On 1/26/2019 12:30 PM, David Mertz wrote:
On Sat, Jan 26, 2019 at 10:31 AM Steven D'Aprano <st...@pearwood.info
<mailto:st...@pearwood.info>> wrote:
In what way is it worse, given that returning a namedtuple with named
fields is backwards compatible with returning a regular tuple? We can
have our cake and eat it too.
Unless the caller does a type-check, there is no difference. Sequence
unpacking will still work, and namedtuples unlike regular tuples can
support optional attributes.
I suppose the one difference is where someone improperly relies on tuple
unpacking.
Old version:
def myfun():
# ...
return a, b, c
# Call site
val1, val2, val3 = myfun()
New version:
def myfun():
# ...
return a, b, c, d
Now the call site will get "ValueError: too many values to unpack".
Namedtuples don't solve this problem, of course. But they don't make
anything worse either.
The better approach, of course, is to document the API as only using
attribute access, not positional. I reckon dataclasses from the start
could address that concern... but so can documentation alone. E.g.:
Old version (improved):
def myfun():
mydata = namedtuple("mydata", "a b c")
# ...
return mydata(a, b, c)
# Call site
ret = myfun()
val1, val2, val3 = ret.a, ret.b, ret.c
New version (improved)
def myfun():
mydata = namedtuple("mydata", "a b c d e")
# ...
return mydata(a, b, c, d, e)
Now the call site is completely happy with no changes (assuming it
doesn't need to care about what values 'ret.d' or 'ret.e' contain... but
presumably those extra values are optional in some way.
Moreover, we are even perfectly fine if we had created
namedtuple("mydata", "e d c b a") for some reason, completely changing
the positions of all the named attributes in the improved namedtuple.
Preventing this automatic unpacking (and preventing iteration in
general) was one of the motivating factors for dataclasses:
https://www.python.org/dev/peps/pep-0557/#id47
Eric
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