fields: preserve containers when copying fields

[T3pp31]

Summary

Fix copying of _FieldContainer instances so wrappers such as Emph remain wrappers instead of being replaced by their contained field.

The root cause was that _FieldContainer delegated missing attributes to fld, so container.copy() resolved to container.fld.copy() when the container class did not define its own copy() method. This also affected packet field copying when fields_desc references another packet class and overrides a copied field default.

Fixes #4657.

Changes

• Add _FieldContainer.copy() to copy the container instance and its stored attributes.
• Delegate unsupported attribute assignment to the contained field, matching the existing read delegation needed by field default replacement.
• Add a regression test for direct Emph(...).copy() and the packet-metaclass copy path from the issue.

Validation

• python3 -m scapy.tools.UTscapy -t test/fields.uts -N -qq -f xUnit -o /tmp/scapy-fields.xml
• /tmp/scapy-flake8-venv/bin/flake8 scapy/

Note: tox is not installed in this local environment, so flake8 was run through a temporary venv using the repository-pinned flake8<6.0.0 version.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.04%. Comparing base (66ef96a) to head (159afb7).
⚠️ Report is 48 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/fields.py	83.33%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5009      +/-   ##
==========================================
- Coverage   80.31%   80.04%   -0.28%     
==========================================
  Files         381      384       +3     
  Lines       93630    95658    +2028     
==========================================
+ Hits        75202    76565    +1363     
- Misses      18428    19093     +665     

Files with missing lines	Coverage Δ
scapy/fields.py	92.60% <83.33%> (-0.50%)	⬇️

... and 62 files with indirect coverage changes

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[Copilot]

Pull request overview

This PR fixes a longstanding copy bug for _FieldContainer wrappers (e.g., Emph) so that copying preserves the container type rather than collapsing to the wrapped field—particularly impacting Packet_metaclass default-replacement when fields_desc references another packet (issue #4657).

Changes:

• Add _FieldContainer.copy() to return a copied container instance (including copying its stored attributes/slots).
• Add _FieldContainer.__setattr__() to delegate unsupported attribute assignment to the wrapped field (mirroring existing read delegation).
• Add regression tests covering Emph(...).copy() and the packet-metaclass copy/default override path.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
scapy/fields.py	Implements _FieldContainer.copy() and adds write-delegation via __setattr__ to preserve wrappers during copy/default replacement.
test/fields.uts	Adds regression coverage for direct container .copy() and packet metaclass field-copy behavior.

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[T3pp31]

Fixed — __setattr__ now checks whether attr is defined on the container class (via the MRO __dict__) before delegating to the wrapped field. Read-only descriptors like MultipleTypeField.fld will raise the original AttributeError instead of being forwarded to the selected underlying field.

[gpotter2]

What this function does looks very funky to me. Couldn't we just call the copy() of the sub field?

[T3pp31]

Good point — I've simplified it significantly.

We can't just return self.fld.copy() though: that was the original bug. Without an explicit copy() on _FieldContainer, the call goes through __getattr__ and ends up as self.fld.copy(), which returns a bare ByteField instead of an Emph wrapper (see #4657).

The new implementation is much shorter: create a new container via __new__, iterate over __slots__, and call .copy() on slot values where available. This keeps the wrapper type while still deep-copying the contained field.

[gpotter2]

Are you commenting with AI?

[T3pp31]

Thanks for the review! I've pushed an update addressing both comments:

• copy(): Simplified the implementation. The issue wasn't that we shouldn't call the sub-field's copy() at all — it's that without an explicit _FieldContainer.copy(), container.copy() was resolved via __getattr__ and returned only container.fld.copy() (a plain ByteField instead of Emph). The new version creates a new container instance and copies each __slots__ entry, calling .copy() on values that support it (e.g. the wrapped field).

• __setattr__: Added a guard so delegation only happens for attributes that aren't defined on the container class. Assignments like mtf.fld = ... on MultipleTypeField now correctly raise AttributeError instead of silently mutating the selected underlying field.

Rebased onto current master. All test/fields.uts tests pass, including the new regression test.