fix missing code contents that should be included in signature of a f…

[ztessler]

…unction action

The code of a python function that is used as a build action is inspected and included in the sconsign signature so the target can be rebuilt if the function changes. However, scons currently does not notice if a constant value in the expression of a list comprehension is changed. This PR fixes that.

In this example:

def test(source, target, env):
    xs = [2*x for x in range(5)]
    with open(str(target[0])) as out:
        out.write(str(xs))

env = Environment()
env.Command(
    source=None,
    target='out.txt',
    action=test)

changing the 2 in the list comprehension in test() would (incorrectly) not cause a rebuild.

The existing code that computes contents of a function (in _code_contents()) strips off the first item from the co_consts tuple. In a function, this first item is the docstring (or None if there is no docstring). List comprehension code is not included directly in a function's co_code, but appears as an object in the co_consts tuple, and is processed on a later recursive invocation of _code_contents(). List comprehensions also have their own co_consts tuple, though theirs do not have a docstring (or a None placeholder). Removing the first object from their co_consts effectively ignores a real constant from the code. This PR changes the logic to only remove values from the co_consts tuple that equal the docstring, if any.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated master/src/CHANGES.txt directory (and read the README.txt in that directory)
• I have updated the appropriate documentation

[bdbaddog]

Is the docstring always passed in?
Barring that if not passed in, can we grab it in this function?

[ztessler]

The callpath to _code_contents() for a function (or method or callable class instance) goes through _function_contents(), which does pass along the docstring. The only other docstring-equipped object that comes to mind are modules - not sure if they could act as a function for the purposes of FunctionAction.

When the docstring doesn't exist, this code strips out any None's from the signature. I can't imagine any circumstances where that would be a problem though, particularly as that is similar to the existing behavior.

[bdbaddog]

Good catch.
Looks good. I added one comment regarding docstring. Please take a look