[Python-Dev] Type hints -- a mediocre programmer's reaction (original) (raw)

Steven D'Aprano steve at pearwood.info
Tue Apr 21 16:06:54 CEST 2015

On Tue, Apr 21, 2015 at 03:08:27PM +0200, Antoine Pitrou wrote:

On Tue, 21 Apr 2015 22:47:23 +1000 Steven D'Aprano <steve at pearwood.info> wrote: > > Ironically, type hinting will reduce the need for intrusive, > anti-duck-testing explicit calls to isinstance() at runtime:

It won't, since as you pointed out yourself, type checks are purely optional and entirely separate from compilation and runtime evaluation.

Perhaps you are thinking of libraries, where the library function has to deal with whatever junk people throw at it. To such libraries, I believe that the major benefit of type hints is not so much in proving the library's correctness in the face of random arguments, but as documentation. In any case, of course you are correct that public library functions and methods will continue to need to check their arguments. (Private functions, perhaps not.)

But for applications, the situation is different. If my application talks to a database and extracts a string which it passes on to its own function spam(), then it will be a string. Not a string-like object. Not something that quacks like a string. A string. Once the type checker is satisfied that spam() always receives a string, then further isinstance checks inside spam() is a waste of time. If spam()'s caller changes and might return something which is not a string, then the type checker will flag that.

Obviously to get this benefit you need to actually use a type checker. I didn't think I needed to mention that.

-- Steve

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