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

[Chris Angelico]

On Mon, Apr 2, 2018 at 7:24 AM, David Foster <davidfstr at gmail.com> 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.
>

It's not as simple as you think. Here's a demo. Using all of the
information available to the compiler, tell me which of these names
are valid and which are not:


import sys

def search_file(fn):
    with open(fn) as f:
        for line in f:
            if "spam" in line and len(line) < 80:
                print("Mmmm delicious spam")

if __name__ == "__main__":
    try:
        search_file(sys.argv[1])
    except IndexError:
        print("Please provide a file name", file=sys.stderr)
    except FileNotFoundError:
        print("File not found", file=sys.stderr)
    except OSError as Exception:
        print("Error %d reading file: %s" %
            (Exception.errno, Exception.strerror))
    except Exception:
        print("Something else went wrong:")
        import traceback
        traceback.print_exc()
    finally:
        print("Goodbye.")


Okay. Reckon you got them all? Look again  there's probably at least
one sneaky one that you missed. List them for me, and tell me what's
valid and what's not.

ChrisA