API, BUG: Raise error on complex input to i0 (#17062)

* BUG, API: Raise error on complex input to np.i0

2 files changed, 17 insertions, 15 deletions

diff --git a/numpy/lib/function_base.py b/numpy/lib/function_base.py
index cd8862c94..b530f0aa1 100644
--- a/numpy/lib/function_base.py
+++ b/numpy/lib/function_base.py
@@ -3193,25 +3193,18 @@ def i0(x):
     """
     Modified Bessel function of the first kind, order 0.
 
-    Usually denoted :math:`I_0`.  This function does broadcast, but will *not*
-    "up-cast" int dtype arguments unless accompanied by at least one float or
-    complex dtype argument (see Raises below).
+    Usually denoted :math:`I_0`.
 
     Parameters
     ----------
-    x : array_like, dtype float or complex
+    x : array_like of float
         Argument of the Bessel function.
 
     Returns
     -------
-    out : ndarray, shape = x.shape, dtype = x.dtype
+    out : ndarray, shape = x.shape, dtype = float
         The modified Bessel function evaluated at each of the elements of `x`.
 
-    Raises
-    ------
-    TypeError: array cannot be safely cast to required type
-        If argument consists exclusively of int dtypes.
-
     See Also
     --------
     scipy.special.i0, scipy.special.iv, scipy.special.ive
@@ -3241,12 +3234,16 @@ def i0(x):
     Examples
     --------
     >>> np.i0(0.)
-    array(1.0)  # may vary
-    >>> np.i0([0., 1. + 2j])
-    array([ 1.00000000+0.j        ,  0.18785373+0.64616944j])  # may vary
+    array(1.0)
+    >>> np.i0([0, 1, 2, 3])
+    array([1.        , 1.26606588, 2.2795853 , 4.88079259])
 
     """
     x = np.asanyarray(x)
+    if x.dtype.kind == 'c':
+        raise TypeError("i0 not supported for complex values")
+    if x.dtype.kind != 'f':
+        x = x.astype(float)
     x = np.abs(x)
     return piecewise(x, [x <= 8.0], [_i0_1, _i0_2])
 
diff --git a/numpy/lib/tests/test_function_base.py b/numpy/lib/tests/test_function_base.py
index 89c1a2d9b..635fe1432 100644
--- a/numpy/lib/tests/test_function_base.py
+++ b/numpy/lib/tests/test_function_base.py
@@ -2111,8 +2111,9 @@ class Test_I0:
             i0(0.5),
             np.array(1.0634833707413234))
 
-        A = np.array([0.49842636, 0.6969809, 0.22011976, 0.0155549])
-        expected = np.array([1.06307822, 1.12518299, 1.01214991, 1.00006049])
+        # need at least one test above 8, as the implementation is piecewise
+        A = np.array([0.49842636, 0.6969809, 0.22011976, 0.0155549, 10.0])
+        expected = np.array([1.06307822, 1.12518299, 1.01214991, 1.00006049, 2815.71662847])
         assert_almost_equal(i0(A), expected)
         assert_almost_equal(i0(-A), expected)
 
@@ -2149,6 +2150,10 @@ class Test_I0:
 
         assert_array_equal(exp, res)
 
+    def test_complex(self):
+        a = np.array([0, 1 + 2j])
+        with pytest.raises(TypeError, match="i0 not supported for complex values"):
+            res = i0(a)
 
 class TestKaiser: