BUG: re-render CSS with styler.apply and applymap non-cleared _todo (#39396)

Author: attack68

doc/source/whatsnew/v1.3.0.rst

@@ -385,6 +385,7 @@ Other
 ^^^^^
 - Bug in :class:`Index` constructor sometimes silently ignorning a specified ``dtype`` (:issue:`38879`)
 - Bug in constructing a :class:`Series` from a list and a :class:`PandasDtype` (:issue:`39357`)
+- Bug in :class:`Styler` which caused CSS to duplicate on multiple renders. (:issue:`39395`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/io/formats/style.py

@@ -830,6 +830,7 @@ def _compute(self):
         r = self
         for func, args, kwargs in self._todo:
             r = func(self)(*args, **kwargs)
+        self._todo = []
         return r
 
     def _apply(
@@ -1454,9 +1455,8 @@ def set_properties(self, subset=None, **kwargs) -> Styler:
         >>> df.style.set_properties(color="white", align="right")
         >>> df.style.set_properties(**{'background-color': 'yellow'})
         """
-        values = ";".join(f"{p}: {v}" for p, v in kwargs.items())
-        f = lambda x: values
-        return self.applymap(f, subset=subset)
+        values = "".join(f"{p}: {v};" for p, v in kwargs.items())
+        return self.applymap(lambda x: values, subset=subset)
 
     @staticmethod
     def _bar(

pandas/tests/io/formats/test_style.py

@@ -102,12 +102,27 @@ def test_deepcopy(self):
         assert self.styler._todo != s2._todo
 
     def test_clear(self):
-        s = self.df.style.highlight_max()._compute()
-        assert len(s.ctx) > 0
+        # updated in GH 39396
+        tt = DataFrame({"A": [None, "tt"]})
+        css = DataFrame({"A": [None, "cls-a"]})
+        s = self.df.style.highlight_max().set_tooltips(tt).set_td_classes(css)
+        # _todo, tooltips and cell_context items added to..
         assert len(s._todo) > 0
+        assert s.tooltips
+        assert len(s.cell_context) > 0
+
+        s = s._compute()
+        # ctx and _todo items affected when a render takes place
+        assert len(s.ctx) > 0
+        assert len(s._todo) == 0  # _todo is emptied after compute.
+
+        s._todo = [1]
         s.clear()
+        # ctx, _todo, tooltips and cell_context items all revert to null state.
         assert len(s.ctx) == 0
         assert len(s._todo) == 0
+        assert not s.tooltips
+        assert len(s.cell_context) == 0
 
     def test_render(self):
         df = DataFrame({"A": [0, 1]})
@@ -116,6 +131,15 @@ def test_render(self):
         s.render()
         # it worked?
 
+    def test_multiple_render(self):
+        # GH 39396
+        s = Styler(self.df, uuid_len=0).applymap(lambda x: "color: red;", subset=["A"])
+        s.render()  # do 2 renders to ensure css styles not duplicated
+        assert (
+            '<style  type="text/css" >\n#T__row0_col0,#T__row1_col0{\n            '
+            "color:  red;\n        }</style>" in s.render()
+        )
+
     def test_render_empty_dfs(self):
         empty_df = DataFrame()
         es = Styler(empty_df)