Fix cotengra bug for empty contraction paths with tests

[refraction-ray]

Fixed a bug in _algebraic_base_contraction where cotengra would fail when given an empty contraction path (0 or 1 tensors). Added explicit handling for 0 and 1 tensor cases to bypass cotengra and return the correct result. Also added a guard for nodes[0] access to prevent IndexError when no nodes are provided. Added comprehensive test cases in tests/test_hyperedge.py covering 0 nodes, 1 node, and 1 node with a self-loop (trace).

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PR created automatically by Jules for task 3005992988580253242 started by @refraction-ray

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[sonarqubecloud]

Quality Gate failed

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[gemini-code-assist]

Code Review

This pull request updates _algebraic_base_contraction in tensorcircuit/cons.py to handle empty node lists and contraction paths by providing a default backend and returning a scalar tensor. It also introduces unit tests for these edge cases in tests/test_hyperedge.py. Reviewers identified a potential crash when handling networks consisting solely of CopyNodes with dangling edges and recommended refining the logic to distinguish between empty node lists and empty tensor lists. Furthermore, moving local imports to the top of the test file was suggested to align with PEP 8 guidelines.

[gemini-code-assist]

The check len(raw_tensors) == 0 correctly handles the case where nodes is empty, but it also catches cases where nodes contains only CopyNodes (hyperedges). In the latter case, if there are dangling edges, returning a scalar ones([]) will cause a crash at line 632 because the tensor rank (0) will not match the number of dangling edges. It is safer to explicitly check if nodes is empty for the scalar 1.0 result, or handle the CopyNode-only case specifically.

Suggested change

	if len(raw_tensors) == 0:
	# Avoid cotengra bug for empty contraction paths
	final_raw_tensor = be.ones([])
	exponent = 0.0
	if len(nodes) == 0:
	# Avoid cotengra bug for empty contraction paths
	final_raw_tensor = be.ones([])
	exponent = 0.0
	elif len(raw_tensors) == 0:
	# Networks with only CopyNodes are not yet supported in the algebraic path
	# as they require constructing a dense delta tensor.
	raise ValueError("Algebraic contraction of networks with only CopyNodes is not supported.")

[gemini-code-assist]

It is generally preferred to place imports at the top of the file to adhere to PEP 8 guidelines and improve readability, unless there is a specific reason for a local import (e.g., avoiding circular dependencies). Since tensorcircuit is already imported as tc, you can also access the function via tc.cons._algebraic_base_contraction.

References

1. Imports should be at the top of the file. ^(link)