BUG: Improve col_space in to_html to allow css length strings (#25941)

doc/source/whatsnew/v0.25.0.rst

@@ -361,6 +361,7 @@ I/O
 - Bug in ``read_csv`` which would not raise ``ValueError`` if a column index in ``usecols`` was out of bounds (:issue:`25623`)
 - Improved the explanation for the failure when value labels are repeated in Stata dta files and suggested work-arounds (:issue:`25772`)
 - Improved :meth:`pandas.read_stata` and :class:`pandas.io.stata.StataReader` to read incorrectly formatted 118 format files saved by Stata (:issue:`25960`)
+- Improved the ``col_space`` parameter in :meth:`DataFrame.to_html` to accept a string so CSS length values can be set correctly (:issue:`25941`)
 
 Plotting
 ^^^^^^^^

pandas/core/frame.py

@@ -663,7 +663,9 @@ def _repr_html_(self):
 
     @Substitution(header='Write out the column names. If a list of strings '
                          'is given, it is assumed to be aliases for the '
-                         'column names')
+                         'column names',
+                  col_space_type='int',
+                  col_space='The minimum width of each column')
     @Substitution(shared_params=fmt.common_docstring,
                   returns=fmt.return_docstring)
     def to_string(self, buf=None, columns=None, col_space=None, header=True,
@@ -2149,7 +2151,12 @@ def to_parquet(self, fname, engine='auto', compression='snappy',
                    compression=compression, index=index,
                    partition_cols=partition_cols, **kwargs)
 
-    @Substitution(header='Whether to print column labels, default True')
+    @Substitution(header='Whether to print column labels, default True',
+                  col_space_type='str or int',
+                  col_space='The minimum width of each column in CSS length '
+                            'units.  An int is assumed to be px units.\n\n'
+                            '            .. versionadded:: 0.25.0\n'
+                            '                Abillity to use str')
     @Substitution(shared_params=fmt.common_docstring,
                   returns=fmt.return_docstring)
     def to_html(self, buf=None, columns=None, col_space=None, header=True,

pandas/io/formats/format.py

@@ -45,8 +45,8 @@
             Buffer to write to.
         columns : sequence, optional, default None
             The subset of columns to write. Writes all columns by default.
-        col_space : int, optional
-            The minimum width of each column.
+        col_space : %(col_space_type)s, optional
+            %(col_space)s.
         header : bool, optional
             %(header)s.
         index : bool, optional, default True

pandas/io/formats/html.py

@@ -85,8 +85,21 @@ def write(self, s, indent=0):
         rs = pprint_thing(s)
         self.elements.append(' ' * indent + rs)
 
-    def write_th(self, s, indent=0, tags=None):
-        if self.fmt.col_space is not None and self.fmt.col_space > 0:
+    def write_th(self, s, header=False, indent=0, tags=None):
+        """
+        Return a formatted <th> cell.
+
+        Most tags are passed in as a parameter, but if col_space is set
+        then that value is used for min-width.
+
+        Since min-width is only set inside <thead>, this function also takes
+        the 'header' parameter.  If 'header' is True, min-width needs to be
+        set as it is for use inside <thead>
+        """
+        if header and self.fmt.col_space is not None:
+            if isinstance(self.fmt.col_space, int):
+                self.fmt.col_space = ('{colspace}px'
+                                      .format(colspace=self.fmt.col_space))
             tags = (tags or "")
             tags += ('style="min-width: {colspace};"'
                      .format(colspace=self.fmt.col_space))
@@ -137,7 +150,7 @@ def write_tr(self, line, indent=0, indent_delta=0, header=False,
         for i, s in enumerate(line):
             val_tag = tags.get(i, None)
             if header or (self.bold_rows and i < nindex_levels):
-                self.write_th(s, indent, tags=val_tag)
+                self.write_th(s, indent=indent, header=header, tags=val_tag)
             else:
                 self.write_td(s, indent, tags=val_tag)
 

pandas/tests/io/formats/test_to_html.py

@@ -643,3 +643,17 @@ def test_to_html_round_column_headers():
         notebook = df.to_html(notebook=True)
     assert "0.55555" in html
     assert "0.556" in notebook
+
+
+@pytest.mark.parametrize("unit", ['100px', '10%', '5em', 150])
+def test_to_html_with_col_space_units(unit):
+    # GH 25941
+    df = DataFrame(np.random.random(size=(1, 3)))
+    result = df.to_html(col_space=unit)
+    result = result.split('tbody')[0]
+    hdrs = [x for x in result.split("\n") if re.search(r"<th[>\s]", x)]
+    if isinstance(unit, int):
+        unit = str(unit) + 'px'
+    for h in hdrs:
+        expected = '<th style="min-width: {unit};">'.format(unit=unit)
+        assert expected in h