I define a class which extends another(that I have previously > defined in some module) , create some instance completely on the fly > and then add/redefine methods to it? > > If affirmative, I've thought of a problem I would maybe have to face: > as the class has been defined by direct input to the python > interpreter, I could only create instances of it on the same session I > entered the definition(because it's not on a module I can load on > future uses) but not afterwards. Is there a way to keep that code? > > Even more newbie paranoia: what would happen if I make that 'on the > fly" object persist via pickle? Where would Python find the code to > handle it once unpickled on another session (once again, i take that no > code with the definition of that instance would exist, as it was never > stored on a module). > > Hope it wasn't too ridiculous an idea. > > Thank you for your time, guys.
It's not ridiculous at all. The easiest way to do what you want would be to build a string containing your new class and then run it with the exec statement: |>> s = 'class foo: pass' |>> exec s |>> foo <class __main__.foo at 0xb7defe9c> If you want to keep your new class around between runs of your script (and if you want to make it pickle-able) then you should write your strings to a .py file (be sure to include an import statement to import the classes you're subclassing) and import that file. If you do that and are going to modify the class(es) during a single session, be sure to call reload() on your new (modified) file (rather than just importing it again) otherwise you won't get the changes. See http://docs.python.org/ref/exec.html for the exec statement and http://docs.python.org/lib/built-in-funcs.html#l2h-59 for the reload() function. Peace, ~Simon -- http://mail.python.org/mailman/listinfo/python-list
