Q: How to acces a list from a Class constructor, for use in another class method, in python? How do I access a list from a class constructor in python, to use in another class method?
For example:   
Class Data():
    def __init__(self):
        list1 = ["a", "e", "i", "o", "u"]

    def __length__():
        length = len(list1)
        print(length)

To print the length of the current list.

A: Pay attention to the self parameter passed to every instance method. It's the instance itself. It's like this in C++ or Java, just passed explicitly. And you should use it explicitly (type import this if in doubt).
So, as other posters mentioned, you need something like this:
class Data():
  def __init__(self):
    self.vowels = ['a', 'e', 'u', 'i', 'o']

  def __len__(self): # if you want to use len() on your object
    return len(self.vowels) # refer to the list defined in constructor 


A: You need use self attribue, according python code style "Always use self for the first argument to instance methods.", Try this :
class Data():
    def __init__(self):
        self.list1 = ["a", "e", "i", "o", "u"]

    def __length__(self):
        length = len(self.list1)
        print(length)


A: To solve your problem, you should use instance attributes:
class Data():
    def __init__(self):
        self.list1 = ["a", "e", "i", "o", "u"]

    def __length__(self):
        length = len(self.list1)
        print length

The idea here is not that you're "accessing the constructor", but you are accessing an attribute of an instance of this class. The class Data might have attributes: list1, 'list2,vowels,consonants`, you name it. You will want to set these in the constructor, and then you can change or access them in class methods.
But perhaps you could do even better. Consider these two thoughts:


*

*Why do you want to call your method __length__? Perhaps something more descriptive (and less abusive of those underscores) would help e.g. def print_number_of_vowels(self).

*The name Data is a pretty vague, considering making it more specific e.g. AlphabetData.


Perhaps something more like:
class AlphabetData():
    def __init__(self):
        self.vowels = ["a", "e", "i", "o", "u"]

    def print_number_of_vowels(self):
        print len(self.vowels)

