Why is the use of an undefined name not a syntax error?

[Terry Reedy]

On 4/1/2018 5:24 PM, David Foster wrote:
> My understanding is that the Python interpreter already has enough information when bytecode-compiling a .py file to determine which names correspond to local variables in functions. That suggests it has enough information to identify all valid names in a .py file and in particular to identify which names are not valid.
> 
> If broken name references were detected at compile time, it would eliminate a huge class of errors before running the program: missing imports, call of misspelled top-level function, reference to misspelled local variable.
> 
> Of course running a full typechecker like mypy would eliminate more errors like misspelled method calls, type mismatch errors, etc. But if it is cheap to detect a wide variety of name errors at compile time, is there any particular reason it is not done?
> 
> - David
> 
> P.S. Here are some uncommon language features that interfere with identifying all valid names. In their absence, one might expect an invalid name to be a syntax error:
> 
> * import *
> * manipulating locals() or globals()
> * manipulating a frame object
> * eval

The CPython parser and compiler are autogenerated from an LL(1) 
context-free grammer and other files.  Context-dependent rules like the 
above are for linters and other whole-program analyses.  A linter that 
makes occasional mistakes in its warning can still be useful.  A 
compiler should be perfect.


-- 
Terry Jan Reedy