On Fri, Apr 08, 2016 at 11:24:15AM -0700, rian@thelig.ht wrote:

I want to live in a world where I can do this:

  while cbs:

      cb = cbs.pop()

      try:

          cb()

      except Exception as e:

          logging.exception("In main loop")

          if is_a_bug(e):

              raise SystemExit() from e

And I want a pony :-)

So long as Python allows people to write code like this:

# A deliberately silly example

if len(seq) == 0:

  raise ImportError("assertion fails")

you cannot expect to automatically know what an exception means without 

any context of where it came from and why it happened. The above example 

is silly, but it doesn't take much effort to come up with more serious 

ones:

- an interpreter written in Python may raise SyntaxError, which 

is not a bug in the interpreter;

- a test framework may raise AssertionError for a failed test,

which is not a bug in the framework;

- a function may raise MemoryError if the call *would* run out of

memory, but without actually running out of memory; consequently

it is not a fatal error, while another function may raise the 

same MemoryError because it actually did fatally run out of memory.

Effectively, you want the compiler to Do What I Mean when it comes to 

exceptions. DWIM may, occasionally, be a good idea in applications, but 

I maintain it is never a good idea in a programming language.