TST: Additional tests for Series ufuncs

This adds a set of tests for ufuncs on Series. The goal is to
establish the correct behavior prior to implementing `Series.__array_ufunc__`.

There are two kinds of xfails right now

1. Series[Sparse] fails because `Series.__array_ufunc__` doesn't
   yet dispatch to `Series.array.__array_ufunc__`
2. `ufunc(series, series)` when the two series are unaligned. It's
   been determined that these should align, but isn't currently
   implemented.

Author: TomAugspurger

pandas/tests/series/test_ufunc.py

@@ -0,0 +1,152 @@
+import string
+
+import numpy as np
+import pytest
+
+import pandas as pd
+import pandas.util.testing as tm
+
+UNARY_UFUNCS = [np.positive, np.floor, np.exp]
+BINARY_UFUNCS = [np.add, np.logaddexp]  # -> dunder op
+SPARSE = [
+    pytest.param(True,
+                 marks=pytest.mark.xfail(reason="Series.__array_ufunc__")),
+    False,
+]
+SPARSE_IDS = ['sparse', 'dense']
+SHUFFLE = [
+    pytest.param(True, marks=pytest.mark.xfail(reason="GH-26945")),
+    False
+]
+
+
+@pytest.fixture
+def arrays_for_binary_ufunc():
+    """
+    A pair of random, length-100 integer-dtype arrays, that are mostly 0.
+    """
+    a1 = np.random.randint(0, 10, 100)
+    a2 = np.random.randint(0, 10, 100)
+    a1[::3] = 0
+    a2[::4] = 0
+    return a1, a2
+
+
+@pytest.mark.parametrize("ufunc", UNARY_UFUNCS)
+@pytest.mark.parametrize("sparse", SPARSE, ids=SPARSE_IDS)
+def test_unary_ufunc(ufunc, sparse):
+    array = np.random.randint(0, 10, 10)
+    array[::2] = 0
+    if sparse:
+        array = pd.SparseArray(array, dtype=pd.SparseDtype('int', 0))
+
+    index = list(string.ascii_letters[:10])
+    name = "name"
+    series = pd.Series(array, index=index, name=name)
+
+    result = ufunc(series)
+    expected = pd.Series(ufunc(array), index=index, name=name)
+    tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize("ufunc", BINARY_UFUNCS)
+@pytest.mark.parametrize("sparse", SPARSE, ids=SPARSE_IDS)
+@pytest.mark.parametrize("shuffle", SHUFFLE)
+@pytest.mark.parametrize("box_other", [True, False])
+def test_binary_ufunc(ufunc, sparse, shuffle, box_other,
+                      arrays_for_binary_ufunc):
+    # Check the invariant that
+    #   ufunc(Series(a), Series(b)) == Series(ufunc(a, b))
+    #   with alignment.
+    a1, a2 = arrays_for_binary_ufunc
+    if sparse:
+        a1 = pd.SparseArray(a1, dtype=pd.SparseDtype('int', 0))
+        a2 = pd.SparseArray(a2, dtype=pd.SparseDtype('int', 0))
+
+    name = "name"
+    # TODO: verify name when the differ? Take the first? Drop?
+    s1 = pd.Series(a1, name=name)
+    s2 = pd.Series(a2, name=name)
+
+    # handle shufling / alignment
+    # If boxing -- ufunc(series, series) -- then we don't need to shuffle
+    # the other array for the expected, since we align.
+    # If not boxing -- ufunc(series, array) -- then we do need to shuffle
+    # the other array, since we *dont'* align
+    idx = np.random.permutation(len(s1))
+    if box_other and shuffle:
+        # ensure we align before applying the ufunc
+        s2 = s2.take(idx)
+    elif shuffle:
+        a2 = a2.take(idx)
+
+    result = ufunc(s1, s2)
+    expected = pd.Series(ufunc(a1, a2), name=name)
+    tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize("ufunc", BINARY_UFUNCS)
+@pytest.mark.parametrize("sparse", SPARSE, ids=SPARSE_IDS)
+@pytest.mark.parametrize("flip", [True, False])
+def test_binary_ufunc_scalar(ufunc, sparse, flip, arrays_for_binary_ufunc):
+    array, _ = arrays_for_binary_ufunc
+    if sparse:
+        array = pd.SparseArray(array)
+    other = 2
+    series = pd.Series(array, name="name")
+
+    a, b = series, other
+    c, d = array, other
+    if flip:
+        c, d = b, c
+        a, b = b, a
+
+    expected = pd.Series(ufunc(a, b), name="name")
+    result = pd.Series(ufunc(c, d), name="name")
+    tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize("ufunc", [np.divmod])  # any others?
+@pytest.mark.parametrize("sparse", SPARSE, ids=SPARSE_IDS)
+@pytest.mark.parametrize("shuffle", SHUFFLE)
+@pytest.mark.filterwarnings("ignore:divide by zero:RuntimeWarning")
+def test_multiple_ouput_binary_ufuncs(ufunc, sparse, shuffle,
+                                      arrays_for_binary_ufunc):
+    a1, a2 = arrays_for_binary_ufunc
+
+    if sparse:
+        a1 = pd.SparseArray(a1, dtype=pd.SparseDtype('int', 0))
+        a2 = pd.SparseArray(a2, dtype=pd.SparseDtype('int', 0))
+
+    s1 = pd.Series(a1)
+    s2 = pd.Series(a2)
+
+    if shuffle:
+        # ensure we align before applying the ufunc
+        s2 = s2.sample(frac=1)
+
+    expected = ufunc(a1, a2)
+    assert isinstance(expected, tuple)
+
+    result = ufunc(s1, s2)
+    assert isinstance(result, tuple)
+    tm.assert_series_equal(result[0], pd.Series(expected[0]))
+    tm.assert_series_equal(result[1], pd.Series(expected[1]))
+
+
+@pytest.mark.parametrize("sparse", SPARSE, ids=SPARSE_IDS)
+def test_multiple_ouput_ufunc(sparse, arrays_for_binary_ufunc):
+    array, _ = arrays_for_binary_ufunc
+
+    if sparse:
+        array = pd.SparseArray(array)
+
+    series = pd.Series(array, name="name")
+    result = np.modf(series)
+    expected = np.modf(array)
+
+    assert isinstance(result, tuple)
+    assert isinstance(expected, tuple)
+
+    tm.assert_series_equal(result[0], pd.Series(expected[0], name="name"))
+    tm.assert_series_equal(result[1], pd.Series(expected[1], name="name"))