An easy way to mock loosely defined Python dict objects

Question

Is there an easy way to mock loosely defined dict objects in Python? For example, how can I easily express that given a dict input, I want to check whether or not each value in it conforms to a particular meta-definition, like minimum and maximum values, lengths, and types?

Being able to do this could be handy, for example, when writing tests.

In mock (unittest.mock in Python versions 3.3+) one can specify that a value can be ANY value, like in

>>> mock = Mock(return_value=None)
>>> mock('foo', bar=object())
>>> mock.assert_called_once_with('foo', bar=ANY)

However, what if bar above should be a dict-like object, like

>>> {'baz': <an integer between -3 and 14>, 'qux': <'yes' or 'no'>}

Answer 1

I've actually written AnyValid, a minimal library that leverages the great work implemented in formencode and unittest.mock, to handle such cases.

For example, testing a dict object like described above, can then be expressed as

>>> from mock import Mock
>>> from any_valid import AnyValid, Int, OneOf
>>> valid_bar = {
...     'baz': AnyValid(Int(min=-3, max=14)),
...     'qux': AnyValid(OneOf(['yes', 'no'])),
...     }
>>> mock = Mock(return_value=None)
>>> mock('foo', bar={'baz': 4, 'qux': 'yes'})
>>> mock.assert_called_once_with('foo', bar=valid_bar)
>>>

Because AnyValid is can take any validator from the large set of validators in formencode, many other conditions can be specified in a similarly expressive manner.