Subtract dict value from a nested dict if the key exists

Question

Basically I made this request to perform the operation efficiently but I guess the data structure I'm using isn't.

First dict:

f_dict = {'n1':{'x':1,'y':1,'z':3},'n2':{'x':6,'y':0, 'z':1}, ...}
s_dict = {'x':3,'t':2, 'w':6, 'y':8, 'j':0, 'z':1}

I want to obtain e such that:

e = {'n1':{'x':-2,'y':-7,'z':1},'n2':{'x':3,'y':-8,'z':0}, ...} 

Answer 1

You could use a nested dictionary comprehension and use dict.get to subtract the value or a default value (in this case 0):

>>> {key: {ikey: ival - s_dict.get(ikey, 0) 
...        for ikey, ival in i_dct.items()} 
...  for key, i_dct in f_dict.items()}
{'n1': {'x': -2, 'y': -7, 'z': 2}, 'n2': {'x': 3, 'y': -8, 'z': 0}}

Or if you prefer explicit loops:

res = {}
for key, i_dict in f_dict.items():
    newdct = {}
    for ikey, ival in i_dict.items():
        newdct[ikey] = ival - s_dict.get(ikey, 0)
    res[key] = newdct

print(res)
# {'n1': {'x': -2, 'y': -7, 'z': 2}, 'n2': {'x': 3, 'y': -8, 'z': 0}}