BUG: cells are missing in the excel file when exporting excel using xlsxwriter with option constant_memory set to True (#15392)
- [x] closes #15392
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @KenTsui! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-07-28 19:03:37 UTC I think all of this logic should be contained within the `write_cells` method of pandas.io.excel._xlsxwriter.py - does moving it there make things easier? > I think all of this logic should be contained within the `write_cells` method of pandas.io.excel._xlsxwriter.py - does moving it there make things easier?

It seems not quite possible as the sequence of generating cells, which is the crux of this bug, is not controlled by `write_cells`.

BTW, I am stuck on how to debug using azure pipelines.  Does anyone have some good resources to learn about that? @KenTsui can you merge master? As far as log failures go if you click on next to failing logs you should be able to click through and find test failure output. Should match what you would see locally in a lot of cases @KenTsui is this still active? Closing as stale but ping if you'd like to pick back up