Repro

Here's self-contained script to reproduce:

from prettytable import TableStyle, PrettyTable

table = PrettyTable()
table.field_names = ["City name", "Area", "Population", "Annual Rainfall"]
table.add_row(["Adelaide", 1295, 1158259, 600.5])
table.add_row(["Brisbane", 5905, 1857594, 1146.4])
table.add_row(["Darwin", 112, 120900, 1714.7])
table.add_row(["Hobart", 1357, 205556, 619.5])
table.add_row(["Sydney", 2058, 4336374, 1214.8])
table.add_row(["Melbourne", 1566, 3806092, 646.9])
table.add_row(["Perth", 5386, 1554769, 869.4])

table.set_style(TableStyle.SINGLE_BORDER)
table.set_style(TableStyle.MARKDOWN)

print(table)

Outputs:

| City name | Area | Population | Annual Rainfall |
├ :───────: | :──: | :────────: | :─────────────: ┤
|  Adelaide | 1295 |  1158259   |      600.5      |
|  Brisbane | 5905 |  1857594   |      1146.4     |
|   Darwin  | 112  |   120900   |      1714.7     |
|   Hobart  | 1357 |   205556   |      619.5      |
|   Sydney  | 2058 |  4336374   |      1214.8     |
| Melbourne | 1566 |  3806092   |      646.9      |
|   Perth   | 5386 |  1554769   |      869.4      |

But doesn't render properly:

| City name | Area | Population | Annual Rainfall |
├ :───────: | :──: | :────────: | :─────────────: ┤
| Adelaide | 1295 | 1158259 | 600.5 |
| Brisbane | 5905 | 1857594 | 1146.4 |
| Darwin | 112 | 120900 | 1714.7 |
| Hobart | 1357 | 205556 | 619.5 |
| Sydney | 2058 | 4336374 | 1214.8 |
| Melbourne | 1566 | 3806092 | 646.9 |
| Perth | 5386 | 1554769 | 869.4 |

Workaround

Set the default style in between:

from prettytable import TableStyle, PrettyTable

table = PrettyTable()
table.field_names = ["City name", "Area", "Population", "Annual Rainfall"]
table.add_row(["Adelaide", 1295, 1158259, 600.5])
table.add_row(["Brisbane", 5905, 1857594, 1146.4])
table.add_row(["Darwin", 112, 120900, 1714.7])
table.add_row(["Hobart", 1357, 205556, 619.5])
table.add_row(["Sydney", 2058, 4336374, 1214.8])
table.add_row(["Melbourne", 1566, 3806092, 646.9])
table.add_row(["Perth", 5386, 1554769, 869.4])

table.set_style(TableStyle.SINGLE_BORDER)
table.set_style(TableStyle.DEFAULT)  # <!-- Add this
table.set_style(TableStyle.MARKDOWN)

print(table)

Outputs:

| City name | Area | Population | Annual Rainfall |
| :-------: | :--: | :--------: | :-------------: |
|  Adelaide | 1295 |  1158259   |      600.5      |
|  Brisbane | 5905 |  1857594   |      1146.4     |
|   Darwin  | 112  |   120900   |      1714.7     |
|   Hobart  | 1357 |   205556   |      619.5      |
|   Sydney  | 2058 |  4336374   |      1214.8     |
| Melbourne | 1566 |  3806092   |      646.9      |
|   Perth   | 5386 |  1554769   |      869.4      |

Renders:

We should probably call _set_default_style() at the start of set_style() for all styles (and remove _set_default_style() from the start of _set_orgmode_style().

Would you like to submit a PR with tests?

Yes that would make sense to make sure all other styles are clean, too.

Regarding doing the PR, sadly I don't have the time for that at the moment, sorry. I added the workaround in my code to call _set_default_style() and moved on to next things. Can try in a couple of weeks but hard to promise a timeline.

Sure, no rush :)

As a workaround, you might prefer to call set_style(TableStyle.DEFAULT) rather than the internal _set_default_style().

@hugovk , I took a look at the code and added some logic for resetting defaultsbefore going to if statement in set_style along with taking out redundant resets in the _set_orgmode method.

Pull request is here. PR-344. Will upload tests shortly.