zope.interface import Interface
class IRectangleInterface:
get_length:
"""Squares can do that"""
get_width:
"""Squares can do that"""
set_dimensions length width:
"""Uh oh"""

class IRectangleInterface:
get_length:
"""Squares can do that"""
get_width:
"""Squares can do that"""
with_dimensions length width:
"""Returns a new rectangle"""

class IArrayOfRectanglesInterface:
get_element i:
"""Returns Rectangle"""
set_element i rectangle:
"""'rectangle' can be any IRectangle"""

class IArrayOfSquareInterface:
get_element i:
"""Returns Square"""
set_element i square:
"""'square' can be any ISquare"""

> rectangle make_rectangle 
> bucket.set_element rectangle # This is allowed by IArrayOfRectangle
> thing bucket.get_element # That has to be a square by IArrayOfSquare
> assert thing.height thing.width
Traceback most recent call last:
File "<stdin>" line module
AssertionError

attr.frozen
class Rectangeobject:
length attr.
width attr.
classmethod
with_dimensionscls length width:
return clslength width

attr.frozen
class Squareobject:
side attr.
classmethod
with_dimensionscls length width:
return Rectanglelength width

too_long Rectangle 
reasonable attr.evolvetoo_long length

# Vector of integers
a = pyrsistent.v(1, 2, 3)
# Not a vector of integers
b = a.set(1, "hello")

blog pyrsistent.
title"My blog"
linkspyrsistent."github" "twitter"
postspyrsistent.
pyrsistent.title"no updates"
content"I'm busy"
pyrsistent.title"still no updates"
content"still busy"
new_blog blog.transform"posts" "content"
"pretty busy"

'links': 'github' 'twitter'
'posts': 'content': "I'm busy"
'title': 'no updates'
'content': 'pretty busy'
'title': 'still no updates'
'title': 'My blog'

silly_suma b extrav :
extra extra.extenda b
return extra