A simple rule to help improve convention and maintainability: Camelcasing should be used when dealing with Objects,  Everything else should use underscores.

Objects

• Class definitions should be CapCase
• Objects should be mixedCase
• Class Methods should be mixedCase

class BookAuthors(object):    # <-- Class Definition
   def getAuthors(self, book_id):  # <-- Class Method
      ...
      return results

bookAuthors = BookAuthors()   # <-- Object

Everything Else

When not dealing with objects we should use underscores and lowercase

• Simple instance variables
• Functions
• Filenames / Modules
• SQL schema, table and column names

sql_pretty(sql):
   ...
   return sql2

num_books = len(books)
fp_debug = open('/tmp/debug.log', 'a') 

sql = 'select book_id, author_id from books.book_authors where book_id = ?'

This varies slightly from the PEP8 Naming conventions.   The PEP8 suggest class methods and non-class methods (or functions) be treated the same way, with underscores. However I prefer naming class methods with mixedCase to remind us we’re in the domain of objects.

With regard to sql schemas, table and column names, because column names can become very long, underscores makes them more readable.