How can I make sure my tests touch every line of my codebase?

Coverage might be the tool you're looking for:

Coverage.py measures code coverage, typically during test execution. It uses the code analysis tools and tracing hooks provided in the Python standard library to determine which lines are executable, and which have been executed.

Coverage measures the lines you're covering and also help you identify pending lines. For example, in coverage documentation you can see the kind of report it generates:

$ coverage report -m 
Name                      Stmts   Miss  Cover   Missing
-------------------------------------------------------
my_program                   20      4    80%   33-35, 39
my_other_module              56      6    89%   17-23
-------------------------------------------------------
TOTAL                        76     10    87%