support tracking changes in requirements.txt without full rebuild

[pytoxbot]

• Bitbucket: https://bitbucket.org/hpk42/tox/issue/296
• Originally reported by: hexadite-maxk
• Originally created at: 2015-12-16T16:50:59.538

Use case

during the development, you often switch between new and old requirements.
This currently forces the user to run tox -r.
Sometimes it's not serious, but for automated tests in CI, using internet - it can be several minutes overhead of ~5 or even more minutes. This becomes problematic for short builds: test suite that runs 1 minutes is waiting for tox ~5 minutes.

scenarios I'd like to handle are:

1. addition of new packages
2. removal of currently installed packages
3. upgrade of existing package(s)
4. downgrade of existing package(s)

Scope

only handle 4 situations, nothing else

Implementation:

Use difflib module's functionality: it already has ndiff() seemingly very useful for this purpose.
based on that diff we should generate 2 lists:

1. Packages_to_remove
2. Packages_to_install

Testing should be done first on the function that properly detects what needs to be removed and what needs to be installed.

1. Initially - just remove anything that is changed, and install it from scratch
2. later - we might want to optimize:
   1. packages to remove - to contain packages that are supposed to be removed or downgraded
   2. packages to install - should contain packages that are supposed to be installed or upgraded with -I option (i.e. older will be uninstalled)

A relatively easy to implement approach would be to compare:
*. current pip freeze output (on the left)

• requirements.txt (on the right)

As a result, if there are diffs (lines with "-" and lines with "+"), those with "-" can be uninstalled, and those with "+" - installed.

[pytoxbot]

Original comment by @The-Compiler

This looks like something to be done after #149, right? It also doesn't seem to be specific to requirements.txt, e.g. when having the dependencies in tox.ini the same would apply as well.

[gaborbernat]

This is now done in tox 4.