Update the default repr() of symbols

dwave/optimization/model.pyx

@@ -40,6 +40,7 @@ import numpy as np
 from cpython cimport Py_buffer
 from cython.operator cimport dereference as deref, preincrement as inc
 from cython.operator cimport typeid
+from libc.stdint cimport uintptr_t
 from libcpp cimport bool
 from libcpp.utility cimport move
 from libcpp.vector cimport vector
@@ -1193,6 +1194,20 @@ cdef class Symbol:
         # via their subclasses.
         raise ValueError("Symbols cannot be constructed directly")
 
+    def __repr__(self):
+        """Return a representation of the symbol.
+
+        The representation refers to the id of the underlying node, rather than
+        the id of the Python symbol.
+        """
+        cls = type(self)
+        # We refer to the node_ptr, which is not necessarily the address of the
+        # C++ node, as it sublasses Node.
+        # But this is unique to each node, and functions as an id rather than
+        # as a pointer, so that's OK.
+        # Otherwise we aim to match Python's default __repr__.
+        return f"<{cls.__module__}.{cls.__qualname__} at {<uintptr_t>self.node_ptr:#x}>"
+
     cdef void initialize_node(self, Model model, cppNode* node_ptr) noexcept:
         self.model = model
 

releasenotes/notes/fix-to_networkx-a78f0f669cc9638c.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Make ``repr()`` of symbols unique to the underlying node, rather than to the Python symbol.
+    See `#52 <https://github.com/dwavesystems/dwave-optimization/issues/52>_.

tests/test_model.py

@@ -424,3 +424,14 @@ def test_abstract(self):
         from dwave.optimization.model import Symbol
         with self.assertRaisesRegex(ValueError, "Symbols cannot be constructed directly"):
             Symbol()
+
+    def test_repr(self):
+        model = Model()
+        c0 = model.constant(5)
+        c1, = model.iter_symbols()
+        c2 = model.constant(5)
+
+        # the specific form is an implementation detail, but different symbols
+        # representing the same underlying node should have the same repr
+        self.assertEqual(repr(c0), repr(c1))
+        self.assertNotEqual(repr(c0), repr(c2))