[Python] Is there a way to set multiple defaults on a Python dict using another dict?

Suppose I've got two dicts in Python:

mydict = { 'a': 0 }

defaults = {
'a': 5,
'b': 10,
'c': 15
}


I want to be able to expand mydict using the default values from defaults, such that 'a' remains the same but 'b' and 'c' are filled in. I know about dict.setdefault() and dict.update(), but each only do half of what I want - with dict.setdefault(), I have to loop over each variable in defaults; but with dict.update(), defaults will blow away any pre-existing values in mydict.

Is there some functionality I'm not finding built into Python that can do this? And if not, is there a more Pythonic way of writing a loop to repeatedly call dict.setdefaults() than this:

for key in defaults.keys():
mydict.setdefault(key, defaults[key])


Context: I'm writing up some data in Python that controls how to parse an XML tree. There's a dict for each node (i.e., how to process each node), and I'd rather the data I write up be sparse, but filled in with defaults. The example code is just an example... real code has many more key/value pairs in the default dict.

(I realize this whole question is but a minor quibble, but it's been bothering me, so I was wondering if there was a better way to do this that I am not aware of.)

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