`omit_defaults` does not omit tuples and frozensets

[jodal]

Description

When a struct has omit_defaults=True set, any fields defaulting to dict, list, or set is omitted from the serialization, as described in the docs.

However, if one is using tuple instead of list and frozenset instead of set as default values, to make the struct as immutable as possible, those defaults are included in the encoded format.

I think that tuple and frozenset should be handled in the same way as dict, list, andsetbyomit_defaultsandrepr_omit_defaults`.

Example

import msgspec

class Backpack(msgspec.Struct, omit_defaults=True):
    a_list: list[str] = msgspec.field(default_factory=list)
    a_tuple: tuple[str, ...] = msgspec.field(default_factory=tuple) 
    a_frozenset: frozenset[str] = msgspec.field(default_factory=frozenset) 

instance = Backpack()
print(msgspec.json.encode(instance))

Output:

b'{"a_tuple":[],"a_frozenset":[]}'

Expected:

b'{}'

[jcrist]

Thanks for opening this - this should be fixed by #653. Note that for these cases specifying the default value by value (rather than via default_factory) already works:

import msgspec

class Backpack(msgspec.Struct, omit_defaults=True):
    a_list: list[str] = []
    a_tuple: tuple[str, ...] = ()
    a_frozenset: frozenset[str] = frozenset()

instance = Backpack()
print(msgspec.json.encode(instance))
#> b'{}'

The reason tuple and frozenset are the special cases here is that they're both immutable types and may always be specified by value. Since there's no functional reason to use a default_factory for these types, we hadn't implemented the logic to check for equivalent defaults based on default_factory for tuple/frozenset.

[jodal]

Thank you for the very swift fix! Especially useful with the tip on how to achieve this without waiting for a new release :-)