[Python-ideas] Why is design-by-contracts not widely adopted?

Paul Moore p.f.moore at gmail.com
Fri Sep 28 08:49:01 EDT 2018

On Fri, 28 Sep 2018 at 13:23, David Mertz <mertz at gnosis.cx> wrote:
> I agree that all the Sphinx documentation examples shown are very nice. Prose descriptions would often be nicer still, but the Boolean expressions are helpful for those unusual cases where I don't want to look at the code.

I'm ambivalent about the Sphinx examples. I find the highly detailed
code needed to express a condition fairly unreadable (and I'm an
experienced Python programmer). For example

@pre(lambda args, result: not any(Path(arg).is_absolute() for arg in args) or
(result == [pth for arg in args for pth in [Path(arg)] if
"When several absolute paths are given, the last is taken as an anchor
(mimicking os.path.join()’s behaviour)")

The only way I'd read that is by looking at the summary text - I'd
never get the sense of what was going on from the code alone. There's
clearly a number of trade-offs going on here:

* Conditions should be short, to avoid clutter
* Writing helper functions that are *only* used in conditions is more
code to test or get wrong
* Sometimes it's just plain hard to express a verbal constraint in code
* Marko isn't that familiar with the codebase, so there may be better
ways to express certain things

But given that *all* the examples I've seen of contracts have this
issue (difficult to read expressions) I suspect the problem is

Another thing that I haven't yet seen clearly explained. How do these
contracts get *run*? Are they checked on every call to the function,
even in production code? Is there a means to turn them off? What's the
runtime overhead of a "turned off" contract (even if it's just an
if-test per condition, that can still add up)? And what happens if a
contract fails - is it an exception/traceback (which is often
unacceptable in production code such as services)? The lack of any
clear feel for the cost of adding contracts is part of what makes
people reluctant to use them (particularly in the face of the
unfortunately still common assertion that "Python is slow" :-()


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