BUG: Fix misleading assert message in assert_almost_equal #12200

Fixes #12200 by making a copy of the matrix before NaN's are excluded.
Add a test for it.

2 files changed, 22 insertions, 5 deletions

diff --git a/numpy/testing/_private/utils.py b/numpy/testing/_private/utils.py
index a3832fcde..24b32cf00 100644
--- a/numpy/testing/_private/utils.py
+++ b/numpy/testing/_private/utils.py
@@ -692,6 +692,9 @@ def assert_array_compare(comparison, x, y, err_msg='', verbose=True,
     x = array(x, copy=False, subok=True)
     y = array(y, copy=False, subok=True)
 
+    # original array for output formating
+    ox, oy = x, y
+
     def isnumber(x):
         return x.dtype.char in '?bhilqpBHILQPefdgFDG'
 
@@ -780,10 +783,10 @@ def assert_array_compare(comparison, x, y, err_msg='', verbose=True,
         # do not trigger a failure (np.ma.masked != True evaluates as
         # np.ma.masked, which is falsy).
         if cond != True:
-            match = 100-100.0*reduced.count(1)/len(reduced)
-            msg = build_err_msg([x, y],
+            mismatch = 100.0 * reduced.count(0) / ox.size
+            msg = build_err_msg([ox, oy],
                                 err_msg
-                                + '\n(mismatch %s%%)' % (match,),
+                                + '\n(mismatch %s%%)' % (mismatch,),
                                 verbose=verbose, header=header,
                                 names=('x', 'y'), precision=precision)
             raise AssertionError(msg)
diff --git a/numpy/testing/tests/test_utils.py b/numpy/testing/tests/test_utils.py
index e0d3414f7..7e6b18631 100644
--- a/numpy/testing/tests/test_utils.py
+++ b/numpy/testing/tests/test_utils.py
@@ -469,7 +469,8 @@ class TestAlmostEqual(_GenericTest):
         self._test_not_equal(x, z)
 
     def test_error_message(self):
-        """Check the message is formatted correctly for the decimal value"""
+        """Check the message is formatted correctly for the decimal value.
+           Also check the message when input includes inf or nan (gh12200)"""
         x = np.array([1.00000000001, 2.00000000002, 3.00003])
         y = np.array([1.00000000002, 2.00000000003, 3.00004])
 
@@ -493,6 +494,19 @@ class TestAlmostEqual(_GenericTest):
             # remove anything that's not the array string
             assert_equal(str(e).split('%)\n ')[1], b)
 
+        # Check the error message when input includes inf or nan
+        x = np.array([np.inf, 0])
+        y = np.array([np.inf, 1])
+        try:
+            self._assert_func(x, y)
+        except AssertionError as e:
+            msgs = str(e).split('\n')
+            # assert error percentage is 50%
+            assert_equal(msgs[3], '(mismatch 50.0%)')
+            # assert output array contains inf
+            assert_equal(msgs[4], ' x: array([inf,  0.])')
+            assert_equal(msgs[5], ' y: array([inf,  1.])')
+
     def test_subclass_that_cannot_be_bool(self):
         # While we cannot guarantee testing functions will always work for
         # subclasses, the tests should ideally rely only on subclasses having
@@ -1077,7 +1091,7 @@ class TestStringEqual(object):
 
         assert_raises(AssertionError,
                       lambda: assert_string_equal("foo", "hello"))
-        
+
     def test_regex(self):
         assert_string_equal("a+*b", "a+*b")