Check-in 3: Fix unit-tested code

24 February 2019

Once you’ve written tests for the numberfmt module, and used them to expose the three bugs,

Try fixing the code for formatInt so that all the tests pass.

For the size=0 bug, it’s okay to require size>0 as a precondition, and then test for the AssertionError.
For the leading-zero bug, the problem is that str(0) produces "0" rather than "000", or "48" when it should be "048". Instead of using str(), you can use Python format strings like this:
```
>>> "%0*d" % (8, 415)
'00000415'
>>> "%0*d" % (5, 415)
'00415'
```
where the first number in the pair is the size.
The trickiest is probably the negative number bug. I suggest checking for a negative number explicitly, and either setting a boolean flag, or else a string to represent the sign.
```
if num < 0:
   sign = "-"
   num = - num                  # Make it positive
else:
   sign = ""
```
Then you can proceed with doing the formatting for the positive number, and when done, stick the sign out front.

Before committing, you should create a file at the top of your repository called .gitignore (notice the leading dot), with the following lines:

__pycache__/
.coverage
htmlcov/

That will ensure that temporary object files and coverage reports do not get committed to the repository.