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
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