Add the possibility to add import and respect the import sorting.

[JonathanPlasse]

• In Add convert exit() to sys.exit() rule #816, we need to add the sys module if missing and keep the imports sorted.

[JonathanPlasse]

@charliermarsh, does #875 facilitate the implementation of this issue?

[charliermarsh]

This would be good to tackle. It has now come up a few times now.

I think the idea would be to track the imports that need to be present for a given code modification, and then do a pass at the end to "add" those imports. It would somehow have to be connected to the autofix API too, to ensure that we only add the import if we end up applying the fix.

You can see how LibCST handles the problem here.

[charliermarsh]

There are a few sub-problems:

• How do we determine where the imports should go? It's probably not necessary to put the imports in the "right" spot, since we can rely on the next autofix iteration fixing up the order. But they should go in the first "block".
• How do we only add imports we don't already have?
• ...

[charliermarsh]

I am returning to this.

[charliermarsh]

Getting close...

diff --git a/foo.py b/foo.py
index 0647ee33b..696a9e873 100644
--- a/foo.py
+++ b/foo.py
@@ -1,3 +1,4 @@
+import sys
 exit = 1

-quit(1)
+sys.exit(1)