Python: Iterate and modify a complex dict including lists

I need to iterate through a complex dictionary and modify it according to several conditions.

This is a very basic example of the dict

a = {
   'x' : ["x1", "x2", 'x3'],
   'y' : [
       {'y1' : 1},
       {'y2' : 2.0},
       {'y3' : True},
       {'y4' : 99},
   ],
   'z' : {
      'x' : ["a1", "a2", 'a3'],
      'y' : [
           {'y1' : 66},
           {'y2' : False},
           {'y3' : 3},
           {'y4' : 4.3},
       ]
   },
   'y3' : "Delete Me"
}

Îterating through it, is also no problem like:

 pTypes = [str, bool, int, float]
 def parse(d, c):
     t = type(d)
     if t == dict:
         for k, v in d.items():
             parse(v, c+[k])
     elif t == list:
         for i, p in enumerate(d):
             parse(p,c+[i])
     elif t in pTypes:
         print(c,'=>',d)
     else:
         print('Error: ',c,d,t)


 parse(a,[])

My problem is, that I need to delete all items which have a key == ‘y3’ or a value > 50

Additionally all strings need to be enclosed by ‘-‘ like ‘x1’ => ‘-x1-‘

Just modifing the data does not work (is not persistent)

...
elif t in pTypes:
   if t == str:
       d = '-'+d+'-'
...

And I have no idea how to remove items while iterating the dict itself.

Since the dict can be huge, i would prefer not to make a "reduced" copy of the original.

Can anybody help?

>Solution :

The exact expected output is unclear, but since you are iterating a nested structure, better generate a new object.

Keeping your original recursive logic, you should add a return at each step to propagate your objects:

def parse(d):
    if isinstance(d, dict):
        return {k: parse(v) for k, v in d.items()
                if k != 'y3' and not
                (isinstance(v, (int, float)) and v>50)}
    elif isinstance(d, list):
        return [parse(x) for x in d]
    elif isinstance(d, str):
        return f'-{d}-'
    else:
        return d

out = parse(a)

Output:

{'x': ['-x1-', '-x2-', '-x3-'],
 'y': [{'y1': 1}, {'y2': 2.0}, {}, {}],
 'z': {'x': ['-a1-', '-a2-', '-a3-'],
       'y': [{}, {'y2': False}, {}, {'y4': 4.3}]}}