More tinkering. Lots of refactoring to get it to do what I want.

git-tidy.py

@@ -0,0 +1,144 @@
+#!/usr/bin/env python
+
+# git-tidy is free software: you can redistribute it and/or modify it under the terms of the GNU
+# General Public License as published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+
+# git-tidy is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
+# the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+# Public License for more details.
+
+# You should have received a copy of the GNU General Public License along with git-tidy. If not, see
+# <https://www.gnu.org/licenses/>.
+"""git-tidy module."""
+
+import subprocess
+import sys
+from typing import List, Tuple, Union
+
+common_default_branches = [
+    "main",
+    "master",
+    "devel",
+    "dev",
+]
+
+
+def run(command: Union[List[str], str]) -> Tuple[int, str]:
+    """Run a subprocess.
+
+    The subprocess is checked for whether it returns successfully, and its
+    stdout or stderr is captured for the caller to make use of.
+
+    Parameters
+    ----------
+    command
+        The command to run as a subprocess.
+
+    Returns
+    -------
+    returncode
+        The (integer) return code from the completed subprocess.
+    stdout / stderr
+        In the case of a zero return code, the stdout from the subprocess is
+        returned. In all other cases, the stderr is returned. In both cases,
+        the bytes are converted to str and stripped of whitespace on either
+        side.
+    """
+    try:
+        process = subprocess.run(command, check=True, capture_output=True)
+    except subprocess.CalledProcessError as exception:
+        return exception.returncode, exception.stderr.decode().strip()
+
+    return process.returncode, process.stdout.decode().strip()
+
+
+def get_default_branch() -> str:
+    """Return the name of the repo's default branch."""
+    returncode, configured_default_branch = run(["git", "config", "--get", "--local", "tidy.defaultbranch"])
+    if returncode == 128:
+        # The --local flag caused this to fail because we're not in a git repository.
+        print(configured_default_branch, file=sys.stderr)
+        sys.exit(returncode)
+
+    # If we're in a repository, we need a list of branches, either as a
+    # sanity check that the configured branch is there, or to choose one
+    # if it's not configured.
+    _, branch_list_raw = run(["git", "branch", "--list"])
+    branch_list = [
+        branch.strip("*").strip()  # Remove the indicator of current branch.
+        for branch in branch_list_raw.split("\n")
+        if len(branch)  # Ignore empty lines.
+    ]
+
+    if returncode == 0:  # The config entry was successfully returned.
+        # But it's worth checking that the branch exists nonetheless.
+        if configured_default_branch in branch_list:
+            return configured_default_branch
+
+        print(
+            f"{configured_default_branch} configured as defauly branch for tidying purposes, but it doesn't exist!",
+            file=sys.stderr,
+        )
+        # TODO: There is probably a clean way to fix this.
+        sys.exit(-1)
+
+    print("No default branch for git-tidy configured, will try to figure out which to use...")
+
+    candidates = []
+
+    # Check whether any branches match our list of common defaults.
+    for branch in branch_list:
+        if branch in common_default_branches:
+            candidates.append(branch)
+
+    if len(candidates) == 1:
+        # Only one branch name matches, so we'll go with that one.
+        run(["git", "config", "--add", "tidy.defaultbranch", candidates[0]])
+        return candidates[0]
+
+    if len(candidates) == 0:
+        print("No candidate branches!", file=sys.stderr)
+        sys.exit(-1)
+
+    # If we don't have one or zero, which do we have?
+    selection = -1
+    while (selection < 0) or (selection >= len(candidates)):
+        print(
+            f"{'Several' if len(candidates) > 1 else 'No'}  potential default branch candidates found. "
+            "Please select one:"
+        )
+        for n, candidate in enumerate(candidates):
+            print(f"{n}: {candidate}")
+
+        # TODO: This breaks if the user is stupid and doesn't input an int.
+        selection = int(input("Which to choose?"))
+
+    return candidates[selection]
+
+
+if __name__ == "__main__":
+
+    default_branch = get_default_branch()
+    print(f"Switching to {default_branch}")
+    run(["git", "checkout", default_branch])
+
+    # Check which remote it's configured with.
+    _, remote = run(["git", "config", "--get", f"branch.{default_branch}.remote"])
+
+    print(f"{default_branch} is configured for remote {remote}")
+
+    # Get latest changes from remote, clear up unnecessary stuff.
+    run(["git", "fetch", "--all", "--prune"])
+    run(
+        ["git", "merge", "--ff-only", f"{remote}/{default_branch}", default_branch]
+    )  # TODO: Handle what happens if the ff fails.
+
+    # Check which branches are `--merged` and clean them up.
+    _, merged_branches_raw = run(["git", "branch", "--merged"])
+    merged_branches = [
+        branch for branch in merged_branches_raw.split("\n") if len(branch)  # Remove zero-length elements.
+    ]
+    for branch in merged_branches:
+        if branch[0] != "*":  # This time we want to explicitly exclude the checked-out branch.
+            subprocess.run(["git", "branch", "-d", branch.strip()], check=True)