how to check if a variable is a property

Question

Suppose there is a class with a @property defined:

class MyClass:
   ...
   @property
   def this_is_a_property(self):
       return self.some_thing
   ...
   def this_is_a_function(self, x):
       ...
       return other_thing

Normally, to check whether an attribute is a function, I can use isfunction from the inspect module.

 import inspect
 if inspect.isfunction(MyClass.__dict__['this_is_a_function']):
    print('this_is_a_function',' is a function')

How can I check a property? There appears to be no inspect.isproperty function.

score 8 · Accepted Answer · answered Apr 05 '15 at 18:31

8

Simply check the type against the property object:

if isinstance(MyClass.this_is_a_property, property):

You don't really have to retrieve it from the class dictionary here; looking up a property on the class as an attribute also returns the property instance.

answered Apr 05 '15 at 18:31

Martijn Pieters

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3

yes, though this is not a certain way to test this; the actual instance could be *another* descriptor that just returns a `property` when looked up on a class ;) (far-fetched yes) – Antti Haapala -- Слава Україні Apr 05 '15 at 19:48

vaultah · Answer 2 · 2015-08-30T13:24:35.347

You could use inspect.isdatadescriptor:

Return true if the object is a data descriptor. ... Examples are properties (defined in Python), getsets, and members.

...

CPython implementation detail: getsets are attributes defined in extension modules via PyGetSetDef structures.

...

CPython implementation detail: Member descriptors are attributes defined in extension modules via PyMemberDef structures

Data descriptors are just types that have certain methods. See 3.3.2.1. Implementing Descriptors:

If the descriptor defines __set__() and/or __delete__(), it is a data descriptor; if it defines neither, it is a non-data descriptor.

Non-data descriptors include classmethod and staticmethod (that is, they're not functions, they're types). For example, inspect.isdatadescriptor(MyClass.this_is_a_classmethod) would return False.

On the other hand, property is a data descriptor:

In [6]: inspect.isdatadescriptor(MyClass.this_is_a_property)
Out[6]: True

The downside of using this function is that it may return True if isinstance(mystery, property) is False.

A better way is to check for object type directly:

In [7]: isinstance(MyClass.this_is_a_property, property)
Out[7]: True

`property` is a *type* of data descriptor; any object with both `__get__` and `__set__` methods would qualify; this includes attributes defined with `__slots__`. — Martijn Pieters, Apr 05 '15 at 21:34

how to check if a variable is a property

2 Answers2