Python Introspection
Internal
Overview
Introspection is Python's equivalent for Java reflection. It is the ability to determine the type of an object at runtime and to dynamically examine Python objects.
Introspect Members of an Object Instance
All members of an object instance can be identified using inspect.getmembers(). It its most generic form, without any argument, the function returns all members of an object instance as (name, value) tuples, sorted by name. The result will include the reference to the object instance's class, as __class__, instance attributes, methods, etc.:
import inspect
class SampleClass:
def __init__(self):
self.color = 'blue'
i = SampleClass()
print(f'{"name".ljust(17)} {"value".ljust(83)} value type')
for (name, value) in inspect.getmembers(i):
print(f'{name.ljust(17)} {str(value).ljust(83)} {str(type(value))}')
produces:
name value value type
__class__ <class '__main__.SampleClass'> <class 'type'>
__delattr__ <method-wrapper '__delattr__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__dict__ {'color': 'blue'} <class 'dict'>
__dir__ <built-in method __dir__ of SampleClass object at 0x109144190> <class 'builtin_function_or_method'>
__doc__ None <class 'NoneType'>
__eq__ <method-wrapper '__eq__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__format__ <built-in method __format__ of SampleClass object at 0x109144190> <class 'builtin_function_or_method'>
__ge__ <method-wrapper '__ge__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__getattribute__ <method-wrapper '__getattribute__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__gt__ <method-wrapper '__gt__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__hash__ <method-wrapper '__hash__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__init__ <bound method SampleClass.__init__ of <__main__.SampleClass object at 0x109144190>> <class 'method'>
__init_subclass__ <built-in method __init_subclass__ of type object at 0x7faabab12f60> <class 'builtin_function_or_method'>
__le__ <method-wrapper '__le__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__lt__ <method-wrapper '__lt__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__module__ __main__ <class 'str'>
__ne__ <method-wrapper '__ne__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__new__ <built-in method __new__ of type object at 0x10984c950> <class 'builtin_function_or_method'>
__reduce__ <built-in method __reduce__ of SampleClass object at 0x109144190> <class 'builtin_function_or_method'>
__reduce_ex__ <built-in method __reduce_ex__ of SampleClass object at 0x109144190> <class 'builtin_function_or_method'>
__repr__ <method-wrapper '__repr__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__setattr__ <method-wrapper '__setattr__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__sizeof__ <built-in method __sizeof__ of SampleClass object at 0x109144190> <class 'builtin_function_or_method'>
__str__ <method-wrapper '__str__' of SampleClass object at 0x109144190> <class 'method-wrapper'>
__subclasshook__ <built-in method __subclasshook__ of type object at 0x7faabab12f60> <class 'builtin_function_or_method'>
__weakref__ None <class 'NoneType'>
color blue <class 'str'>
Introspect Members of an Object Instance Filtering with Predicates
inspect.getmembers() accepts a predicate as the second argument. The predicate is a function applied to the value that must return true or false. The inspect package comes with several predefined predicates:
| Predicate | Description |
|---|---|
inspect.isclass |
Return true if the value of the member is a class |
inspect.ismethod |
Return true if the value of the member is an instance method |
inspect.ismodule |
Return true if the value of the member is a module |
import inspect
class SampleClass:
def __init__(self):
self.color = 'blue'
def some_method(self):
print('some_method')
i = SampleClass()
for (name, value) in inspect.getmembers(i, inspect.ismethod):
print(f' method name: {name}')
will display:
method name: __init__ method name: some_method
inspect Standard Library Module
inspect is a Standard Library module.
getattr()
getattr() is a built-in function that returns the value of the specified attribute from the specified object. If an attribute with such name does not exist, an AttributeError exception is thrown.
Class Introspection with getattr()
A specific static method can be identified by querying the class attributes with inspect.getmembers() and selecting the matching method, or by name with getattr() builtin.
cls = ...
method_name = 'some_static_method'
try:
method = getattr(cls, method_name)
print(method)
except AttributeError:
print(f"{method_name} method not found in class {cls}")