I would like to create a subclass that inherits methods from the arcpy Describe class, as in:

import arcpy

class my_sublass(arcpy.Describe):

However, arcpy.Describe is a function (that returns a class object, according to ESRI), not a class in and of itself. How can I code my_subclass such that it inherits from the Describe class?

For reference, I am in Python 3, using a cloned environment from Pro's default conda distribution.

1 Answer 1


I don't think you can. If you dig into the arcpy source you'll find that arcpy.Describe essentially returns a wrapper around a non-python binary object (see line 369 in <install dir>\ArcGIS\Pro\Resources\ArcPy\arcpy\geoprocessing\_base.py) which doesn't have a class you can inherit from.

However, you could fake a subclass, by creating a stand-alone class that:

  • in __init__, accepts a value to describe then creates and stores the describe instance
  • defines any custom properties or methods you want
  • automatically passes through any attempts to access Describe object properties to the real describe instance and returns the result using a __getattr__ method.

A simple example:

import arcpy

class CustomDescribe(object):

    def __init__(self, thing):
        self._desc = arcpy.Describe(thing)
        self.test_value = 'Hello'

    def __getattr__(self, attr):
        """Pass any other attribute or method calls through to the underlying Describe object"""
        return getattr(self._desc, attr)

    def test_method(self):
        return 'World!'

a = CustomDescribe(r'C:\temp\Default.gdb')

  • I'm just learning about coding with classes, and this seems wonderfully sneaky. I think I understand the logic of __getattr__: it's kind of synthesizing the class tree lookup without directly inheriting attributes of Describe, right? Is that what you mean by "faking" it? Also, why the leading underscore for self._desc?
    – jesnes
    Sep 24, 2019 at 3:57
  • Yes, any call to a property or method that isn't explicitly defined by your class will get passed automatically to __getattr__ (if you define a __getattr__ method). It's not a subclass of Describe, it's just pretending to be, so it's "faking it". The underscore prefix is a hint to programmers/IDEs/code analysers etc.. that a variable or method is intended for internal use, i.e it's private. This convention is defined in PEP 8, though python doesn't enforce it.
    – user2856
    Sep 24, 2019 at 5:51
  • 1
    Ah, now that I'm looking through arcpy source, I see tons of those leading underscores. Why would ESRI code their software like this? Is there an inherent advantage to structuring fake inheritance in this way?
    – jesnes
    Sep 24, 2019 at 13:32
  • Not sure what you mean. Whether a variable is supposed to be private/internal has nothing to do with what I called faking a subclass. The issue with Describe and other objects that arcpy wraps is that they have no class to inherit from, they're just an interface to non-python objects, kind of like what SWIG does en.m.wikipedia.org/wiki/SWIG
    – user2856
    Sep 24, 2019 at 20:24
  • 1
    Oh, because an ArcObject isn't a Python class! Ok, I get it. Thank you for your help, this has been educational in the extreme.
    – jesnes
    Sep 24, 2019 at 21:30

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