Implementing a unit of work in Python can be an interesting challenge. Consider the following code:

	class Holder(object):
	def __init__(self):
	self.items = dict()
	def try_set(self, key, name):
	if key in self.items:
	return
	self.items[key] = name
	def try_get(self, key):
	if key in self.items:
	return self.items[key]
	return None

view raw Holder.py hosted with ❤ by GitHub

This is about as simple a code as possible, to associate a tag to an object, right?

However, this code will fail for the following scenario:

	@dataclass
	class Item:
	name: str
	holder = Holder()
	cup = Item(name="Cup")
	holder.try_set(cup, "cups/1")

view raw cups.py hosted with ❤ by GitHub

You’ll get a lovely: “TypeError: unhashable type: 'Item'” when you try this. This is because data classes in Python has a complicated relationship with __hash__().

An obvious solution to the problem is to use:

	class Holder(object):
	def __init__(self):
	self.items = dict()
	# this is bad
	def try_set(self, key, name):
	if id(key) in self.items:
	return
	self.items[id(key)] = name
	# this is bad
	def try_get(self, key):
	if id(key) in self.items:
	return self.items[id(key)]
	return None

view raw Holder2.py hosted with ❤ by GitHub

However, the id() in Python is not guaranteed to be unique. Consider the following code:

	cup = Item(name="Cup")
	print(id(cup))
	cup = None
	cup = Item(name="Cup")
	print(id(cup)) # different instance

view raw opps.py hosted with ❤ by GitHub

On my machine, running this code gives me:

124597181219840
124597181219840

In other words, the id has been reused. This makes sense, since this is just the pointer to the value. We can fix that by holding on to the object reference, like so:

	class RefEq(object):
	def __init__(self, ref):
	self.ref = ref
	def __eq__(self, other):
	if id(self.ref) == id(other):
	return True
	if not isinstance(other, RefEq):
	return False
	return id(self.ref) == id(other.ref)
	def __hash__(self):
	return id(self.ref)
	class Holder(object):
	def __init__(self):
	self.items = dict()
	def try_set(self, key, name):
	if RefEq(key) in self.items:
	return
	self.items[RefEq(key)] = name
	def try_get(self, key):
	if RefEq(key) in self.items:
	return self.items[RefEq(key)]
	return None

view raw Holder3.py hosted with ❤ by GitHub

With this approach, we are able to implement proper reference equality and make sure that we aren’t mixing different values.