Source: 📖 Python Cookbook ch1.9 p15

Set operations can be used on dictionary .keys() and .items() calls

[Set operations](https://miro.medium.com/max/3200/0*a02OPI3-TnbKXyub.png) such as unions, intersections and differences can be used on the return values of .keys() and .items().

This is useful for identifying commonalities and differences between two dictionaries.

a = {
	'x': 1,
	'y': 2,
	'z': 3,
}

b = {
	'w': 10,
	'x': 11,
	'y': 2,
}


# Find keys in common
a.keys() & b.keys()		# {'x', 'y'}

# Find keys in a that are not in b
a.keys() - b.keys()		# {'z'}

# Find (key, value) pairs in common
a.items() & b.items()	# {('y', 2)}

This is handy for filtering and altering dictionary contents, e.g. within a dict comprehension.

c = {key:a[key] for key in a.keys() - {'z', 'w'}}
# c is {'x': 1, 'y': 2}

The return of the .values() call does not support the same operation as the items contained in a dictionary's values are not guaranteed to be unique. If this kind of functionality is required for the values, a set of the values can be created first.