1. Names. They should mean something. Argument names shouldn't be d or a, more likely user_id_to_instance_dict or something like that.

2. Docstrings. Functions and classes can have documentation strings, you can explain types and structure in them

   class UserRegistry: """ Here we tell what does this class do.

    Fields:
    - some_public_field - this field is used to store such and such data
   
    Private fields:
    - _user_id_to_instance - this is dictionary mapping user ID to User instance.
    """
    <class code>
   

3. Annotations. Python3 allows us to annotate function arguments and output with arbitrary python expression.

   def count_women(self, id_to_instance_dict: "dict( user id -> User instance)") -> int:

   def average_age(self, id_to_instance_dict: dict) -> float:

4. Comments. You can also describe fields with comments: class UserRegistry:

    some_field = {} # dict(str -> int)
   
    def __init__(self):
        self.another_field = [] # list(tuple(str, str))
   

PS. Can someone help me with formatting?

1. Names. They should mean something. Argument names shouldn't be d or a, more likely user_id_to_instance_dict or something like that.

2. Docstrings. Functions and classes can have documentation strings, you can explain types and structure in them

    class UserRegistry:
        """
        Here we tell what does this class do.
   
        Fields:
        - some_public_field - this field is used to store such and such data
   
        Private fields:
        - _user_id_to_instance - this is dictionary mapping user ID to User instance.
        """
        <class code>
   

3. Annotations. Python3 allows us to annotate function arguments and output with arbitrary python expression.

    def count_women(self, id_to_instance_dict: "dict( user id -> User instance)") -> int:
   
    def average_age(self, id_to_instance_dict: dict) -> float:
   

4. Comments. You can also describe fields with comments: class UserRegistry:

    some_field = {} # dict(str -> int)
   
    def __init__(self):
        self.another_field = [] # list(tuple(str, str))
   

PS. Can someone help me with formatting?