A wrapper function that validates that some JSON object matches a spec, something like a validating XML parser. This is particularly useful if you expose a JSON API to third-party developers and want to return a useful error if they provide malformed input.
Python
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README
json_validate.py

README

Provides a Python function decorator, @json_validate, which validates
JSON input against a specification.

Example session:

>>> json_structure = required({'must_be_here':str}) + {
...     'a': int,
...     'b': float,
...     'c': str,
...     'd': [int],
...     'e': anytype,
...     'f': {'nested_structure': float},
...     'g': re.compile('^fo*$'),
...     'h': one_of({'a': int, 'b': list}),
...     'i': atleast_one({'a': int, 'b': list}),
... } + atleast_one({'more_keys': str})
>>> class C:
...     @json_validate(json_structure)
...     def fn(self, input):
...         print input
... 
>>> c = C()
>>> c.fn({
...     'must_be_here': 'is here',
...     'a': 1.0,
...     'b': 2.5,
...     'c': 'Hi!',
...     'd': [1, 2, 3],
...     'e': [],
...     'f': {'nested_structure': 5},
...     'extra_key': 'this is ignored by json_validate',
...     'g': 'fooooooooo',
...     'h': {'b':['anything','is','OK','here']},
...     'i': {'a':1},
...     'more_keys': ''
... })
{'must_be_here': 'is here', 'a': 1.0, 'c': 'Hi!', 'b': 2.5, 'e': [], 'd': [1, 2, 3], 'g': 'fooooooooo', 'f': {'nested_structure': 5}, 'i': {'a': 1}, 'h': {'b': ['anything', 'is', 'OK', 'here']}, 'more_keys': '', 'extra_key': 'this is ignored by json_validate'}
>>> 
>>> c.fn({
...     'must_be_here': 'is here',
...     'a': 1.0,
...     'b': 2.5,
...     'c': 'Hi!',
...     'd': [1, 2, 3],
...     'e': [],
...     'f': {'nested_structure': 5},
...     'extra_key': 'this is ignored by json_validate',
...     'g': 'fooooooooo',
...     'h': {'b':['anything','is','OK','here']},
...     'i': {'a':1},
... })
Traceback (most recent call last):
  File "<stdin>", line 12, in <module>
  File "json_validate.py", line 455, in validator
    'h': {'b':['anything','is','OK','here']},
json_validate.JSONException: client_json requires *at least* one of these keys, but found none: ['more_keys']
>>> 
>>> c.fn({
...     'must_be_here': 'is here',
...     'a': 1.0,
...     'b': 2.5,
...     'c': 'Hi!',
...     'd': [1, 2, 3],
...     'e': [],
...     'f': {'nested_structure': 5},
...     'extra_key': 'this is ignored by json_validate',
...     'g': 'fooooooooo',
...     'h': {'a':1,'b':["shouldn't have both a and b"]},
...     'i': {'a':1},
...     'more_keys': ''
... })
Traceback (most recent call last):
  File "<stdin>", line 13, in <module>
  File "json_validate.py", line 455, in validator
    'h': {'b':['anything','is','OK','here']},
json_validate.JSONException: client_json['h'] requires one of these keys: ['a', 'b'], but found several: ['a', 'b']