Create Nested Python Dictionary From Tuple of Keys

Question

I have an iterable of keys like ('a', 'b', 'c') that I want to correspond to an entry in a dictionary I am building. I am looking for a one-linerish way to build this dictionary from multiple tuples.

Input:

keys = ('a', 'b', 'c')
value = 'bar'

Output:

{'a': {'b': {'c': 'bar'}}}

I have been trying things using defaultdict, but haven't really gotten the grasp of it. The one-liner requirement is pretty soft, but I was hoping to get a really small footprint for this.

Answer 1

You can use reduce ¹ here:

reduce(lambda x, y: {y: x}, keys[::-1], value)

Or, use the iterator returned by reversed :

reduce(lambda x, y: {y: x}, reversed(keys), value)

A lot of people might not like this too much. Unless you're really into functional programming in python, this is a bit opaque. A simpler solution takes only 3 lines:

>>> keys = ('a', 'b', 'c')
>>> value = 'bar'
>>> for c in reversed(keys):
...   value = {c: value}
... 
>>> value
{'a': {'b': {'c': 'bar'}}}

and doesn't rely on any slightly obscure builtin functions.

FWIW, I'm not sure which version I prefer ...

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^{1 functools.reduce in python3.x which also exists in python2.6+ to ease py3k transitions IIRC}