select numerical keys in dictionary

numdict.py

my_dict = dict({
      1     : "",
      2     : "",
      3     : "oyy",
      "oyy" : "",
      1990  : 's kid',
      2000  : "a",
      2014  : "b",
      2015  : "c",
      "foo" : "bar",
      2016  : "d",
      2017  : "e",
      2018  : "f",
      4     : ""
      })

class ndict(dict):
    def num_keys(self):
        return filter(lambda x: isinstance(x, (int, long, float)), self.keys())
    def slice_key(self, min=float("-inf"), max=float("inf")):
        if (min > max):
            return None
        return filter(lambda x: (x >= min) & (x <= max), self.num_keys())


my_dict = ndict(my_dict)
print(my_dict.num_keys()
## >> [2016, 1, 2, 3, 4, 1990, 2017, 2018, 2000, 2014, 2015]  

print(my_dict.slice_key(2014,2016))
## >> [2016, 2014, 2015]