Derive requiredness from the remaining defaults

[elinscott]

Giving a socket a default still left it reported as required, so a caller who relied on the default was told the input was missing. Given

from node_graph import socket_spec as ss

spec = ss.namespace(a=int, b=(int, 5))
print(spec.fields["b"].default)        # 5
print(spec.fields["b"].meta.required)  # True

b carries the default it was given and is still required, so nothing downstream can tell it apart from an input the caller simply never supplied.

This stacks on #173, which fixed the same defect for a dataclass or pydantic field. That PR listed three paths it did not cover; these are those three:

• the tuple form of a namespace entry, namespace(b=(int, 5)), which is also how a dynamic namespace's fixed fields are declared
• set_default(spec, "b", 5)
• the mapping default of a namespace parameter, def f(cfg: ns(x=int, y=int) = {"y": 3}), which sets defaults on the leaves the mapping names

Changes

Mark a leaf optional when any of those three gives it a default. That leaves SocketSpec(identifier, default=...), built by hand, as the only remaining way to end up with a default and required=True — see below.

set_default is a mutation rather than a construction, so it needs an inverse: unset_default makes the leaf required again. It restores requiredness by the same rule that removed it, so it only reverses optionality this mechanism introduced; a leaf whose optionality was declared some other way — a NotRequired TypedDict key, or an explicit required=False — is currently made required too, which is wrong and worth fixing separately.

Inner namespaces are left alone here, departing from #173, which marked a defaulted namespace field optional. The two construction paths refuse a namespace default outright, so no namespace case arises; and a mapping default names only some of the leaves below a namespace, so descending into one says nothing about whether the caller may skip it. #173's namespaces were different: a field annotated Optional[Sub] = None is a statement about the whole namespace.

Following #173, requiredness is set to False rather than left as None, since SocketMeta.to_dict drops None and from_dict then restores the True default.

What is left

A SocketSpec constructed by hand with a default — as While.max_iterations and If.invert_condition are in tasks/builtins.py — still comes out required. Deriving requiredness in SocketSpec.__post_init__ would catch that case and every other one, but it would also rewrite specs restored by from_dict, changing how an already-serialized graph deserializes. Decided to separate that decision from this PR.

[codecov]

Codecov Report

❌ Patch coverage is 96.92308% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.82%. Comparing base (8d85e61) to head (19e03b5).

Files with missing lines	Patch %	Lines
tests/test_socket_spec.py	96.22%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
+ Coverage   89.68%   89.82%   +0.14%     
==========================================
  Files          81       81              
  Lines        8984     9109     +125     
==========================================
+ Hits         8057     8182     +125     
  Misses        927      927              

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