Set nested dict value and create intermediate keys

I feel like I saw a way to do this recently. Say I've got an empty dict and I want to set a value in a nested dict inside that empty dict, but obviously that nested dict hasn't been created yet. Is there a 1-line way to create the intermediate keys? This is what I want to do:

mydict = {}
mydict['foo']['bar']['foobar'] = 25

If you execute this code you'll get a KeyError exception for 'foo'. Is there a function to create intermediate keys?

Thanks.