Auto-Vivification in Python

We saw in the earlier post what is auto-vivification and how we can implement it in JavaScript.
Can we do it in Python as well? Yes, Sure.

Python does not have built in auto-vivification

Let's see if it supports it

	univ = {}
	univ.college.stream.year = 1
	''' Output
	univ.college.stream.year = 1
	AttributeError: 'dict' object has no attribute 'college'
	'''

view raw NoAutoViv.py hosted with ❤ by GitHub

No AutoViv in Python

Nope!
So what can we do?
Well, Python is a dynamic language, which allows us not only to add properties (attributes) at runtime but we can also ‘intercept’ calls to attributes which do not exist.

	class Univ:
	def __init__(self):
	# define an attribute
	self.name = "ABC"
	univ = Univ()
	print(univ.name) # This works
	print(univ.college) # This gives error

view raw NoAttrib.py hosted with ❤ by GitHub

No Attribute Error

So, can we intercept univ.college access and do something different? Sure

	class Univ:
	def __init__(self):
	self.name = "ABC"
	# This gets called when attribute is missing
	def __getattr__(self, attribute):
	try:
	return self[attribute]
	except:
	return "We are very sorry, we cannot server you that yet"
	univ = Univ()
	print(univ.name) # Actual attribute
	print(univ.college) # Custom response
	''' Output
	ABC
	We are very sorry, we cannot server you that yet
	'''

view raw InterceptAttr.py hosted with ❤ by GitHub

Intercept Attribute

Great, we can intercept calls for missing attributes, but can we create the attribute on the fly? Sure

Magic of dunders ( double underscores)
Here we override __getattr__/__setattr__ for univ.college.stream.year = “A+”
and we override __getitem__ for univ[‘college’][‘stream’][‘year’] = “A+”

auto inherits a dictionary

This way we can store the attributes in “dict” we are inheriting which simplifies our code to bare minimum. We can also show “bravado” and build this WITHOUT inheriting from dict and manage everything ourselves. Perhaps, another blog post?

Over to final code

	# auto inherits a dictionary
	class auto(dict):
	# if caller did univ.college
	def __getattr__(self, name):
	try:
	# check if its there
	return dict.__setitem__(self, name)
	except:
	# if not, create it on the fly
	dict.__setitem__(self, name, auto())
	return dict.__getitem__(self, name)
	# if caller did univ.college = value
	def __setattr__(self, name, value):
	# set it in internal dict
	dict.__setitem__(self, name, value)
	# if caller did univ['college]
	def __getitem__(self, value):
	try:
	# if found return it
	return dict.__getitem__(self, value)
	except:
	# if not, create it on the fly
	dict.__setitem__(self, value, auto())
	return dict.__getitem__(self, value)
	univ = auto()
	univ.college.stream.year = "A+"
	print(univ)
	univ2 = auto()
	univ2['college']['stream']['year'] = "A+"
	print(univ2)

view raw AutoViv.py hosted with ❤ by GitHub

Auto Vivification in Python

And here is the output

univ1 {'college': {'stream': {'year': 'A+'}}}
univ2 {'college': {'stream': {'year': 'A+'}}}

Auto Vivification Series

1. Auto vivification in JavaScript
2. Auto vivification in Python
3. Auto vivification in JavaScript (Upgraded)