如何通过通用的 pythonic 方法获取 class 实例属性的列表？

Question

The goal is to retrieve a list of class instance attributes. Perferably would like to find some way to do this without having to instantiate an instance of the class (some kind of pre-instantiation inspection). However, this may not be possible. Alternatively, what would be the most pythonic way to do this, given the need to apply it to generic classes.

Here is what I have put together so far and an example of the hurdles to get past to achieve the goal:

First, a basic class and a function that instantiates a class instance, inspects it and returns the attribues successfully:

from typing import List
import inspect
import datetime as dt

class Apple:

    def __init__(self,color:str,size:int) -> None:

        self.log_time = dt.datetime.now()
        self.color = color
        self.size = size

def get_attributes(class_obj:object) -> List[str]:

    '''Returns instantiated class attributes.'''

    params = list(inspect.signature(class_obj).parameters.keys())
    kwargs = {param:None for param in params}
    instance = class_obj(**kwargs)
    attributes = list(vars(instance))

    return attributes

get_attributes(Apple)

>>>

['log_time', 'color', 'size']

This works fine since there are no issues generating attributes (with None populating the parameters) in the init method.

However, for another class:

class Orange:

    def __init__(self,color:str,size:int) -> None:

        self.log_time = dt.datetime.now()
        self.color = color
        self.size = size
        self.another_attribute = size * 2

get_attributes(Orange)

>>>

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[63], line 1
----> 1 get_attributes(Orange)

Cell In[60], line 7, in get_attributes(class_obj)
      5 params = list(inspect.signature(class_obj).parameters.keys())
      6 kwargs = {param:None for param in params}
----> 7 instance = class_obj(**kwargs)
      8 attributes = list(vars(instance))
     10 return attributes

Cell In[62], line 8, in Orange.__init__(self, color, size)
      6 self.color = color
      7 self.size = size
----> 8 self.another_attribute = size * 2

TypeError: unsupported operand type(s) for *: 'NoneType' and 'int'

The deployment of None creates the issue. Entering specific dummy parameters that explicitly permit the class to generate is not the route that I want to take, since this is not a generic enough solution.

Looking for solutions as to how to achieve the goal generically and in the most pythonic manner.

Thanks!

Answer 1

Knowing that general class instance can be dynamically assigned with a new attributes during its life cycle we shouldn't chase an instantiated object with inspect to get its original state.
But in context of inspection of initialization phase (instance varnames/attributes) of potential "designed" instance you can apply the following approach (digging into inspect.getmembers ):

from typing import List
import inspect
import datetime as dt

class Apple:

    def __init__(self,color:str,size:int) -> None:

        self.log_time = dt.datetime.now()
        self.color = color
        self.size = size

def get_init_varnames(class_obj:object) -> List[str]:
    '''Finds instanse varnames/attributes (on initialization phase).'''
    init_code_obj = next(filter(lambda m: m[0] == '__init__', inspect.getmembers(class_obj)))[1]
    init_code = init_code_obj.__code__
    return init_code.co_names[-init_code.co_nlocals:]

print(get_init_varnames(Apple))

---

('log_time', 'color', 'size')