From 5351732faf4b7186628579709439e1b29494a560 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Mon, 16 Mar 2020 18:34:15 -0700
Subject: [PATCH] BUG: resample.agg with read-only data (#32758)

---
 doc/source/whatsnew/v1.0.3.rst             |  1 +
 pandas/_libs/groupby.pyx                   | 12 +++++++----
 pandas/tests/resample/test_resample_api.py | 24 ++++++++++++++++++++++
 3 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/doc/source/whatsnew/v1.0.3.rst b/doc/source/whatsnew/v1.0.3.rst
index 17f1bdc365518..482222fbddbb8 100644
--- a/doc/source/whatsnew/v1.0.3.rst
+++ b/doc/source/whatsnew/v1.0.3.rst
@@ -15,6 +15,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
+- Fixed regression in ``resample.agg`` when the underlying data is non-writeable (:issue:`31710`)
 
 .. _whatsnew_103.bug_fixes:
 
diff --git a/pandas/_libs/groupby.pyx b/pandas/_libs/groupby.pyx
index 27b3095d8cb4f..35a6963165194 100644
--- a/pandas/_libs/groupby.pyx
+++ b/pandas/_libs/groupby.pyx
@@ -848,11 +848,13 @@ cdef inline bint _treat_as_na(rank_t val, bint is_datetimelike) nogil:
         return val != val
 
 
+# GH#31710 use memorviews once cython 0.30 is released so we can
+#  use `const rank_t[:, :] values`
 @cython.wraparound(False)
 @cython.boundscheck(False)
 def group_last(rank_t[:, :] out,
                int64_t[:] counts,
-               rank_t[:, :] values,
+               ndarray[rank_t, ndim=2] values,
                const int64_t[:] labels,
                Py_ssize_t min_count=-1):
     """
@@ -937,11 +939,13 @@ def group_last(rank_t[:, :] out,
         raise RuntimeError("empty group with uint64_t")
 
 
+# GH#31710 use memorviews once cython 0.30 is released so we can
+#  use `const rank_t[:, :] values`
 @cython.wraparound(False)
 @cython.boundscheck(False)
 def group_nth(rank_t[:, :] out,
               int64_t[:] counts,
-              rank_t[:, :] values,
+              ndarray[rank_t, ndim=2] values,
               const int64_t[:] labels, int64_t rank=1,
               Py_ssize_t min_count=-1):
     """
@@ -1235,7 +1239,7 @@ ctypedef fused groupby_t:
 @cython.boundscheck(False)
 def group_max(groupby_t[:, :] out,
               int64_t[:] counts,
-              groupby_t[:, :] values,
+              ndarray[groupby_t, ndim=2] values,
               const int64_t[:] labels,
               Py_ssize_t min_count=-1):
     """
@@ -1308,7 +1312,7 @@ def group_max(groupby_t[:, :] out,
 @cython.boundscheck(False)
 def group_min(groupby_t[:, :] out,
               int64_t[:] counts,
-              groupby_t[:, :] values,
+              ndarray[groupby_t, ndim=2] values,
               const int64_t[:] labels,
               Py_ssize_t min_count=-1):
     """
diff --git a/pandas/tests/resample/test_resample_api.py b/pandas/tests/resample/test_resample_api.py
index d552241f9126f..6389c88c99f73 100644
--- a/pandas/tests/resample/test_resample_api.py
+++ b/pandas/tests/resample/test_resample_api.py
@@ -580,3 +580,27 @@ def test_agg_with_datetime_index_list_agg_func(col_name):
         columns=pd.MultiIndex(levels=[[col_name], ["mean"]], codes=[[0], [0]]),
     )
     tm.assert_frame_equal(result, expected)
+
+
+def test_resample_agg_readonly():
+    # GH#31710 cython needs to allow readonly data
+    index = pd.date_range("2020-01-01", "2020-01-02", freq="1h")
+    arr = np.zeros_like(index)
+    arr.setflags(write=False)
+
+    ser = pd.Series(arr, index=index)
+    rs = ser.resample("1D")
+
+    expected = pd.Series([pd.Timestamp(0), pd.Timestamp(0)], index=index[::24])
+
+    result = rs.agg("last")
+    tm.assert_series_equal(result, expected)
+
+    result = rs.agg("first")
+    tm.assert_series_equal(result, expected)
+
+    result = rs.agg("max")
+    tm.assert_series_equal(result, expected)
+
+    result = rs.agg("min")
+    tm.assert_series_equal(result, expected)