How do I fix wrongly nested / unclosed HTML tags?

using BeautifulSoup:

from BeautifulSoup import BeautifulSoup
html = "<p><ul><li>Foo"
soup = BeautifulSoup(html)
print soup.prettify()

gets you

<p>
 <ul>
  <li>
   Foo
  </li>
 </ul>
</p>

As far as I know, you can’t control putting the <li></li> tags on separate lines from Foo.

using Tidy:

import tidy
html = "<p><ul><li>Foo"
print tidy.parseString(html, show_body_only=True)

gets you

<ul>
<li>Foo</li>
</ul>

Unfortunately, I know of no way to keep the <p> tag in the example. Tidy interprets it as an empty paragraph rather than an unclosed one, so doing

print tidy.parseString(html, show_body_only=True, drop_empty_paras=False)

comes out as

<p></p>
<ul>
<li>Foo</li>
</ul>

Ultimately, of course, the <p> tag in your example is redundant, so you might be fine with losing it.

Finally, Tidy can also do indenting:

print tidy.parseString(html, show_body_only=True, indent=True)

becomes

<ul>
  <li>Foo
  </li>
</ul>

All of these have their ups and downs, but hopefully one of them is close enough.