test: incidence() is checked cell by cell

[petercorke]

Summary

Tests for incidence() that compare the matrix cell by cell. No source change.

0.6.5 returned the wrong matrix for every graph with an edge, from two bugs visible in its source:

for i, vertex in enumerate(self):            # i = vertex index
    for i, e in enumerate(vertex.edges()):   # reuses i: the edge's position in the vertex's list
        I[i, edict[e]] = 1

1. the loop variable i is overwritten, so edges are marked in the wrong rows;
2. only each vertex's own edges() are visited, so in a directed graph the vertex at the end of an edge is never marked.

Both were fixed in this cycle (the matrix is now built from edges() and both endpoints), but nothing pinned them: the existing tests check only column sums and row sums, because edges() was a set and the column order was undefined. It is now in the order the edges were added, so the matrix can be checked by position.

Tests

tests/test_incidence.py (32, both graph kinds), with each expectation written out by hand rather than derived from the code under test:

• empty graph and vertices with no edges (shape)
• a path, a star, edges pointing backwards, and a vertex that is only ever an edge's end (the two bugs)
• rows follow vertex creation order, not names; columns follow the order edges were added, and are edges() order
• parallel and antiparallel edges, undirected and directed loops (2 and 1)
• columns close up after remove_edge, rows after remove_vertex, and an isolated vertex is a row of zeros

They discriminate: run against the real v0.6.5 source, 16 of the 16 that do not need newer API fail, all by assertion.

Also corrects a stale comment in test_graph.py that said the column order was not deterministic.

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

🟢 Metrics 22 complexity · 0 duplication

Metric	Results
Complexity	22
Duplication	0

View in Codacy

_{NEW Get contextual insights on your PRs based on Codacy's metrics, along with PR and Jira context, without leaving GitHub. Enable AI reviewer
TIP This summary will be updated as you push new changes.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.37%. Comparing base (6e2e81a) to head (96e5479).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #50   +/-   ##
=======================================
  Coverage   95.37%   95.37%           
=======================================
  Files           3        3           
  Lines        1277     1277           
=======================================
  Hits         1218     1218           
  Misses         59       59           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.