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#!/usr/bin/env python3
"""
git_merge_report.py
Merges a remote branch into a local branch, and builds a summary report of
the merge (divergence point, commit count, line changes, file changes).
The rendered summary is used as the merge commit message.
Usage:
python git_merge_report.py <local_branch> <remote_branch> (-m "message" | -f path/to/message.txt)
Examples:
python git_merge_report.py main feature/login -m "Add login flow"
python git_merge_report.py main feature/login -f commit_msg.txt
python git_merge_report.py main feature/login -m "Add login flow" --dry-run
"""
import argparse
import re
import subprocess
import sys
from datetime import datetime, timezone
TEMPLATE = """Merge {remote} into {local}:
Merge commit message:
{msg}
-----------------------------BEGIN SUMMARY OF MERGE----------------------------
Merging {n} commits into {local}.
{remote} diverged from {local} at commit {commit_hash} {days} days ago
Commit makes {add_chg} additions, {rem_chg} deletions
Commit adds {add_file} files, removes {rem_file} files
------------------------------END SUMMARY OF MERGE-----------------------------
"""
class GitError(RuntimeError):
pass
def run_git(args, repo="."):
"""Run a git command and return stripped stdout. Raises GitError on failure."""
try:
result = subprocess.run(
["git", "-C", repo] + args,
check=True,
capture_output=True,
text=True,
)
return result.stdout.strip()
except subprocess.CalledProcessError as e:
raise GitError(
f"git {' '.join(args)} failed:\n{e.stderr.strip()}"
) from e
except FileNotFoundError as e:
raise GitError("git executable not found on PATH") from e
def verify_branch_exists(branch, repo="."):
try:
run_git(["rev-parse", "--verify", "--quiet", branch], repo)
except GitError:
raise GitError(f"Branch '{branch}' does not exist in this repository.")
def get_merge_base(local, remote, repo="."):
"""Commit where remote diverged from local."""
return run_git(["merge-base", local, remote], repo)
def get_commit_date(commit_hash, repo="."):
"""ISO8601 commit date of a commit, as a timezone-aware datetime."""
iso = run_git(["show", "-s", "--format=%cI", commit_hash], repo)
return datetime.fromisoformat(iso)
def days_since(commit_hash, repo="."):
commit_dt = get_commit_date(commit_hash, repo)
now = datetime.now(timezone.utc)
delta = now - commit_dt.astimezone(timezone.utc)
return delta.days
def get_commit_count(local, remote, repo="."):
"""Number of commits on remote not yet on local (i.e. being merged in)."""
out = run_git(["rev-list", "--count", f"{local}..{remote}"], repo)
return int(out)
def get_diff_shortstat(local, remote, repo="."):
"""Lines added/removed between merge-base(local,remote) and remote."""
out = run_git(["diff", "--shortstat", f"{local}...{remote}"], repo)
added = removed = 0
if out:
add_match = re.search(r"(\d+) insertion", out)
rem_match = re.search(r"(\d+) deletion", out)
if add_match:
added = int(add_match.group(1))
if rem_match:
removed = int(rem_match.group(1))
return added, removed
def get_file_changes(local, remote, repo="."):
"""Count of files added (A) and removed (D) between local and remote."""
out = run_git(["diff", "--name-status", f"{local}...{remote}"], repo)
added_files = removed_files = 0
for line in out.splitlines():
if not line.strip():
continue
status = line.split("\t", 1)[0]
# status can be 'A', 'D', 'M', or rename/copy scores like 'R100'
if status.startswith("A"):
added_files += 1
elif status.startswith("D"):
removed_files += 1
return added_files, removed_files
def read_message(args):
if args.message is not None:
return args.message
with open(args.message_file, "r", encoding="utf-8") as f:
return f.read().strip()
return "None"
def build_summary(local, remote, msg, repo="."):
merge_base_hash = get_merge_base(local, remote, repo)
short_hash = run_git(["rev-parse", "--short", merge_base_hash], repo)
n_days = days_since(merge_base_hash, repo)
n_commits = get_commit_count(local, remote, repo)
add_chg, rem_chg = get_diff_shortstat(local, remote, repo)
add_file, rem_file = get_file_changes(local, remote, repo)
summary = TEMPLATE.format(
remote=remote,
local=local,
msg=msg,
n=n_commits,
commit_hash=short_hash,
days=n_days,
add_chg=add_chg,
rem_chg=rem_chg,
add_file=add_file,
rem_file=rem_file,
)
return summary
def perform_merge(local, remote, commit_message, repo="."):
run_git(["checkout", local], repo)
run_git(["merge", "--no-ff", remote, "-m", commit_message], repo)
def parse_args(argv=None):
parser = argparse.ArgumentParser(
description="Merge a remote branch into a local branch and report a summary."
)
parser.add_argument("local_branch", help="Branch to merge INTO")
parser.add_argument("remote_branch", help="Branch to merge (source)")
msg_group = parser.add_mutually_exclusive_group(required=True)
msg_group.add_argument(
"-m", "--message", help="Merge commit message, given directly as text"
)
msg_group.add_argument(
"-f",
"--message-file",
help="Path to a file containing the merge commit message",
)
parser.add_argument(
"--repo",
default=".",
help="Path to the git repository (default: current directory)",
)
parser.add_argument(
"--dry-run",
action="store_true",
help="Only print the summary report; do not perform the actual merge",
)
return parser.parse_args(argv)
def main(argv=None):
args = parse_args(argv)
repo = args.repo
try:
verify_branch_exists(args.local_branch, repo)
verify_branch_exists(args.remote_branch, repo)
msg = read_message(args)
summary = build_summary(args.local_branch, args.remote_branch, msg, repo)
print(summary)
if args.dry_run:
print("[dry-run] No merge performed.")
return 0
perform_merge(args.local_branch, args.remote_branch, summary, repo)
print(f"Merge complete: '{args.remote_branch}' merged into '{args.local_branch}'.")
return 0
except GitError as e:
print(f"Error: {e}", file=sys.stderr)
return 1
except FileNotFoundError:
print(f"Error: message file '{args.message_file}' not found.", file=sys.stderr)
return 1
if __name__ == "__main__":
sys.exit(main())
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