Everyone who's done Python for a while soon learns that dicts are mutable. I.e. that they can change.
One way of "forking" a dictionary into two different ones is to create a new dictionary object with dict(). E.g:
>>> first = {'key': 'value'}
>>> second = dict(first)
>>> second['key'] = 'other'
>>> first
{'key': 'value'}
>>> second
{'key': 'other'}
See, you can change the value of a key without affecting the dictionary it came from.
But, if one of the values is also mutable, beware!
>>> first = {'key': ['value']}
>>> second = dict(first)
>>> second['key'].append('second value')
>>> first
{'key': ['value', 'second value']}
>>> second
{'key': ['value', 'second value']}
This is where you need to use the built in copy.deepcopy.
>>> import copy
>>> first = {'key': ['value']}
>>> second = copy.deepcopy(first)
>>> second['key'].append('second value')
>>> first
{'key': ['value']}
>>> second
{'key': ['value', 'second value']}
Yay! Hope it helps someone avoid some possibly confusing bugs some day.
UPDATE
As ëRiC reminded me, there are actually three ways to make a "shallow copy" of a dictionary:
1) some_copy = dict(some_dict)
2) some_copy = some_dict.copy()
3) some_copy = copy.copy(some_dict) # after importing 'copy'
