TST: Branch coverage improvement for `np.polynomial` (#18499)

* added tests for vander_nd in test_polyutils to cover some of the missed branches

* added tests to polyline and vander to improve branch coverage

* added tests to test_legendre.py to improve branch coverage

3 files changed, 34 insertions, 0 deletions

diff --git a/numpy/polynomial/tests/test_legendre.py b/numpy/polynomial/tests/test_legendre.py
index a2a212c24..92399c160 100644
--- a/numpy/polynomial/tests/test_legendre.py
+++ b/numpy/polynomial/tests/test_legendre.py
@@ -305,6 +305,9 @@ class TestIntegral:
         res = leg.legint(c2d, k=3, axis=1)
         assert_almost_equal(res, tgt)
 
+    def test_legint_zerointord(self):
+        assert_equal(leg.legint((1, 2, 3), 0), (1, 2, 3))
+
 
 class TestDerivative:
 
@@ -345,6 +348,9 @@ class TestDerivative:
         res = leg.legder(c2d, axis=1)
         assert_almost_equal(res, tgt)
 
+    def test_legder_orderhigherthancoeff(self):
+        c = (1, 2, 3, 4)
+        assert_equal(leg.legder(c, 4), [0])
 
 class TestVander:
     # some random values in [-1, 1)
@@ -393,6 +399,9 @@ class TestVander:
         van = leg.legvander3d([x1], [x2], [x3], [1, 2, 3])
         assert_(van.shape == (1, 5, 24))
 
+    def test_legvander_negdeg(self):
+        assert_raises(ValueError, leg.legvander, (1, 2, 3), -1)
+
 
 class TestFitting:
 
@@ -541,6 +550,9 @@ class TestMisc:
     def test_legline(self):
         assert_equal(leg.legline(3, 4), [3, 4])
 
+    def test_legline_zeroscl(self):
+        assert_equal(leg.legline(3, 0), [3])
+
     def test_leg2poly(self):
         for i in range(10):
             assert_almost_equal(leg.leg2poly([0]*i + [1]), Llist[i])
diff --git a/numpy/polynomial/tests/test_polynomial.py b/numpy/polynomial/tests/test_polynomial.py
index 5fd1a82a2..a0a09fcf4 100644
--- a/numpy/polynomial/tests/test_polynomial.py
+++ b/numpy/polynomial/tests/test_polynomial.py
@@ -473,6 +473,10 @@ class TestVander:
         van = poly.polyvander3d([x1], [x2], [x3], [1, 2, 3])
         assert_(van.shape == (1, 5, 24))
 
+    def test_polyvandernegdeg(self):
+        x = np.arange(3)
+        assert_raises(ValueError, poly.polyvander, x, -1)
+
 
 class TestCompanion:
 
@@ -591,3 +595,6 @@ class TestMisc:
 
     def test_polyline(self):
         assert_equal(poly.polyline(3, 4), [3, 4])
+
+    def test_polyline_zero(self):
+        assert_equal(poly.polyline(3, 0), [3])
diff --git a/numpy/polynomial/tests/test_polyutils.py b/numpy/polynomial/tests/test_polyutils.py
index 1b27f53b5..cc630790d 100644
--- a/numpy/polynomial/tests/test_polyutils.py
+++ b/numpy/polynomial/tests/test_polyutils.py
@@ -40,6 +40,21 @@ class TestMisc:
         assert_equal(pu.trimcoef(coef, 1), coef[:-3])
         assert_equal(pu.trimcoef(coef, 2), [0])
 
+    def test_vander_nd_exception(self):
+        # n_dims != len(points)
+        assert_raises(ValueError, pu._vander_nd, (), (1, 2, 3), [90])
+        # n_dims != len(degrees)
+        assert_raises(ValueError, pu._vander_nd, (), (), [90.65])
+        # n_dims == 0
+        assert_raises(ValueError, pu._vander_nd, (), (), [])
+
+    def test_div_zerodiv(self):
+        # c2[-1] == 0
+        assert_raises(ZeroDivisionError, pu._div, pu._div, (1, 2, 3), [0])
+
+    def test_pow_too_large(self):
+        # power > maxpower
+        assert_raises(ValueError, pu._pow, (), [1, 2, 3], 5, 4)
 
 class TestDomain: