Add script to ease migration to black (#3038)

* Add script to ease migration to black

* Update CHANGES.md

Co-authored-by: Cooper Lees <me@cooperlees.com>

CHANGES.md

@@ -39,6 +39,8 @@
 
 <!-- For example, Docker, GitHub Actions, pre-commit, editors -->
 
+- Add migrate-black.py script to ease migration to black formatted git project (#3038)
+
 ### Output
 
 <!-- Changes to Black's terminal output and error messages -->

scripts/migrate-black.py

@@ -0,0 +1,95 @@
+#!/usr/bin/env python3
+# check out every commit added by the current branch, blackify them,
+# and generate diffs to reconstruct the original commits, but then
+# blackified
+import logging
+import os
+import sys
+from subprocess import check_output, run, Popen, PIPE
+
+
+def git(*args: str) -> str:
+    return check_output(["git"] + list(args)).decode("utf8").strip()
+
+
+def blackify(base_branch: str, black_command: str, logger: logging.Logger) -> int:
+    current_branch = git("branch", "--show-current")
+
+    if not current_branch or base_branch == current_branch:
+        logger.error("You need to check out a feature brach to work on")
+        return 1
+
+    if not os.path.exists(".git"):
+        logger.error("Run me in the root of your repo")
+        return 1
+
+    merge_base = git("merge-base", "HEAD", base_branch)
+    if not merge_base:
+        logger.error(
+            "Could not find a common commit for current head and %s" % base_branch
+        )
+        return 1
+
+    commits = git(
+        "log", "--reverse", "--pretty=format:%H", "%s~1..HEAD" % merge_base
+    ).split()
+    for commit in commits:
+        git("checkout", commit, "-b%s-black" % commit)
+        check_output(black_command, shell=True)
+        git("commit", "-aqm", "blackify")
+
+    git("checkout", base_branch, "-b%s-black" % current_branch)
+
+    for last_commit, commit in zip(commits, commits[1:]):
+        allow_empty = (
+            b"--allow-empty" in run(["git", "apply", "-h"], stdout=PIPE).stdout
+        )
+        quiet = b"--quiet" in run(["git", "apply", "-h"], stdout=PIPE).stdout
+        git_diff = Popen(
+            [
+                "git",
+                "diff",
+                "--find-copies",
+                "%s-black..%s-black" % (last_commit, commit),
+            ],
+            stdout=PIPE,
+        )
+        git_apply = Popen(
+            [
+                "git",
+                "apply",
+            ]
+            + (["--quiet"] if quiet else [])
+            + [
+                "-3",
+                "--intent-to-add",
+            ]
+            + (["--allow-empty"] if allow_empty else [])
+            + [
+                "-",
+            ],
+            stdin=git_diff.stdout,
+        )
+        if git_diff.stdout is not None:
+            git_diff.stdout.close()
+        git_apply.communicate()
+        git("commit", "--allow-empty", "-aqC", commit)
+
+    for commit in commits:
+        git("branch", "-qD", "%s-black" % commit)
+
+    return 0
+
+
+if __name__ == "__main__":
+    import argparse
+
+    parser = argparse.ArgumentParser()
+    parser.add_argument("base_branch")
+    parser.add_argument("--black_command", default="black -q .")
+    parser.add_argument("--logfile", type=argparse.FileType("w"), default=sys.stdout)
+    args = parser.parse_args()
+    logger = logging.getLogger(__name__)
+    logger.addHandler(logging.StreamHandler(args.logfile))
+    logger.setLevel(logging.INFO)
+    sys.exit(blackify(args.base_branch, args.black_command, logger))