ChainMap

$ python3
Python 3.4.5 (default, Dec 11 2017, 14:22:24)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-16)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from collections import ChainMap
>>> a = {'x':1, 'z':3}
>>> b = {'y':1, 'z':4}
>>> c = ChainMap(a, b)
>>> c
ChainMap({'z': 3, 'x': 1}, {'z': 4, 'y': 1})
>>> c['x']
1
>>> c['y']
1
>>> c['z']
3
>>> len(c)
3
>>> c.keys()
KeysView(ChainMap({'z': 3, 'x': 1}, {'z': 4, 'y': 1}))
>>> list(c.keys())
['y', 'z', 'x']
>>> list(c.values())
[1, 3, 1]
>>> del c['y']
Traceback (most recent call last):
  File "/usr/lib64/python3.4/collections/__init__.py", line 856, in __delitem__
    del self.maps[0][key]
KeyError: 'y'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "", line 1, in
  File "/usr/lib64/python3.4/collections/__init__.py", line 858, in __delitem__
    raise KeyError('Key not found in the first mapping: {!r}'.format(key))
KeyError: "Key not found in the first mapping: 'y'"
>>> del c['x']
>>> c
ChainMap({'z': 3}, {'z': 4, 'y': 1})
>>> a
{'z': 3}
>>> c['w'] = 34
>>> a
{'z': 3, 'w': 34}
>>> c
ChainMap({'z': 3, 'w': 34}, {'z': 4, 'y': 1})

>>> # merging dict using update() method
>>> a = {'x':1, 'z':3}
>>> b = {'y':2, 'z':4}
>>> merged = dict(b)
>>> merged.update(a)
>>> merged
{'z': 3, 'x': 1, 'y': 2}