Add codemod command for removing unused imports

[zsol]

Test Plan

Ran on an earlier version of this file:

❯ python -m libcst.tool codemod remove_unused_imports.RemoveUnusedImportsCommand libcst/codemod/commands/remove_unused_imports.py -u                                                                      normal
Calculating full-repo metadata...
Executing codemod...
0.01s 0% complete, [calculating] estimated for 1 files to go...--- /home/zsol/libcst/libcst/codemod/commands/remove_unused_imports.py
+++ /home/zsol/libcst/libcst/codemod/commands/remove_unused_imports.py
@@ -3,16 +3,14 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 #
 # pyre-strict
 
-from typing import Type, Generator
-from libcst.codemod import Codemod, VisitorBasedCodemodCommand
+from libcst.codemod import VisitorBasedCodemodCommand
 
 from libcst.codemod.visitors import RemoveImportsVisitor
 from libcst import Import, ImportFrom
-import asyncio
 
 
 class RemoveUnusedImportsCommand(VisitorBasedCodemodCommand):
     DESCRIPTION: str = "Remove all imports that are not used in a file."
 
Finished codemodding 1 files!
 - Transformed 1 files successfully.
 - Skipped 0 files.
 - Failed to codemod 0 files.
 - 0 warnings were generated.

[codecov-io]

Codecov Report

Merging #266 into master will decrease coverage by <.01%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #266      +/-   ##
==========================================
- Coverage   93.89%   93.89%   -0.01%     
==========================================
  Files         214      216       +2     
  Lines       20822    20854      +32     
==========================================
+ Hits        19550    19580      +30     
- Misses       1272     1274       +2

Impacted Files	Coverage Δ
...demod/commands/tests/test_remove_unused_imports.py	100% <100%> (ø)
libcst/codemod/commands/remove_unused_imports.py	81.81% <81.81%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f612604...610de7e. Read the comment docs.

[jimmylai]

This looks good!
Can you add test cases in this directory like the other codemods?
https://github.com/Instagram/LibCST/tree/master/libcst/codemod/commands/tests

[zsol]

Done! Thanks for the review. I'm wondering if we could have some better documentation about what these built-in codemods do.

[jimmylai]

@zsol , your change has a lint error which the updated imports are not sorted. You can tox -e autofix to fix them.
Do you mean better documentation for RemoveImportsVisitor? We have detailed documentation for each of codemod helper:
https://libcst.readthedocs.io/en/latest/codemods.html#libcst.codemod.visitors.RemoveImportsVisitor

[zsol]

Do you mean better documentation for RemoveImportsVisitor?

No I meant for the Commands themselves. It's great that they have online docs python -m libcst.tool mycodemod --help, but I'm wondering if we can somehow put those in the generate sphinx docs.

Linters fixed, sorry :)

[jimmylai]

Btw, RemoveImportsVisitor uses scope analysis and it doesn't analyze string type annotation.
That means, in this example code, import a will be removed by this codemod.

import a
x: "a" = 1

In general, RemoveUnusedImportsCommand should be used with cautions since it has limitations including it doesn't consider imports used by other modules (indirect imports) and it doesn't consider references in string type annotations.
@zsol, would you like to include those as warnings as document? Probably short description in DESCRIPTION attr and longer explanation in docstring of RemoveUnusedImportsCommand.

[jimmylai]

No I meant for the Commands themselves. It's great that they have online docs python -m libcst.tool mycodemod --help, but I'm wondering if we can somehow put those in the generate sphinx docs.

We have a codemod tutorial presents usage of libcst CLI.
https://libcst.readthedocs.io/en/latest/codemods_tutorial.html
Do you think it's not clear? If you have good idea about how to restructure the document to improve readability, I'm more than happy to update it.

[zsol]

If you have good idea about how to restructure the document to improve readability, I'm more than happy to update it.

I was suggesting to generate an actual list of available codemods (along with their docs) in either the tutorial there, or the reference manual part. This would allow people to explore them without needing to install libcst. I'll look at how hard this is to do.

[jimmylai]

If you have good idea about how to restructure the document to improve readability, I'm more than happy to update it.

I was suggesting to generate an actual list of available codemods (along with their docs) in either the tutorial there, or the reference manual part. This would allow people to explore them without needing to install libcst. I'll look at how hard this is to do.

That's great idea and can make it more clear about available commands. One way to do it is to generate codemod command list reStructuredText in source/conf.py, e.g. Instagram/Fixit#7

[javierhonduco]

Seems like imports that are used in f-strings are not tracked either, guess it's similar to the string type annotation issue!