Checking an object type

[bvdet]

Python 2.3 Windows XP
We have a builtin module 'point'. I am trying to determine a better way to check the type of a list of 'point' objects. A point object is created:

from point import Point
pt1 = Point(x_coord, y_coord, z_coord)
print type(pt1)
<type 'point'>
print dir(pt1)
['dist']

Original code:

        def chk_type(p_list):
            ret_list = []
            for p in p_list:
                if type(p) ==  type(Point(0,0,0))):
                    ret_list.append(True)
                else:
                    ret_list.append(None)
            return ret_list

        if None not in chk_type([p1, p2, p3]):
            .....do stuff

Improved code:

        def chk_type(p_list):
            for p in p_list:
                if not isinstance(p, type(Point(0,0,0))):
                    return False
            return True     
        
        if chk_type([p1, p2, p3]):
            ........do stuff

This does not work:

if not isinstance(p, Point):

I am confused about what 'type' of object 'pt1' actually is.

[fuffens]

Hi bvdet!

Can you specify what's not working with the isinstance expression? I do not have the point module so I created this simple script

class A:
    pass

a1=A()
aList = [a1, a1, a1]
for a in aList:
    print isinstance(a, A)

Here all a in the list are instances of A. I am using version 2.2.3 of Python.
Best regards
/Fredrik

[bvdet]

Fredrik,

I do not have access to the source code for the point module, so I cannot determine what is going on. Since a point object has a method (pt.dist()), I thought it must be an instance. It's not though. Here's part of a script that I use to check types:

# use to check type of objects
from point import Point, PointLocate

p1 = Point(0.0, 0.0, 0.0)
p2 = Point(1.0, 1.0, 1.0)

print type(p2)
print type(Point)
print type(p1.dist)

print dir(p1)

if isinstance(p1, type(Point(0,0,0))):
    print "Success"
else:
    print "Failure"

if isinstance(p1, object):
    print "SUCCESS"
else:
    print "FAILURE"
    
print    
print "Distance between point p1 and point p2 = %s" % (p1.dist(p2))
print
print "Attribute listing for object 'p1':"
for i in dir(p1):
    try:
        _value_ = eval('p1.' + i)
    except:
        'p1.' + i, "is not a valid attribute or method for this instance."
    else:
        print 'p1.' + i, "=", _value_
        
import types
print isinstance(p1, types.ObjectType)

if isinstance(p1, Point):
    print "Success"
else:
    print "Failure"

Output from the above script:

<type 'point'>
<type 'builtin_function_or_method'>
<type 'builtin_function_or_method'>
['dist']
Success
SUCCESS

Distance between point p1 and point p2 = 1.73205080757

Attribute listing for object 'p1':
p1.dist = <built-in method dist of point object at 0x0470CDC0>
True
Traceback (most recent call last):
  File "C:/SDS2_7.0/macro/Work In Progress/TypeCheck.py", line 42, in ?
    if isinstance(p1, Point):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

The only builtin type that returns true is 'ObjectType'. My code works, but I dislike not knowing what is going on. Thanks for the post Fredrick.

Have a good one!
BV

[fuffens]

This is quite strange. Obviously p1 is an object from the result of isinstance(p1, types.ObjectTyp e) and one would think that Point is a class. But it could be that Point() is an object factory function that returns an object. Unfortunately, type does only say instance and not the type of class.

BR
/Fredrik

[bartonc]

bv, check out the inspect module. There a lots of introspection methods there. I hope it exists in 2.3.

[bvdet]

bv, check out the inspect module. There a lots of introspection methods there. I hope it exists in 2.3.

That's a good thought Barton, but it does not help much. Module 'point' is compiled and embedded in sds2.exe, the main executable file for SDS/2 www.sds2.com.

inspect.getmodulename(Point)

The above call fails, and an odd directory path is listed in the traceback. I found out that module 'point' was written in C and is probably 6 years old. There are several other modules as well. THANKS for your help guys. I will settle on this type check code:

isinstance(pt, type(Point(0,0,0)))