reloading modules and isinstance()

[Diez B. Roggisch]

Tlis wrote:

> I am using a software system with an embedded Python interpreter
> (version 2.3) for scripting. The KcsPoint2D.py module contains a
> Point2D class with the following method:
> 
> def SetFromMidpoint(self, p1, p2):
>    if not isinstance(p1, Point2D) or not isinstance(p2, Point2D):
>       raise TypeError, 'some error message'
>    ---
> 
> The problem is that after I launch my script once, and then use the
> 'Reload modules' button (presumably it calls reload() on all loaded
> modules), the isinstance() tests shown above fails, even though the
> expressions:
> 
> type(self)
> type(p1)
> type(p2)
> 
> all show: class 'KcsPoint2D.Point2D'
> 
> Could you explain, please what is going on? Why the isinstance()
> function returns False?

You just discovered one reason why reload() is a bad idea and IMHO shouldn't
be used at all - as tempting it might be.

Reload will re-execute the importing of the module and perform all necessary
initializations there, overwrite names in the module namespace and such.

But it _won't_ magically change classes or other objects being referred to
from other code that have been created prior to the reloading. Consider
this example:

class A(object):
   foo = 10

a = A()

class A(object):
   foo = 20

print a.foo

>>> 10

All that happens is that the name A is bound to a new class. So all
subsequent instantiations are of the new type. But not the old instances -
because the refer to their class not by NAME, but just have a pointer to
it.

Diez