Re: A strange behavior for isinstance method

Happy New Year from 2014.

 From the docs for reload():

    When a module is reloaded, its dictionary (containing the module's
    global variables) is retained.

    If a module instantiates instances of a class, reloading the module
    that defines the class does not affect the method definitions of the
    instances — they continue to use the old class definition.


When you reload a module you execute its body afresh. In your case, this 
means that point.Point is redefined to reference a new class object. But 
in your main program you have retained a reference "Point" to the old 
class object and p1, p2, p3 are instances of the old class. So when you 
call __add__ by p2 + p2, you call the add method of the old class.

However, within that class, when make reference to "Point", Python looks 
up the meaning of the symbol in that module's dictionary. This 
dictionary has been refilled by the reload, so it finds the _new_ class 
called Point. Tricky, eh?

If you rewrite as below, you can see it happen between the second and 
third calls.

Jeff

|# point.py||
||# ---------||
||class Point():||
||    def __init__(self, x=0, y=0):||
||        self.x = x||
||        self.y = y||
||
||    def __str__(self):||
||        return "({0},{1})".format(self.x, self.y) ||
||
||    def __add__(self, other):||
||        print 'Inside __add__:'||
||        print '  Type test (Point): ' + str(isinstance(other, Point)) ||
||        print '  Type test (self) : ' + str(isinstance(other, 
self.__class__)) ||
||        x = self.x + other.x ||
||        y = self.y + other.y ||
||        return Point(x, y) ||
||
||
||#Run test in module||
||print '-------------Test inside module ---------------'||
||print 'In point module:'||
||p1 = Point(2,3)||
||p2 = Point(-1,2)||
||print '  Type test (Point): '+ str(isinstance(p2, Point))||
||print '  Type test (p1): '+ str(isinstance(p2, p1.__class__))||
||p3 = p1 + p2||
||print ||
||
||# test_type.py||
||# -------------||
||||import point||
||from point import Point||
||
||reload(point)||
||
||#Run test||
||print '-------------Test outside module ---------------'||
||print 'In __main__ module:'||
||p1 = Point(2,3)||
||p2 = Point(-1,2)||
||print '  Type test (Point): '+ str(isinstance(p2, Point))||
||print '  Type test (p1)   : '+ str(isinstance(p2, p1.__class__))||
||p3 = p1 + p2|

|-------------Test inside module ---------------||
||In point module:||
||  Type test (Point): True||
||  Type test (p1): True||
||Inside __add__:||
||  Type test (Point): True||
||  Type test (self) : True||
||
||-------------Test inside module ---------------||
||In point module:||
||  Type test (Point): True||
||  Type test (p1): True||
||Inside __add__:||
||  Type test (Point): True||
||  Type test (self) : True||
||
||-------------Test outside module ---------------||
||In __main__ module:||
||  Type test (Point): True||
||  Type test (p1)   : True||
||Inside __add__:||
||  Type test (Point): False||
||  Type test (self) : True|

Jeff Allen

On 31/12/2014 07:47, [email protected] wrote:
> Dear all,
>
> Happy new year!
>
> The isinstance method return diffrent value for a certain object with 
> differenct conditions.
> <snip>

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