[MRG+2] Fix repr on isotropic kernels when a 1-D length scale is given (scikit-learn#7259)

Author: MechCoder

sklearn/gaussian_process/kernels.py

@@ -1204,7 +1204,7 @@ def __repr__(self):
                                                    self.length_scale)))
         else:  # isotropic
             return "{0}(length_scale={1:.3g})".format(
-                self.__class__.__name__, self.length_scale)
+                self.__class__.__name__, np.ravel(self.length_scale)[0])
 
 
 class Matern(RBF):
@@ -1348,9 +1348,10 @@ def __repr__(self):
                 self.__class__.__name__,
                 ", ".join(map("{0:.3g}".format, self.length_scale)),
                 self.nu)
-        else:  # isotropic
+        else:
             return "{0}(length_scale={1:.3g}, nu={2:.3g})".format(
-                self.__class__.__name__, self.length_scale, self.nu)
+                self.__class__.__name__, np.ravel(self.length_scale)[0],
+                self.nu)
 
 
 class RationalQuadratic(StationaryKernelMixin, NormalizedKernelMixin, Kernel):

sklearn/gaussian_process/tests/test_kernels.py

@@ -41,7 +41,8 @@
            4.0 * Matern(length_scale=[0.5, 0.5], nu=2.5),
            RationalQuadratic(length_scale=0.5, alpha=1.5),
            ExpSineSquared(length_scale=0.5, periodicity=1.5),
-           DotProduct(sigma_0=2.0), DotProduct(sigma_0=2.0) ** 2]
+           DotProduct(sigma_0=2.0), DotProduct(sigma_0=2.0) ** 2,
+           RBF(length_scale=[2.0]), Matern(length_scale=[2.0])]
 for metric in PAIRWISE_KERNEL_FUNCTIONS:
     if metric in ["additive_chi2", "chi2"]:
         continue
@@ -304,3 +305,10 @@ def test_set_get_params():
                 kernel.set_params(**{hyperparameter.name: value})
                 assert_almost_equal(np.exp(kernel.theta[index]), value)
                 index += 1
+
+
+def test_repr_kernels():
+    """Smoke-test for repr in kernels."""
+
+    for kernel in kernels:
+        repr(kernel)