I'm not sure what to do with the repeated assurance that that various things "are obvious."  It really is NOT the case that me, or Paul Moore, or Hugh Fisher, or Greg Ewing are simply too simple minded to understand what DbC is.

The off-putting evangelical quality around DbC is something like the similar evangelical insistence that OOP solves all problems that one tended to hear in the late-1980s to mid-1990s especially.  The fact that no one can quite pin down just what this special quality of DbC is doesn't help... the reality is that they really ARE NOT much different from assertions, in either practice or theory.

Actually, the evangelical OOP thing connects with the evangelical DbC.  One of the "advantages" of DbC is often stated as support for inheritance.  But the truth is, I hardly ever use inheritance in my code, or at most very shallow inheritance in ways where invariants are mostly not preserved.  My use of OOP in Python is basically mixins plus magic methods... and no, it's not because I don't understand OOP (I wrote, for example, some of the most widely read papers on metaclass programming in Python, and designed and wrote about making a—toy, admittedly—method resolution order and OOP system for R; my frequent co-author wrote the canonical paper on C3 linearization, in which I'm acknowledged for my edits, but am not an author).

I also wrote an article or two about DbC in Python in the early 2000s.  None of this is very new.

On Mon, Sep 24, 2018 at 3:47 AM Marko Ristin-Kaufmann <marko.ristin@gmail.com> wrote:
Obvious benefits
You both seem to misconceive the contracts. The goal of the design-by-contract is not reduced to testing the correctness of the code, as I reiterated already a couple of times in the previous thread. The contracts document formally what the caller and the callee expect and need to satisfy when using a method, a function or a class. This is meant for a module that is used by multiple people which are not necessarily familiar with the code. They are not a niche. There are 150K projects on pypi.org. Each one of them would benefit if annotated with the contracts.

Greg Ewing, Chris Angelica, and Paul Moore make the point very well that MOST code is simply not the sort of thing that is amenable to having contracts.  What one needs to state is either Turing complete or trivially reducible to type declarations.  Or the behavior is duck typed loosely enough that it can do the right thing without being able to specify the pre- and post-conditions more precisely than "do what this function does."  Very often, that looseness is simply NOT a problem, and is part of what makes Python flexible and successful.

Contracts are difficult to read.
David wrote:
To me, as I've said, DbC imposes a very large cost for both writers and readers of code.

This is again something that eludes me and I would be really thankful if you could clarify. Please consider for an example, pypackagery (https://pypackagery.readthedocs.io/en/latest/packagery.html) and the documentation of its function resolve_initial_paths:

The documentation you show below is definitely beautiful  I guess that's generated by your Sphinx enhancement, right?  There are similar systems for pulling things out of docstrings that follow conventions, but there's a small incremental value in making them live tests at the same time.

But reading the end-user documentation is not the reading I'm talking about.  The reading I mean is looking at the actual source code files.  Stating all the invariants you want code to follow makes the definitions of functions/methods longer and more cognitive effort to parse.  A ten line function body is likely to be accompanied by 15 lines of invariants that one can state about hopes for the function behavior.  That's not always bad... but it's definitely not always good.

This is why unit tests are often much better, organizationally.  The function definitions can remain relatively concise and clean because most of the time when you are reading or modifying them you don't WANT to think about those precise contracts.  Sure, maybe some tool support like folding of contracts could make the burden less; but not less than putting them in entirely separate files.

Most of the time, I'd like to look at the simplest expression of the code possible.  Then on a different day, or with a different set of eyes, I can look at the completely distinct file that has arbitrarily many unit tests to check invariants I'd like functions to maintain.  Yes, the issues are a little bit different in classes and nested hierarchies... but like I said, I never write those, and tend not to like code that does. 

Resolve the initial paths of the dependency graph by recursively adding *.py files beneath given directories.


initial_paths (List[Path]) – initial paths as absolute paths

Return type:



list of initial files (i.e. no directories)

  • all(pth.is_absolute() for pth in initial_paths)
  • len(result) >= len(initial_paths) if initial_paths else result == []
  • all(pth.is_absolute() for pth in result)
  • all(pth.is_file() for pth in result)
Again, this is a straw man.
Writing contracts is difficult.
David wrote:
To me, as I've said, DbC imposes a very large cost for both writers and readers of code.

The effort of writing contracts include as of now:
* include icontract (or any other design-by-contract library) to setup.py (or requirements.txt), one line one-off
* include sphinx-icontract to docs/source/conf.py and docs/source/requirements.txt, two lines, one-off
* write your contracts (usually one line per contract).

It's not much work to add `import icontract` of course.  But writing contracts is a lot of work.  Usually they work best only for pure functions, and only for ones that deal with rather complex data structures (flat is better than nested).  In Python, you usually simply do not have to think about the issues that contracts guard against.
I think that ignorance plays a major role here. Many people have misconceptions about the design-by-contract. They just use 2) for more complex methods, or 3) for rather trivial methods. They are not aware that it's easy to use the contracts (1) and fear using them for non-rational reasons (e.g., habits).

Again, this evangelical spirit to "burn the heretics" really isn't going to win folks over.  No one replying here is ignorant.  We all do not see any "silver bullet" in DbC that you advocate.  It's a moderately useful style that I wouldn't object to using if a codebase style guide demanded it.  But it's rarely the tool I would reach for on my own.  I just find it easier to understand and use a combination of assertions and unit tests.  Neither is exactly the same thing as DbC, but it's pretty darn close in practice.

And no, my hesitance isn't because I don't understand boolean logic.  I've also studied a bit of graduate logic and model theory in a long ago life.

Yours, David...

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