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PyADBC: Design by Contract in Python

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PyADBC: Design by Contract in Python

PyADBC provides pure python support for `Design by
Contract <>`__.
Invariants, pre- and post-conditions are added using decorators.

* Free software: BSD license



To define an invariant on a class (a condition that always holds after
the object is instantiated), use the ``@invariant`` decorator. The
``self`` passed into the passed lambda is the same ``self`` that would
be passed to a method.


@invariant(lambda self: self.capacity >= 0)
class List(object):
# ...


Preconditions can be specified with ``@requires``. These are properties
that must evaluate to ``True`` *before* the method is run.


@requires(lambda self: self.size() < self.capacity)
def append(self, bla):
self._size += 1


Postconditions can be specified with ``@ensures``:


@ensures(lambda self: self.size() == len(self.things))
def append(self, bla):
self._size += 1

These are properties that must evaluate to ``True`` *after* the method
has run.

``@old`` for before/after comparisons

In order to compare properties of the object before and after an
operation, PyADBC provides the ``@old`` decorator. You can use it to
"cache" values for use in the postcondition.

For example, the following method's postcondition guarantees that the
``x`` instance attribute is increased by one. Dictionaries returned in
the functions passed to ``@old`` will be merged and passed as a second
argument to the postcondition functions.


@ensures(lambda self, old: old['x'] + 1 == self.x,
lambda self, old: old['size'] < 0)
@old(lambda self: {'x': self.x, 'size': self._size})
def doThing(self):
self.x -= 1


If any conditions defined by the above decorators evaluate to ``False``,
one of the following exceptions will be raised based on what kind of
condition it is:

- ``PreconditionFailedException``, which implies that the client of the
class failed to satisfy the class's contract.
- ``PostconditionFailedException``, which implies that the object
itself failed to satisfy its class's contract.
- ``InvariantFailedException``, which implies that the object has
entered an illegal state.

``@dbcinherit`` for inheriting classes

An important feature of DBC is that it can validate the `Liskov
Principle <>`__.
That is, if a child class ``CoolList`` inherits from the base class
``List``, its operations should satsify ``List``'s contracts. This gives
some assurance that the principle holds for the child class.

To do this with PyADBC, you need to explicitly decorate the child class,


class CoolList(List):
# ...

If ``CoolList`` overrides the ``append()`` method, the contract of
``List``'s ``append()`` method will be applied to ``CoolList``'s

This also currently works with multiple inheritance.

Other solutions

- `PyDBC <>`__, which uses a metaclass.
- `pycontract <>`__, which uses
docstrings. See the Python Enhancement Proposal (PEP) that references
this implementation
`here <>`__.


See ```` for missing features. Suggestions and bug reports are
always welcome.


Thanks to

- Prof. Constantinos Constantinides, for his feedback regarding Liskov
and for teaching me about DBC in the first place


0.1.0 (2013-08-11)

* First release on PyPI.

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