Where is WindowsError defined?

Question

I am beginner. Here to learn.

A piece of code is throwing a WindowsError. I catch it and rename it to exception.

I introspect the object, through dir(exception), type(exception), print(exception.__dict__) trying to find a reference to run some documentation command on: e.g. help(areference)`.

I can't find where WindowsError was defined. I tried help(os.WindowsError) but that obviously lead to nowhere.

The ultimate intent is to locate where WindowsError comes from but for this question, as I got the answer from Google somehow, I would like to learn how to find this out through Python commands.

Specifically:

• Get the module in which a type is defined
• Get the supertype of a class I get in an exception (or even... any class?!)

Am I introspecting wrong? Are there better ways to do that?

Answer 1

You probably want to give a look at the inspect module. In particular the inspect.getmodule function.

To obtain the parent classes of a class you can simply look at a class __bases__ attribute:

>>> IOError.__bases__
(<type 'exceptions.EnvironmentError'>,)

Or you can obtain the hierarchy of classes through the __mro__ attribute:

>>> IOError.__mro__
(<type 'exceptions.IOError'>, <type 'exceptions.EnvironmentError'>, <type 'exceptions.StandardError'>, <type 'exceptions.Exception'>, <type 'exceptions.BaseException'>, <type 'object'>)

The __mro__ tells you in which order python looks methods and attributes in the class hierarchy.

Note that, since python allows multiple inheritance, you cannot define the concept of "super-class". Each class can have more than one parent class, and the hierarchy can become a quite complicated graph, and trying to think of super-classes will only lead to pitfalls(e.g. calls to methods fails when the subclasses redefine the signature etc. etc.)

Answer 2

Question 1: Module in which a type is defined

According to my Python, it's defined in the built in module named exceptions:

>>> WindowsError
<type 'exceptions.WindowsError'>

Programmatically you can find out the module in which a type is defined by reading its __module__ attribute:

>>> WindowsError.__module__
'exceptions'
>>> sys.modules[WindowsError.__module__]
<module 'exceptions' (built-in)>

Question 2: Find the superclasses of a type

The __bases__ attribute of a type list its superclasses.

>>> WindowsError.__bases__
(<type 'exceptions.OSError'>,)

Question 3: What is the type of a given object

You didn't actually ask this question, but your comments indicate that you should have done so. To find the type of an object use the type() function.

>>> i = 666
>>> type(i)
<type 'int'>
>>> type(i).__bases__
(<type 'object'>,)
>>> type(i).__module__
'__builtin__'

So, when you catch an exception, you have an object rather than a type, that is you have an instance of the class. So to learn about the type you must first use type() to obtain the type.