getting attributes and methods of class without creating object

[shuvro]

Suppose I have a class like this -

class myClass(object):

def __init__(self):
self.a = 10
self.b = 20

def my_method(self,var = 20):
self.local_var = var

I want to know about its method(__init__ and my_method) and
variables(a,b, local_var) without creating the object of it. I tried
getmembers function of inspect module. But it can do this with an
object of myClass as argument. Like

import inspect
var = myClass()
inspect.getmembers(var)

I have to know about this without creating the object of myClass.
Can anyone help me please?

 

[Patrick Maupin]

Suppose I have a class like this -

class myClass(object):

    def __init__(self):
        self.a = 10
        self.b = 20

    def my_method(self,var = 20):
        self.local_var = var

I want to know about its method(__init__ and my_method) and
variables(a,b, local_var) without creating the object of it. I tried
getmembers function of inspect module. But it can do this with an
object of myClass as argument. Like

import inspect
var = myClass()
inspect.getmembers(var)

I have to know about this without creating the object of myClass.
Can anyone help me please?

Well, you can do the same thing with myClass itself:

inspect.getmembers(myClass)

But that won't show you a,b, or local_var, because those don't belong
to the class. They only belong to class instances, and maybe even not
all class instances. (For example, if my_method is not called, then
the instance won't have local_var defined.)

Python is a very dynamic language, and class instances (and even the
classes themselves!) can be modified at any time during execution.
You don't even have to be executing inside a class's function to add
stuff to it:

class foo:
pass

bar = foo()
bar.a = 3


So to "know" about the attributes of an instance of a class without
actually creating that instance is very difficult. You didn't specify
why you want to do this, but often the reason is to try to catch
potential errors without actually running code. This is typically
called "linting", and there are a few Python packages designed to do
this, such as pylint and pychecker. Probably others, but I don't know
anything about them, because I find that in most cases, the best way
to test or to reason about Python code is to actually run it.

Regards,
Pat

 

[shuvro]

Well, you can do the same thing with myClass itself:

inspect.getmembers(myClass)

But that won't show you a,b, or local_var, because those don't belong
to the class.  They only belong to class instances, and maybe even not
all class instances.  (For example, if my_method is not called, then
the instance won't have local_var defined.)

Python is a very dynamic language, and class instances (and even the
classes themselves!) can be modified at any time during execution.
You don't even have to be executing inside a class's function to add
stuff to it:

class foo:
    pass

bar = foo()
bar.a = 3

So to "know" about the attributes of an instance of a class without
actually creating that instance is very difficult.  You didn't specify
why you want to do this, but often the reason is to try to catch
potential errors without actually running code.  This is typically
called "linting", and there are a few Python packages designed to do
this, such as pylint and pychecker.  Probably others, but I don't know
anything about them, because I find that in most cases, the best way
to test or to reason about Python code is to actually run it.

Regards,
Pat

Thanks Pat for your informative reply.
Actually what I am trying to do is to write an external exporter for
the data types an open source software(Blender). So, I have to collect
the properties of the classes.
Here, I have no chance to edit the class and add inspect codes there,
neither I can create objects of the
existing classes (that will do some additional things which I not
want).
Is it not possible with the existing python facilities (without adding
external libraries) ?


Shuvro

 

[Terry Reedy]

Is it not possible with the existing python facilities (without adding
external libraries) ?

To know what a function would do if you were to call it, without
actually calling it, look at the function code with the dis (assembler)
module.

#3.1 def __init__(self, a): self.a = a
2 0 LOAD_FAST 1 (a)
3 LOAD_FAST 0 (self)
6 STORE_ATTR 0 (a)
9 LOAD_CONST 0 (None)
12 RETURN_VALUE

If you read the doc, or know assembler, you would see that the instance
gets attribute a. Use dir(C) to find all the methods to disassemble.

Terry Jan Reedy