On Mon, Sep 12, 2016 at 1:05 AM, Michel Desmoulin
<desmoulinmic...@gmail.com> wrote:
> There is also an alternative to this operator, and it's allowing a
> shortcut to do:
>
> try:
> val = do_thing()
> except ThingError:
> val = "default"
>
> In the form of:
>
> val = do_thing() except ThingError: "default"
Note that there's a subtle difference here when multiple lookups are
involved. Given:
def f(spam):
return spam().eggs().ham
With null-coalescing:
def f(spam):
return spam()?.eggs()?.ham
This is roughly equivalent to:
def f(spam):
_spam = spam()
try:
eggs = _spam.eggs
except AttributeError:
return None
_eggs = eggs()
try:
return _eggs.ham
except AttributeError:
return None
With PEP 463 it doesn't work out so well. The "obvious" spelling would be:
def f(spam):
return (spam().eggs().ham except AttributeError: None)
This is roughly equivalent to:
def f(spam):
try:
return spam().eggs().ham
except AttributeError:
return None
Note how it's different. For one thing, it could mask AttributeError
coming from the calls. For another, you no longer explicitly identify
which lookups to handle. I would expect both to lead to subtle bugs,
whereas with null-coalescing you don't have those problems.
-eric
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