Re: How to pop the interpreter's stack?



On Wed, 22 Dec 2010 13:53:20 -0800, Carl Banks wrote:

On Dec 22, 8:52 am, kj <no.em...@xxxxxxxxxxx> wrote:
In <mailman.65.1292517591.6505.python-l...@xxxxxxxxxx> Robert Kern
<robert.k...@xxxxxxxxx> writes:

Obfuscating the location that an exception gets raised prevents a lot
of debugging...

The Python interpreter does a lot of that "obfuscation" already, and I
find the resulting tracebacks more useful for it.

An error message is only useful to a given audience if that audience
can use the information in the message to modify what they are doing to
avoid the error.

So when the audience files a bug report it's not useful for them to
include the whole traceback?


Well, given the type of error KJ has been discussing, no, it isn't useful.

Fault: function raises documented exception when passed input that
is documented as being invalid

What steps will reproduce the problem?
1. call the function with invalid input
2. read the exception that is raised
3. note that it is the same exception as documented

What is the expected output? What do you see instead?

Excepted somebody to hit me on the back of the head and tell me
not to call the function with invalid input. Instead I just got
an exception.


You seem to have completely missed that there will be no bug report,
because this isn't a bug. (Or if it is a bug, the bug is elsewhere,
external to the function that raises the exception.) It is part of the
promised API. The fact that the exception is generated deep down some
chain of function calls is irrelevant.

The analogy is this: imagine a function that delegates processing of the
return result to different subroutines:

def func(arg):
if arg > 0:
return _inner1(arg)
else:
return _inner2(arg)


This is entirely irrelevant to the caller. When they receive the return
result from calling func(), they have no way of knowing where the result
came from, and wouldn't care even if they could. Return results hide
information about where the result was calculated, as they should. Why
shouldn't deliberate, explicit, documented exceptions be treated the same?

Tracebacks expose the implementation details of where the exception was
generated. This is the right behaviour if the exception is unexpected --
a bug internal to func -- since you need knowledge of the implementation
of func in order to fix the unexpected exception. So far so good -- we
accept that Python's behaviour under these circumstances is correct.

But this is not the right behaviour when the exception is expected, e.g.
an explicitly raised exception in response to an invalid argument. In
this case, the traceback exposes internal details of no possible use to
the caller. What does the caller care if func() delegates (e.g.) input
checking to a subroutine? The subroutine is an irrelevant implementation
detail. The exception is promised output of the function, just as much so
as if it were a return value.

Consider the principle that exceptions should be dealt with as close as
possible to the actual source of the problem:

f('good input')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in f
File "<stdin>", line 2, in g
File "<stdin>", line 2, in h
File "<stdin>", line 2, in i
File "<stdin>", line 2, in j
File "<stdin>", line 2, in k <=== error occurs here, and shown here
ValueError


But now consider the scenario where the error is not internal to f, but
external. The deeper down the stack trace you go, the further away from
the source of the error you get. The stack trace now obscures the source
of the error, rather than illuminating it:

f('bad input') <=== error occurs here
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in f
File "<stdin>", line 2, in g
File "<stdin>", line 2, in h
File "<stdin>", line 2, in i
File "<stdin>", line 2, in j
File "<stdin>", line 2, in k <=== far from the source of error
ValueError


There's no point in inspecting function k for a bug when the problem has
nothing to do with k. The problem is that the input fails to match the
pre-conditions for f. From the perspective of the caller, the error has
nothing to do with k, k is a meaningless implementation detail, and the
source of the error is the mismatch between the input and what f expects.
And so by the principle of dealing with exceptions as close as possible
to the source of the error, we should desire this traceback instead:


f('bad input') <=== error occurs here
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in f <=== matches where the error occurs
ValueError


In the absence of any practical way for function f to know whether an
arbitrary exception in a subroutine is a bug or not, the least-worst
decision is Python's current behaviour: take the conservative, risk-
adverse path and assume the worst, treat the exception as a bug in the
subroutine, and expose the entire stack trace.

But, I suggest, we can do better using the usual Python strategy of
implementing sensible default behaviour but allowing objects to customize
themselves. Objects can declare themselves to be instances of some other
class, or manipulate what names are reported by dir. Why shouldn't a
function deliberately and explicitly take ownership of an exception
raised by a subroutine?

There should be a mechanism for Python functions to distinguish between
unexpected exceptions (commonly known as "bugs"), which should be
reported as coming from wherever they come from, and documented, expected
exceptions, which should be reported as coming from the function
regardless of how deep the function call stack really is.



--
Steven
.



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