Question

Good day pythonians,

I want to make a custom dictionary with two main features:

1. All keys are declared on creation
2. It is impossible to add new keys or modify current ones (values are still modifiable)

Right now code is this:

class pick(dict):
   """This will make delicious toffee when finished"""
   def __init__(self, *args):
       dict.__init__(self)
       for arg in args:
           self[arg] = None

Any help is much appreciated.

upd:

While solution is what I was looking for there is one problem:

dictionary calls the __setitem__ to add the items on the initialization and not finding the keys it raises the error.

cupboard = pick('milk')  #raises error

upd1:

all solved, thank you very much.

Answer 1

Override the __setitem__ method with your desired behavior, call dict.__setitem__(self, key, value) to modify the base dictionary without going through your base logic.

class ImmutableDict(dict):
    def __setitem__(self, key, value):
        if key not in self:
             raise KeyError("Immutable dict")
        dict.__setitem__(self, key, value)

d = ImmutableDict(foo=1, bar=2)
d['foo'] = 3
print(d)
d['baz'] = 4 # Raises error

You’ll also need to override dict.update() and setdefault() to avoid addition of keys. And possibly dict.__delitem__(), dict.clear(), dict.pop() and dict.popitem() to avoid removal of keys.