Allow negative weights from networkx input

src/pymatching/matching.py

@@ -305,8 +305,6 @@ def load_from_networkx(self, graph: nx.Graph) -> None:
                         " (or convertible to a set), not {}".format(fault_ids))
             all_fault_ids = all_fault_ids | fault_ids
             weight = attr.get("weight", 1) # Default weight is 1 if not provided
-            if weight < 0:
-                raise ValueError("Weights cannot be negative.")
             e_prob = attr.get("error_probability", -1)
             g.add_edge(u, v, fault_ids, weight, e_prob, 0 <= e_prob <= 1)
         self.matching_graph = g

tests/test_matching.py

@@ -91,15 +91,6 @@ def test_too_many_checks_per_qubit_raises_value_error():
         Matching(H)
 
 
-def test_negative_weight_raises_value_error():
-    g = nx.Graph()
-    g.add_edge(0,1,weight=-1)
-    with pytest.raises(ValueError):
-        Matching(g)
-    with pytest.raises(ValueError):
-        Matching(csr_matrix([[1,1,0],[0,1,1]]), spacelike_weights=np.array([1,1,-1]))
-
-
 def test_wrong_check_matrix_type_raises_type_error():
     with pytest.raises(TypeError):
         Matching("test")

tests/test_negative_weghts.py

@@ -1,5 +1,6 @@
 import pytest
 import numpy as np
+import networkx as nx
 
 from pymatching import Matching
 
@@ -32,11 +33,12 @@ def test_isolated_negative_weight(nn):
 
 @pytest.mark.parametrize("nn", (None, 30))
 def test_negative_and_positive_in_matching(nn):
-    m = Matching()
-    m.add_edge(0, 1, 0, 1)
-    m.add_edge(1, 2, 1, -10)
-    m.add_edge(2, 3, 2, 1)
-    m.add_edge(3, 0, 3, 1)
+    g = nx.Graph()
+    g.add_edge(0, 1, fault_ids=0, weight=1)
+    g.add_edge(1, 2, fault_ids=1, weight=-10)
+    g.add_edge(2, 3, fault_ids=2, weight=1)
+    g.add_edge(3, 0, fault_ids=3, weight=1)
+    m = Matching(g)
     c, w = m.decode([0, 1, 0, 1], return_weight=True, num_neighbours=nn)
     assert np.array_equal(c, np.array([0, 1, 1, 0]))
     assert w == -9