From 6977085e356e1ebb969491ec9d1d2b38799413e6 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Fri, 26 Jul 2019 16:32:09 -0700
Subject: [PATCH] Comments

---
 pandas/core/internals/blocks.py | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 6e5a2aab298c7..0a631afa7d1b3 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -799,8 +799,11 @@ def replace(
         # If we cannot replace with own dtype, convert to ObjectBlock and
         # retry
         if not self._can_hold_element(to_replace):
-            # TODO: we should be able to infer at this point that there is
-            #  nothing to replace
+            if not isinstance(to_replace, list):
+                if inplace:
+                    return [self]
+                return [self.copy()]
+
             # GH 22083, TypeError or ValueError occurred within error handling
             # causes infinite loop. Cast and retry only if not objectblock.
             if is_object_dtype(self):
@@ -825,14 +828,27 @@ def replace(
             filtered_out = ~self.mgr_locs.isin(filter)
             mask[filtered_out.nonzero()[0]] = False
 
+        if not mask.any():
+            if inplace:
+                return [self]
+            return [self.copy()]
+
         try:
             blocks = self.putmask(mask, value, inplace=inplace)
+            # Note: it is _not_ the case that self._can_hold_element(value)
+            #  is always true at this point.  In particular, that can fail
+            #  for:
+            #   "2u" with bool-dtype, float-dtype
+            #   0.5 with int64-dtype
+            #   np.nan with int64-dtype
         except (TypeError, ValueError):
             # GH 22083, TypeError or ValueError occurred within error handling
             # causes infinite loop. Cast and retry only if not objectblock.
             if is_object_dtype(self):
                 raise
 
+            assert not self._can_hold_element(value), value
+
             # try again with a compatible block
             block = self.astype(object)
             return block.replace(
@@ -988,6 +1004,7 @@ def putmask(self, mask, new, align=True, inplace=False, axis=0, transpose=False)
 
         # if we are passed a scalar None, convert it here
         if not is_list_like(new) and isna(new) and not self.is_object:
+            # FIXME: make sure we have compatible NA
             new = self.fill_value
 
         if self._can_hold_element(new):