Add a raise_invalid_type helper for completeness checking.

 






 

 


  Review Request #14588
  —
  Created Sept. 4, 2025 and updated 

 




















 

  

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  david
 









 
 

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Type checkers for Python are great, but one of the things they do is

they'll flag code as unreachable if there are branches which are

checking the type of arguments, even if that branch exists in order to

provide runtime type-checking of arguments for public APIs.


typing.assert_never is designed for this case, but annoyingly it

always raises an AssertionError, and does not allow customization of the

message.


This change adds a new method, typelets.runtime.raise_invalid_type,

which can be used in the same way as assert_never, but defaults to

raising a ValueError (and allows different exception types if

desired), as well as taking in a message to include in the exception.

This allows us to write APIs which perform runtime type checking on

arguments without having the type checkers flag said checking as

unreachable based on the type hints.

 








  


 
 

  
Used this in some other code.

 







 









 





  

 


 
 

  



 

  
   


    Summary
    ID

   

  
  

   


    

     

      Add a raise_invalid_type helper for completeness checking.
      
Type checkers for Python are great, but one of the things they do is
they'll flag code as unreachable if there are branches which are
checking the type of arguments, even if that branch exists in order to
provide runtime type-checking of arguments for public APIs.

`typing.assert_never` is designed for this case, but annoyingly it
always raises an AssertionError, and does not allow customization of the
message.

This change adds a new method, `typelets.runtime.raise_invalid_type`,
which can be used in the same way as `assert_never`, but defaults to
raising a `ValueError` (and allows different exception types if
desired), as well as taking in a message to include in the exception.
This allows us to write APIs which perform runtime type checking on
arguments without having the type checkers flag said checking as
unreachable based on the type hints.

Testing Done:
Used this in some other code.

     


    		
    lxzzuunlmunktlyoysxnxooumqzulpsp

   



  
 




 













 






 
 
 

  

   

    

     
    

    
    
     
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      Description
      From
      Last Updated
     

    
    



     

      
       
       
Can you add this to docs/coderef/index.rst, docs/index.rst, and README.md?

      		
      
       chipx86chipx86
      
      
       
      
     




     

      
       
       
We should lock this behind a TYPE_CHECKING import.

      		
      
       chipx86chipx86
      
      
       
      
     


    
   

  

 







 






  

 



















 




 




 














 





 

  

   




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    Can you add this to docs/coderef/index.rst, docs/index.rst, and README.md?
    
  
    
 
    

 
    
  


    
 
    
 
      

  

 
    


    

 
    

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       typelets/runtime.py
       


        (Diff revision 1)


       
      
     
    
        
    
     
     
    

     
     
    

        
   
    
  
    
 
    

   
    
    
    




    
 
    
  
  
  

  
  
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    We should lock this behind a TYPE_CHECKING import.
    
  
    
 
    

 
    
  


    
 
    
 
      

  

 
    


    

 
    

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david




   

   

  

 



 

  


   

   

    
 Review request changed

    
   


  

  

  















  

   

    Commits:
   

   

    





 

  
   


    

    Summary
    ID

   

  
  

   


    

    

     

      Add a raise_invalid_type helper for completeness checking.
      
Type checkers for Python are great, but one of the things they do is
they'll flag code as unreachable if there are branches which are
checking the type of arguments, even if that branch exists in order to
provide runtime type-checking of arguments for public APIs.

`typing.assert_never` is designed for this case, but annoyingly it
always raises an AssertionError, and does not allow customization of the
message.

This change adds a new method, `typelets.runtime.raise_invalid_type`,
which can be used in the same way as `assert_never`, but defaults to
raising a `ValueError` (and allows different exception types if
desired), as well as taking in a message to include in the exception.
This allows us to write APIs which perform runtime type checking on
arguments without having the type checkers flag said checking as
unreachable based on the type hints.

Testing Done:
Used this in some other code.

     


    		
    lxzzuunlmunktlyoysxnxooumqzulpsp

   




   


    

    

     

      Add a raise_invalid_type helper for completeness checking.
      
Type checkers for Python are great, but one of the things they do is
they'll flag code as unreachable if there are branches which are
checking the type of arguments, even if that branch exists in order to
provide runtime type-checking of arguments for public APIs.

`typing.assert_never` is designed for this case, but annoyingly it
always raises an AssertionError, and does not allow customization of the
message.

This change adds a new method, `typelets.runtime.raise_invalid_type`,
which can be used in the same way as `assert_never`, but defaults to
raising a `ValueError` (and allows different exception types if
desired), as well as taking in a message to include in the exception.
This allows us to write APIs which perform runtime type checking on
arguments without having the type checkers flag said checking as
unreachable based on the type hints.

Testing Done:
Used this in some other code.

     


    		
    lxzzuunlmunktlyoysxnxooumqzulpsp

   



  
 







   

  





  

   

    Diff:
   

   

    
 Revision 2 (+108)
Show changes
 
    README.md 
    docs/index.rst 
    docs/coderef/index.rst 
    typelets/runtime.py 
 

   

  











 

  
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