ENH: _to_str_columns accepts now optional header

Hereby also to_latex and to_string accept optional headers

Author: mcocdawc

pandas/formats/format.py

@@ -489,30 +489,48 @@ def _to_str_columns(self):
         str_index = self._get_formatted_index(frame)
         str_columns = self._get_formatted_column_labels(frame)
 
-        if self.header:
+        has_aliases = isinstance(self.header, (tuple, list, np.ndarray, Index))
+        if not (has_aliases or self.header):
             stringified = []
             for i, c in enumerate(frame):
-                cheader = str_columns[i]
-                max_colwidth = max(self.col_space or 0, *(self.adj.len(x)
-                                                          for x in cheader))
                 fmt_values = self._format_col(i)
                 fmt_values = _make_fixed_width(fmt_values, self.justify,
-                                               minimum=max_colwidth,
+                                               minimum=(self.col_space or 0),
+                                               adj=self.adj)
+                stringified.append(fmt_values)
+        elif has_aliases:
+            if len(self.header) != len(self.columns):
+                raise ValueError(('Writing %d cols but got %d aliases'
+                                  % (len(self.columns), len(self.header))))
+            stringified = []
+            for i, c in enumerate(frame):
+                cheader = [self.header[i]]
+                header_colwidth = max(self.col_space or 0,
+                                      *(self.adj.len(x) for x in cheader))
+                fmt_values = self._format_col(i)
+                fmt_values = _make_fixed_width(fmt_values, self.justify,
+                                               minimum=header_colwidth,
                                                adj=self.adj)
 
                 max_len = max(np.max([self.adj.len(x) for x in fmt_values]),
-                              max_colwidth)
+                              header_colwidth)
                 cheader = self.adj.justify(cheader, max_len, mode=self.justify)
                 stringified.append(cheader + fmt_values)
         else:
             stringified = []
             for i, c in enumerate(frame):
+                cheader = str_columns[i]
+                header_colwidth = max(self.col_space or 0,
+                                      *(self.adj.len(x) for x in cheader))
                 fmt_values = self._format_col(i)
                 fmt_values = _make_fixed_width(fmt_values, self.justify,
-                                               minimum=(self.col_space or 0),
+                                               minimum=header_colwidth,
                                                adj=self.adj)
 
-                stringified.append(fmt_values)
+                max_len = max(np.max([self.adj.len(x) for x in fmt_values]),
+                              header_colwidth)
+                cheader = self.adj.justify(cheader, max_len, mode=self.justify)
+                stringified.append(cheader + fmt_values)
 
         strcols = stringified
         if self.index: