too dynamic - how to avoid?

[mic]

I've spent last hour trying to debug my code, when I found out that instead
of executing object's method I accidentaly overridden it with variable (see
below example). My stupid! But as the system becomes more complex, I begun
to wonder how to avoid such situations in the future. Does anybody have some
experiences about that?

Simple example:

class test:
    def a(self, value):
        print value
t = test()
t.a('my test') <= returns obviously 'my test'
t.a = 'my test' <= creates object's attribute a, so I'm loosing my method

Regards,

Michal