How to make a dictonary not have delete methods?-CodePudding

I'm working on a project and I have a dictionary in Python. Due to this being an "append-only" dictionary, I need a way to disable .pop, .popitem, .clear, etc. Is this possible in python?

I have tried:

mydict = dict()
#attempt 1:
mydict.pop = None
#Get error pop is read-only
#attempt 2:
del mydict.pop
#Get error pop is read-only

I have tried this on all delete methods with the same results.

CodePudding user response：

Subclass the dict() class:

class MyDict(dict):
    def clear(self):
        pass
    def pop(self):
        pass
    def popitem(self, *args):
        pass

Now when you create your dict, create it using:

myDict = NewDict()

I quickly dumped this into the command line interpreter and it seems to work.

CodePudding user response：

Using inheritance to remove functionality violates Liskov substitution (a hypothetical dict subclass would behave erroneously when treated as a dict instance). And besides, I can easily just dict.clear(my_subclassed_dict_instance). The thing you're asking for is not, in Python terminology, a dict and shouldn't masquerade as a subclass of one.

What you're looking for is a fresh class entirely. You want a class that contains a dict, not one that is a dict.

import collections.abc.Mapping

class MyAppendOnlyContainer(Mapping):

  def __init__(self, **kwargs):
    self._impl = kwargs

  def __getitem__(self, key):
    return self._impl[key]

  def __setitem__(self, key, value):
    self._impl[key] = value

  def __iter__(self):
    return iter(self._impl)

  def __len__(self):
    return len(self._impl)

  # Plus whatever other functionality you need ...

Note that collections.abc.Mapping requires __getitem__, __iter__, and __len__ (which your use case faithfully implements) and gives you get, __contains__, and several other useful dict-like helper functions. What you have is not a dict. What you have is a Mapping with benefits.