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[Python-ideas] namedtuple fields with default values
Guido van Rossumguido at python.org
Fri Jul 17 18:59:21 CEST 2015
FWIW, PEP 484 (and hence typing.py) uses this syntax with a differentmeaning:* NamedTuple, used as ``NamedTuple(type_name, [(field_name, field_type), ...])`` and equivalent to ``collections.namedtuple(type_name, [field_name, ...])``. This is useful to declare the types of the fields of a a named tuple type.On Fri, Jul 17, 2015 at 4:59 PM, Eric V. Smith <eric at trueblade.com> wrote:> -----BEGIN PGP SIGNED MESSAGE-----> Hash: SHA1>> On 07/17/2015 10:44 AM, Barry Warsaw wrote:> > On Jul 16, 2015, at 01:03 PM, Russell Kaplan wrote:> >> >> I'm using a namedtuple to keep track of several fields, only> >> some of which ever need to be specified during instantiation.> >> However, there is no Pythonic way to create a namedtuple with> >> fields that have default values.> >> > I don't know about "Pythonic" but there's a not too horrible way> > to do it:> >> > _Record = namedtuple('Record', 'url destination checksum')('', '',> > '')> >> > def Record(url, destination, checksum=''): return _Record._replace(> > url=url, destination=destination, checksum=checksum)> >> > Now you only need to provide 'url', and 'destination' when you> > create a Record. Okay, sure _Record is the actual namedtuple, but> > I usually don't care.> >> >> Having to assign to Foo.__new__.__defaults__ is a bit ugly. I> >> think it would be easier and more readable to support syntax> >> like:> >>>>> Foo = namedtuple('Foo', ['optional_bar=None',> >>>>> 'optional_baz=None'])> >> > That would mean you couldn't ever have an actual parameter called> > 'optional_bar'.> >> >> This suggestion is fully backwards compatible and allows for> >> cleaner definitions of nametuples with default-value fields.> >> Thanks for considering.> >> > Not that I think anything really needs to be done, but a few other> > approaches could be:> >> > * Allow for arbitrary keyword arguments at the end of the> > signature to define default values.> >> > Record = namedtuple('Record', 'url destination checksum',> > checksum='')> >> > * Extend the semantics of field_names to allow for a dictionary of> > attributes mapping to their default values, though you'd need a> > marker to be able to specify a required field:> >> > Record = namedtuple('Record', dict(url=Required,> > destination=Required, checksum=''))> >> > I suppose I'd prefer the former, although that might cut off the> > ability to add other controlling arguments to the namedtuple()> > API.>> I've implemented default parameters to namedtuples (and namedlists a> mutable version) inhttps://pypi.python.org/pypi/namedlist>> The syntax is not great, but none of the options are awesome.>> >>> Point = namedlist.namedtuple('Point', [('x', 0), ('y', 100)])> >>> p = Point() assert p.x == 0 assert p.y == 100>>>> -----BEGIN PGP SIGNATURE-----> Version: GnuPG v1.4.14 (GNU/Linux)>> iQEcBAEBAgAGBQJVqRg5AAoJENxauZFcKtNxLSYH/iC2Twf2xYhAbtKdMF7nSvG2> xOhhePDOBWl4K9cUVgg0z4jRsnGgL8O8FPCGVmsrBu8arseVAQbsFiAcOsmlKy3k> XZGYQ61KPlDUcqxIO661cOWDiRJG/I9ltQHlafODEs4qGJfox2BeM5aCo+cIpyhk> uC7wJ/t9mJ9uKvv6mG/e1GixzKtgcCO86NfYwIqiwaZWsKnjkqNzQ1peKm6pGRTK> 34u7oCu6EGbqNCUdAJx9Re6Umcs37FH/f3uyc2luJyP9z1p1zz2Ndb00NwOvsGKK> NJw5enVB6/qC+sGZDicE1lJmij/MwdTbu/+Fl+JxjZeMGeRWJ9PEyHaxmuZAzw8=> =ZXBa> -----END PGP SIGNATURE-----> _______________________________________________> Python-ideas mailing list>Python-ideas at python.org>https://mail.python.org/mailman/listinfo/python-ideas> Code of Conduct:http://python.org/psf/codeofconduct/>-- --Guido van Rossum (python.org/~guido)-------------- next part --------------An HTML attachment was scrubbed...URL: <http://mail.python.org/pipermail/python-ideas/attachments/20150717/691225ac/attachment.html>
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