assert_context

The assert_context module exposes a context manager that verifies whether code within a with block behaves as expected. It checks whether a exception of a specific type matching specific value was (or wasn't) raised within the block and whether the result of the block (settable by the user) matches the expected value.

Module exposes ExceptionMatch class which allows to specify not only type of expected exception, but also a string which shall match raised exception value.

Additionally, it provides the AssertContext class, which can be useful for typing in more complex scenarios, as it is the type returned by the __enter__ method.

Basic Example

The following code snippet demonstrates how to use assert_context. It asserts that the result of fun() will be 'something' and that bar() will raise a ValueError.

with assert_context(exception=ValueError, result='something') as context:
    context.set(fun())
    bar()

Parameters

The context manager defines three optional keyword parameters: exception, result, and behaviour. The pair of parameters (exception, result) is mutually exclusive with the behaviour parameter. If behaviour is set, the other parameters must remain unset.

exception Parameter

• If the exception parameter is set to exception type, the with block is expected to raise an exception of the provided type.
• If the exception parameter is set to an instance of ExceptionMatch class, the with block is expected to raise an exception of type specified in type_ field and the value that matches the string specified in match_ field. For exact matching logic see match parameter of the pytest.raises.
• If the exception parameter is NOT set or is set to None, the with block is expected NOT to raise any exceptions.

Examples

Code not raising any exceptions:

with assert_context():
    ...

with assert_context(exception=ValueError):
    raise ValueError

Code raising a ValueError:

with assert_context():
    raise ValueError

with assert_context(exception=TypeError):
    raise ValueError

Code calling pytest.fail (equivalent to pytest.raises(ValueError)):

with assert_context(exception=ValueError):
    ...

result Parameter

• If the result parameter is set, its value is expected to be equal to the result of the with block.
• The with block result is set using the set method of an object yielded by the context manager.
• The comparison is performed using the == operator, with the result argument on the left side.
• If the result parameter is NOT set, the set method of the yielded object is expected not to be called at all.

Examples

Code not raising any exceptions:

with assert_context() as context:
    ...

with assert_context(result='something') as context:
    context.set('something')

Code raising an AssertionError:

with assert_context() as context:
    context.set('else')

with assert_context(result='something') as context:
    context.set('else')

with assert_context(result='something') as context:
    ...

Combining exception and result Parameters

Both exception and result parameters can be set simultaneously to check more complex code.

Example

Code not raising any exceptions:

with assert_context(exception=ValueError, result='something') as context:
    context.set('something')
    raise ValueError

behaviour Parameter

• If behaviour is set to an exception type or ExceptionMatch instance, it is equivalent to setting exception to the same value and leaving result unset.
• If behaviour is set to any other value, it is treated as the value of result, and the exception parameter remains unset.

Examples

Code not raising any exceptions:

with assert_context(behaviour=ValueError) as context:
    raise ValueError

with assert_context(result='something') as context:
    context.set('something')

Code raising exceptions:

with assert_context(behaviour=ValueError) as context:
    context.set('else')

with assert_context(result='something') as context:
    raise ValueError