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

[David Foster]

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