Do built-in Python classes have attributes, and if so, how do I find those that are available to the class?

This feels like a simple question, but I can't seem to figure out the answer after much searching. I'm wondering if, for instance, lists have attributes. By attributes I mean values that are accessed by dot notation (not methods). Do strings have them?

If I assign a string value to a variable:

test = 'test'

I tried dir(test), which returned a long list that included stuff like:

['__add__',
 '__class__',
 '__contains__',
 '__delattr__',
 '__dir__',
 '__doc__',
 '__eq__',
 '__format__',
 '__ge__',
 '__getattribute__',
 '__getitem__',
 '__getnewargs__',
 '__gt__',
 '__hash__',
 '__init__',
 '__init_subclass__',
 '__iter__',
 '__le__',
 '__len__',
 'capitalize',
 'casefold',
 'center',
 'count',
 'encode',
 'endswith',
 'expandtabs',
 'find',
 'format',
 'format_map',
 'index',
 ...]

(Note that I cut items off this list to abridge it.) What are the items with underscores? The other items seem to be methods. Are there any attributes? How would I identify an attribute?

Same question for instances of the list class. How do I see all the available attributes of the class?

Both methods and attributes are considered attributes in python, dir() lists them all. You can check what they are by doing

test = 'test'
for attr in dir(test):
  print(f"{attr} : {type(getattr(test,attr))}")

Try it on this example, it will be more clear:

class TestClass:
  def __init__(self,a):
    self.a = a    
  def add_b(self, b):
    return self.a + b

test = TestClass(10)
for attr in dir(test):
  print(f"{attr} : {type(getattr(test,attr))}")

I will add to the previous answer. The underscore methods are known as dunder methods (double underscore). Double underscore is used to prevent name collision with any functions names that a programmer might use.

These methods are usually called by python interpreter. For example __init__ is called by python during object instantiation. You can override these methods to change the functionality of the object. For example you can override __add__ method to add summation functionality to custom data types.