Allow ExcelWriter() to add sheets to existing workbook

The ability of ExcelWriter to save different dataframes to different worksheets is great for sharing those dfs with the python-deficient. But this quickly leads to a need to add worksheets to an existing workbook, not just creating one from scratch; something like:

df0=pd.DataFrame(np.arange(3))
df0.to_excel('foo.xlsx','Data 0')

df1=pd.DataFrame(np.arange(2))
df1.to_excel('foo.xlsx','Data 1')

The following little diff to io/parsers.py implements this behavior for *.xlsx files:

diff --git a/pandas/io/parsers.py b/pandas/io/parsers.py
index 89f892d..7f010ee 100644
--- a/pandas/io/parsers.py
+++ b/pandas/io/parsers.py
@@ -2099,12 +2099,19 @@ class ExcelWriter(object):
             self.fm_date = xlwt.easyxf(num_format_str='YYYY-MM-DD')
         else:
             from openpyxl.workbook import Workbook
-            self.book = Workbook()  # optimized_write=True)
-            # open pyxl 1.6.1 adds a dummy sheet remove it
-            if self.book.worksheets:
-                self.book.remove_sheet(self.book.worksheets[0])
+            from openpyxl.reader.excel import load_workbook
+
+            try:
+               self.book=load_workbook(filename = path)
+               self.sheets={wks.title:wks for wks in self.book.worksheets}
+            except InvalidFileException:
+                self.book = Workbook()  # optimized_write=True)
+                # open pyxl 1.6.1 adds a dummy sheet remove it
+                if self.book.worksheets:
+                    self.book.remove_sheet(self.book.worksheets[0])
+                self.sheets = {}
+
         self.path = path
-        self.sheets = {}
         self.cur_sheet = None

Doing this for *.xls files is a little harder.

[ankostis]

This stackoverflow workaround, which is based in openpyxl, may work (EDIT: indeed works, checked with pandas-0.17.0):

import pandas
from openpyxl import load_workbook

book = load_workbook('Masterfile.xlsx')
writer = pandas.ExcelWriter('Masterfile.xlsx', engine='openpyxl') 
writer.book = book
writer.sheets = dict((ws.title, ws) for ws in book.worksheets)

data_filtered.to_excel(writer, "Main", cols=['Diff1', 'Diff2'])

writer.save()

[jtratner]

Because of how to_excel is set up, this would mean reading in and then writing the file each time (because to_excel with a path argument saves the file). The right way to do this is to use ExcelWriter:

import pandas as pd
writer = pd.ExcelWriter('foo.xlsx')
df.to_excel(writer, 'Data 0')
df.to_excel(writer, 'Data 1')
writer.save()

I could see (eventually) adding an option to ExcelWriter that doesn’t overwrite the file. But, yet again, that may mean writing in the entire file first. I don’t know.