Add dictionary intersection

[groutr]

An efficient way to intersect dictionaries based on their keys. The motivating case was to replace the below line with something nicer and more general.

rv = {i: (d1[i], d2[i]) for i in d1.keys() & d2.keys()}

This function is generalized to compute the intersection of more than two dictionaries and handle generic mappings.

@eriknw, I think other operations on dictionary views would be useful. However, those other operations don't seem to fit the calling conventions for the functions here because they are only well-defined for two dictionaries (operations like difference, symmetric difference, etc). Union would be similar to merge, but preserves all the values from each input dictionary.

[eriknw]

This seems like a useful and intuitive function to have.

With the addition of https://www.python.org/dev/peps/pep-0584/ in Python 3.9, is this still useful? Perhaps.

Advantages:

• may be able to merge multiple mappings more efficiently
• accepts factory= argument like other functions in dicttoolz

I'm curious how efficient we can do this in Cython.

[eriknw]

oops, this is intersect d & e (not supported by dicts), not union d | e. my bad. I'll play around with this.

[groutr]

Regarding how efficiently this can be done in cython, it breaks down to how efficiently 1) an intersection of the keys can be computed and 2) pulling each of those common keys from each dictionary. I don't think it could be more efficient than O(nm) where n is the number of dictionaries and m is the number of common keys. This is assuming that inserting and retrieving from the mapping is O(1).

[groutr]

I think I need to test without the sort. Sorting might be slowing things down here.

[eriknw]

I wonder if this functionality would be better added to merge (and merge_with) by adding an optional keyword argument.

[groutr]

I wonder if this functionality would be better added to merge (and merge_with) by adding an optional keyword argument.

would that look something like merge_with(max, mappings, method='intersect', factory=dict)
The functionality could certainly be added to merge/merge_with. One thing I would want to think about is how complex the implementation might become with different merge methods.