fix grad accumulation, not overriding

micrograd/engine.py

@@ -34,8 +34,8 @@ class Value:
                     _children=(self, other), _op='+')
 
         def _backward():
-            self.grad = 1.0 * out.grad
-            other.grad = 1.0 * out.grad
+            self.grad += 1.0 * out.grad
+            other.grad += 1.0 * out.grad
         out._backward = _backward
         return out
 
@@ -49,8 +49,8 @@ class Value:
                     _children=(self, other), _op='*')
 
         def _backward():
-            self.grad = other.data * out.grad
-            other.grad = self.data * out.grad
+            self.grad += other.data * out.grad
+            other.grad += self.data * out.grad
         out._backward = _backward
         return out
 
@@ -61,6 +61,6 @@ class Value:
                     _op='tanh', label='tanh')
 
         def _backward():
-            self.grad = (1 - t**2) * out.grad
+            self.grad += (1 - t**2) * out.grad
         out._backward = _backward
         return out

test/test_backprop.py

@@ -61,7 +61,6 @@ def test_large_backprop():
     assert pytest.approx(w2.grad, 0.001) == 0.0
 
 
-@pytest.mark.skip(reason="non-deterministic")
 def test_auto_diff():
     # inputs
     x1 = Value(2.0, label='x1')
@@ -89,7 +88,6 @@ def test_auto_diff():
 
     y.backward()
     assert pytest.approx(n.grad, 0.001) == 0.5
-    assert h.grad == 0.0
 
     assert pytest.approx(b.grad, 0.001) == 0.5
     assert pytest.approx(h.grad, 0.001) == 0.5
@@ -102,3 +100,10 @@ def test_auto_diff():
 
     assert pytest.approx(x2.grad, 0.001) == 0.5
     assert pytest.approx(w2.grad, 0.001) == 0.0
+
+
+def test_accumulation():
+    a = Value(2.0, label='x1')
+    b = a + a
+    b.backward()
+    assert a.grad == 2.0