[Python-ideas] "old" values in postconditions

Marko Ristin-Kaufmann marko.ristin at gmail.com
Sun Sep 23 02:04:49 EDT 2018


(I'd like to fork from a previous thread, "Pre-conditions and
post-conditions", since it got long and we started discussing a couple of
different things. Let's discuss in this thread the implementation of a
library for design-by-contract and how to push it forward to hopefully add
it to the standard library one day.)

For those unfamiliar with contracts and current state of the discussion in
the previous thread, here's a short summary. The discussion started by me
inquiring about the possibility to add design-by-contract concepts into the
core language. The idea was rejected by the participants mainly because
they thought that the merit of the feature does not merit its costs. This
is quite debatable and seems to reflect many a discussion about
design-by-contract in general. Please see the other thread, "Why is
design-by-contract not widely adopted?" if you are interested in that

We (a colleague of mine and I) decided to implement a library to bring
design-by-contract to Python since we don't believe that the concept will
make it into the core language anytime soon and we needed badly a tool to
facilitate our work with a growing code base.

The library is available at http://github.com/Parquery/icontract. The hope
is to polish it so that the wider community could use it and once the
quality is high enough, make a proposal to add it to the standard Python
libraries. We do need a standard library for contracts, otherwise projects
with *conflicting* contract libraries can not integrate (*e.g., *the
contracts can not be inherited between two different contract libraries).

So far, the most important bits have been implemented in icontract:

   - Preconditions, postconditions, class invariants
   - Inheritance of the contracts (including strengthening and weakening of
   the inherited contracts)
   - Informative violation messages (including information about the values
   involved in the contract condition)
   - Sphinx extension to include contracts in the automatically generated
   documentation (sphinx-icontract)
   - Linter to statically check that the arguments of the conditions are
   correct (pyicontract-lint)

We are successfully using it in our code base and have been quite happy
about the implementation so far.

There is one bit still missing: accessing "old" values in the postcondition
(*i.e., *shallow copies of the values prior to the execution of the
function). This feature is necessary in order to allow us to verify state

For example, consider a new dictionary class that has "get" and "put"

from typing import Optional

from icontract import post

class NovelDict:
    def length(self)->int:

    def get(self, key: str) -> Optional[str]:

    @post(lambda self, key, value: self.get(key) == value)
    @post(lambda self, key: old(self.get(key)) is None and
old(self.length()) + 1 == self.length(),
          "length increased with a new key")
    @post(lambda self, key: old(self.get(key)) is not None and
old(self.length()) == self.length(),
          "length stable with an existing key")
    def put(self, key: str, value: str) -> None:

How could we possible implement this "old" function?

Here is my suggestion. I'd introduce a decorator "before" that would allow
you to store whatever values in a dictionary object "old" (*i.e. *an object
whose properties correspond to the key/value pairs). The "old" is then
passed to the condition. Here is it in code:

# omitted contracts for brevity
class NovelDict:
    def length(self)->int:

    # omitted contracts for brevity
    def get(self, key: str) -> Optional[str]:

    @before(lambda self, key: {"length": self.length(), "get": self.get(key)})
    @post(lambda self, key, value: self.get(key) == value)
    @post(lambda self, key, old: old.get is None and old.length + 1 ==
          "length increased with a new key")
    @post(lambda self, key, old: old.get is not None and old.length ==
          "length stable with an existing key")
    def put(self, key: str, value: str) -> None:

The linter would statically check that all attributes accessed in "old"
have to be defined in the decorator "before" so that attribute errors would
be caught early. The current implementation of the linter is fast enough to
be run at save time so such errors should usually not happen with a
properly set IDE.

"before" decorator would also have "enabled" property, so that you can turn
it off (*e.g., *if you only want to run a postcondition in testing). The
"before" decorators can be stacked so that you can also have a more
fine-grained control when each one of them is running (some during test,
some during test and in production). The linter would enforce that before's
"enabled" is a disjunction of all the "enabled"'s of the corresponding
postconditions where the old value appears.

Is this a sane approach to "old" values? Any alternative approach you would
prefer? What about better naming? Is "before" a confusing name?

Thanks a lot for your thoughts!


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