[Python-ideas] Consistent programming error handling idiom

Rian Hunter rian at thelig.ht
Fri Apr 8 17:25:47 EDT 2016

>> On Fri, Apr 08, 2016 at 11:24:15AM -0700, rian at 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. 

I think you're misinterpreting me. I don't want a pony and I don't want a sufficiently smart compiler.

I want a consistent opt-in idiom with community consensus. I want a clear way to express that an exception is an error and not an internal bug. It doesn't have to catch 100% of cases, the idiom just needs to approach consistency across all Python libraries that I may import.

If the programmer doesn't pay attention to the idiom, then is_a_bug() will never return true (or not True if it's is_not_a_bug()). AssertionError is already unambiguous, I'm sure there are other candidates as well.

I'm not the first or only one to want something like this http://blog.tsunanet.net/2012/10/pythons-screwed-up-exception-hierarchy.html

But I can see the pile on has begun and my point is lost. Maybe this will get added in ten or twenty years.
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