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Allow callable defaults for any sealer #6
Description
First let me say - great library! :) I've been evaluating it as well as attrs for a while now.
I'd like to suggest a possible feature extension: allow callable defaults, which will let you do things like this:
from datetime import datetime
import fields
class Record(fields.Tuple.Username[str].Created[datetime.now].Revision[long].Active[bool]):
passUsing types as defaults IMO makes the code more readable. Also assuming the callable takes no arguments, it allows simple initialization at run-time (like with datetime.now).
It's easy to do that with a simple sealer wrapper, like the one below:
from functools import wraps
from fields import factory, tuple_sealer
def with_callable_defaults(sealer):
"""
Wraps the given `sealer` to support `callable` for any specified default.
"""
@wraps(sealer)
def wrapped_sealer(fields, defaults):
for field, default in defaults.iteritems():
if callable(default):
try:
defaults[field] = default()
except TypeError:
pass
return sealer(fields, defaults)
return wrapped_sealer
Tuple = factory(with_callable_defaults(tuple_sealer))What do you think?
Activity
ionelmc commentedon May 15, 2017
Another way, that can also keep backwards compat a bit is to (ab)use slices for defaults as callables, eg:
or even with strings for more possibilities (the init function is made from a string, alike namedtuple, so this is possible):
ionelmc commentedon May 15, 2017
Actually both forms could be supported.
Anyway, doesn't
attrsalready support default values from callables?dimitern commentedon May 16, 2017
I was thinking of abusing the slice syntax at first :) Could be useful for things like validators, converters, or even docstrings, esp. if combined with your suggestion about supporting strings:
Yeah,
attrssupports callable defaults and lots more, but I like the concise declarative style offieldsbetter I think :)I'll look into it some more, cheers!
ionelmc commentedon May 16, 2017
Well yes,
fieldsis not meant to replaceattrs, or even compete with it. As a matter of fact it precedesattrs(it's practically a jab at characteristic). You may have guessed it,attrsprobably wouldn't exist if I hadn't donefieldsxDIntended use is very small classes (5 fields or less). If you can't fit it on one line then probably you shouldn't use
fieldsat all - the whole idea of extreme brevity become a moot point if you split the definition on multiple lines in ugly jquery chain style.