is it safe and/or pythonic to directly overwrite dict entries in the constructor?

Question

I'm storing some configuration state in a dictionary, where a bunch of separate FFMPEG filters share a common set of parameters, and then we change/override those for each specific filter. My current code looks like this, and appears to work....

    default['common']['fontfile'] = '/usr/share/fonts/truetype/liberation/LiberationSans-Regular.ttf'
    default['common']['rate'] = framerate
    default['common']['fontcolor'] = 'yellow'
    default['common']['box'] = 1
    default['common']['y'] = 0       # x and y are offsets from the top left of frame
    default['common']['x'] = 0

then I'm just calling another constructor with the ** operator to pull all those common keys/values into my new dict, as well as add some new k/v pairs and make some changes to the existing k/v pairs.

In this example I'm changing the values for 'x' and 'y' and adding 'start_number' : 0 to the dict:

    default['framectr'] = {
        **default['common'], 
        'y' : (vid_h * 0.2), 
        'x' : (vid_w * 0.2),
        'start_number' : 0
        }

So far this appears to work, but are there potential side effects here I should be worried about? I considered just making a deepcopy() of the original/common dict and then adding/modifying the entries that way, but at least to my inexperienced eye the constructor call seems cleaner.

Thoughts?

Answer 1

Semantically,

default['framectr'] = {
    **default['common'], 
    'y' : (vid_h * 0.2), 
    'x' : (vid_w * 0.2),
    'start_number' : 0
    }

is equivalent to

default['framectr'] = {
    'fontfile': default['common']['fontfile'],
    'rate': default['common']['rate'],
    'fontcolor': default['common']['fontcolor'],
    'box': default['common']['box'],
    'y': default['common']['y'],
    'x': default['common']['x'],
    'y' : (vid_h * 0.2), 
    'x' : (vid_w * 0.2),
    'start_number' : 0
    }

Neither is safer or more dangerous than the other in terms of side effects. Given that you aren't using any mutable values in the definition of default['common'] , making a deep copy won't buy you anything.