Override a class method when it is returned from a function call

Question

I know how to override class methods (an-introduction-to-classes-and-inheritance-in-python) but sometimes you get a class object as an instance returned from a function call (do I say this correctly?) Now want to change one of the methods of such returned instance (or the whole class for that matter). How do I do this? I know one can override instance methods as described in this answer, but it's said that is not a clean way of doing it.

My case:

import pyodbc
import logging

class MyDataBase(object):

    def __init__(self):
        self.db = pyodbc.connect(cnxn_str)
        self.cur = self.db.cursor()
    # Now I want to override the cursor method execute

    def self.cur.execute(self, sql_str):
        logging.debug(sql_str)
        super(cursor.execute)

Obviously this code is not quite right, consider it pseudo code. Question really is how to enhance the execute method for the cursor object. Thanks for your help!

Why do you think you need to do this? Im certain there is a better way. — Daniel Roseman, Oct 13 '14 at 17:37
As the accepted answer to the other question says, what is "not clean" is the *concept* of changing a method on just one instance of a class. If you need to do it, you can do it as shown in another answer to that question, but you're not going to find a "nice" way to do it because it's not really a nice thing to do. You could instead create your own class that wraps `self.cur`. — BrenBarn, Oct 13 '14 at 17:43
@BrenBarn, I'd like to change the whole class, but how do I do if its only returned upon a function call? — Bastiaan, Oct 13 '14 at 19:28
@Bastiaan: Why do you need to change `cur.execute` instead of just giving your MyDataBase class its own `execute` method that internally does the logging and then calls `cur.execute` behind the scenes? — BrenBarn, Oct 13 '14 at 19:30
@BrenBarn I'd like to pass on all other methods of the cursor object including all those that I don't quite know of. But ok, considering there isn't much of an answer I guess that's the only way there is. — Bastiaan, Oct 14 '14 at 02:55
Strange though, all tutorials on class inheritance bloat about how great it is one can add on to existing classes. Now I want to add to pyodbc but can't, just because the class cannot be called directly. — Bastiaan, Oct 14 '14 at 18:48

score 1 · Answer 1 · answered Oct 13 '14 at 17:47

This sounds like a prime case for using decorators, or you could just write a method in MyDataBase that takes the SQL string as an argument, logs it, then calls self.cur.execute. This would be the preferred way, in my opinion, over extending/overriding a third party object.

Override a class method when it is returned from a function call

1 Answers1