Replace null values in Json file

[buran]

As I said - you have complex structure, so you need to check if the value is (for example) list/dict, then you need to check values within this data structure too.
In your example you have

Output:
u'user_mentions': [{u'id': 254599256,
u'id_str': u'254599256',
u'indices': [3, 11],
u'name': u'Alejandro Fargosi',
u'screen_name': u'fargosi'}]},
u'favorite_count': 0,
u'favorited': False,
u'filter_level': u'low',
u'geo': None,
u'id': 1005817329459097600L,
u'id_str': u'1005817329459097600',
u'in_reply_to_screen_name': None,
u'in_reply_to_status_id': None}]

the value is list of dicts, and there are elements in the dict that has value None.
so it is at least two levels deep that you have to check

what is the best way to do it depends on your data and you know your data best

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can you post example of your json file so that we can get better understanding of the data structure that you work with?