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Python 3.11: “Zero cost” exception handling

bugs.python.org

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Re: Python 3.11: “Zero cost” exception handling

#11

Post by brianwawok »

ubercore wrote: post #4
zohch wrote: post #3

Earlier quoted context omitted.

> I've felt weird using exceptions like that How should they be used instead? Maybe I don't understand what you mean by "idiomatic control flow uses exceptions" - could you give an example. Maybe there is some use of exceptions that I'm not quite familiar with in Python.

The first that came to mind is how `get()` is handled in Django's ORM. The idiomatic way to look for a single object is to use `get`, then catch a `DoesNotExist` exception: From https://docs.djangoproject.com/en/3.2/ref/models/querysets/#... from django.core.exceptions import ObjectDoesNotExist try: blog = Blog.objects.get(id=1) entry = Entry.objects.get(blog=blog, entry_number=1) except ObjectDoesNotExist: print("Ei…

Right, but the better way to actually write this is something like

  entry = Entry.objects.filter(blog__id=1, 
  entry_number=1).first()
  if entry is None:
    # deal with does not exist
Maybe it's my scala/Java background shining through, but we are big Django users and we ban the "catch exceptions as standard" workflow, because there is almost always a cleaner way...

Re: Python 3.11: “Zero cost” exception handling

#12

Post by zohch »

openasocket wrote: post #8
zohch wrote: post #3

Earlier quoted context omitted.

> I've felt weird using exceptions like that How should they be used instead? Maybe I don't understand what you mean by "idiomatic control flow uses exceptions" - could you give an example. Maybe there is some use of exceptions that I'm not quite familiar with in Python.

When using a for-loop over an iterator, the iterator protocol in Python says to keep returning elements until you run out, at which point you throw an exception. So every loop over an iterator or iterable object in python throws an exception when it is done. https://docs.python.org/3/library/stdtypes.html#iterator.__n...

TIL, thanks for explaining, indeed would have expected exceptions to be pretty cheap also if I knew this.

Re: Python 3.11: “Zero cost” exception handling

#13

Post by zohch »

brianwawok wrote: post #11
ubercore wrote: post #4

Earlier quoted context omitted.

The first that came to mind is how `get()` is handled in Django's ORM. The idiomatic way to look for a single object is to use `get`, then catch a `DoesNotExist` exception: From https://docs.djangoproject.com/en/3.2/ref/models/querysets/#... from django.core.exceptions import ObjectDoesNotExist try: blog = Blog.objects.get(id=1) entry = Entry.objects.get(blog=blog, entry_number=1) except ObjectDoesNotExist: print("Ei…

Right, but the better way to actually write this is something like entry = Entry.objects.filter(blog__id=1, entry_number=1).first() if entry is None: # deal with does not exist Maybe it's my scala/Java background shining through, but we are big Django users and we ban the "catch exceptions as standard" workflow, because there is almost always a cleaner way...

> Right, but the better way to actually write this is something like

Maybe for some cases, but it does not do the exact same thing, see the comment here: https://stackoverflow.com/a/29455777/1598080

And if we are talking about idiomatic, I think it is maybe a stretch to count this as idiomatic for Python, but given it is documented for Django I think it is fair to call it idiomatic for Django.

Re: Python 3.11: “Zero cost” exception handling

#14

Post by zohch »

foobarbecue wrote: post #6
zohch wrote: post #3

Earlier quoted context omitted.

> I've felt weird using exceptions like that How should they be used instead? Maybe I don't understand what you mean by "idiomatic control flow uses exceptions" - could you give an example. Maybe there is some use of exceptions that I'm not quite familiar with in Python.

In python, it's normal to use exceptions in place of type checking, e.g. in polymorphic functions.

But that would not be for control flow.

Re: Python 3.11: “Zero cost” exception handling

#15

Post by pca006132 »

ziml77 wrote: post #2

They weren't zero cost before? In a language where idiomatic control flow uses exceptions? That's crazy! I've felt weird using exceptions like that but I always assumed that CPython was optimized to minimize overhead of exceptions and exception handlers.

Zero cost refers to the cost when no exception is thrown, not the overhead of exceptions. It may be more expensive throwing an exception under "zero cost" exception model, as throwing an exception may require parsing some data in the executable. (I'm not sure about the implementation, so this is just a may...)

Re: Python 3.11: “Zero cost” exception handling

#16

Post by ziml77 »

brianwawok wrote: post #11
ubercore wrote: post #4

Earlier quoted context omitted.

The first that came to mind is how `get()` is handled in Django's ORM. The idiomatic way to look for a single object is to use `get`, then catch a `DoesNotExist` exception: From https://docs.djangoproject.com/en/3.2/ref/models/querysets/#... from django.core.exceptions import ObjectDoesNotExist try: blog = Blog.objects.get(id=1) entry = Entry.objects.get(blog=blog, entry_number=1) except ObjectDoesNotExist: print("Ei…

Right, but the better way to actually write this is something like entry = Entry.objects.filter(blog__id=1, entry_number=1).first() if entry is None: # deal with does not exist Maybe it's my scala/Java background shining through, but we are big Django users and we ban the "catch exceptions as standard" workflow, because there is almost always a cleaner way...

That is the sort of method that I prefer if it's available. I'm a C# dev so this way also feels far cleaner to me.

Re: Python 3.11: “Zero cost” exception handling

#20

Post by zohch »

foobarbecue wrote: post #17
zohch wrote: post #14

Earlier quoted context omitted.

But that would not be for control flow.

On a smaller scale, it is.

To me sending wrong arguments is an error condition, not control flow, see this for more info: https://softwareengineering.stackexchange.com/questions/1892...
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