[Python-ideas] Possible enhancement to typing

Steven D'Aprano steve at pearwood.info
Mon Nov 6 02:13:57 EST 2017

On Sun, Nov 05, 2017 at 07:18:30PM +0000, Steve Barnes wrote:

> If a group of iterators were to be added to the typing module it would 
> be reasonably simple to automatically add and assert to any decorated 
> modules to ensure that such modules were always called with the 
> documented types.


> I am thinking of decorators such as:
>   - @typing.assert_params_mismatch - this would provide a wrapper that 
> had auto-generated asserts that all the parameters were of designated types.
>   - @typing.debug_assert_params_mismatch - this would provide a wrapper 
> that had auto-generated asserts that all the parameters were of 
> designated types only if a DEBUG environmental variable was set or similar.

That's what assert does: assert only runs when __DEBUG__ is true. That's 
not controlled by an environment variable, but by the -O flag to the 

So your assert_params_mismatch and debug_assert_params_mismatch are 
effectively the same thing.

But using assert to check to perform argument checks is often an abuse 
of assert. To be more specific, using assert to check the value of 
public arguments in library code (where the arguments come from outside the 
library) is wrong, since you (the library author) cannot guarantee 
that your type tests will even run.

Using asserts for argument checking inside application code is more of a 
grey area, with arguments for and against using assert.

But in my opinion, the deciding factor is nearly always that an 
AssertionError is the wrong sort of exception. Outside of some fairly 
limited circumstances, most of which don't involve type-checking 
function arguments, using assert robs the caller of some useful 
information: the *kind* of error. (TypeError, ValueError, etc.)

See here for further discussion: 


In general, I don't think we want to encourage such runtime type 
testing. Obviously there are exceptions -- library code should 
probably type check arguments, applications perhaps not -- and 
we're not exactly discouraging it either. There are already a number of 
third-party libraries that provide argument type tests at runtime, and 
I think that's probably the right place for them.

> I also think that this might increase the uptake of typing by giving 
> some clear benefits outside of documentation and static type checking.

Problem is, the benefits of runtime type checking aren't clear. But the 
costs certainly are: if you want slow code, do lots and lots of runtime 
type checks.


More information about the Python-ideas mailing list