fix: failing post-checkout on single file checkout

pre_commit/commands/run.py

@@ -344,8 +344,20 @@ def run(
 
     # Check if we have unresolved merge conflict files and fail fast.
     if stash and _has_unmerged_paths():
-        logger.error('Unmerged files.  Resolve before committing.')
-        return 1
+        # Do not fail on unmerged files if this is a post-checkout hooks.
+        # This handles cases when there are unmerged paths and:
+        #   * resolving conflict with `git checkout --ours/--theirs -- <paths>`
+        #   * or checking out single files with `git checkout <b> -- <paths>`.
+        if args.hook_stage == 'post-checkout':
+            output.write_line(
+                f'Skipping `{args.hook_stage}` hooks since '
+                f'it\'s a file checkout or same head ref',
+            )
+            return 0
+        else:
+            logger.error('Unmerged files.  Resolve before committing.')
+            return 1
+
     if bool(args.from_ref) != bool(args.to_ref):
         logger.error('Specify both --from-ref and --to-ref.')
         return 1

tests/commands/run_test.py

@@ -536,6 +536,22 @@ def test_merge_conflict(cap_out, store, in_merge_conflict):
     assert b'Unmerged files.  Resolve before committing.' in printed
 
 
+def test_merge_conflict_post_checkout(cap_out, store, in_merge_conflict):
+    ret, printed = _do_run(
+        cap_out,
+        store,
+        in_merge_conflict,
+        run_opts(hook_stage='post-checkout'),
+    )
+    assert ret == 0
+    assert b'Unmerged files.  Resolve before committing.' not in printed
+    msg = (
+        b'Skipping `post-checkout` hooks since '
+        b'it\'s a file checkout or same head ref'
+    )
+    assert msg in printed
+
+
 def test_files_during_merge_conflict(cap_out, store, in_merge_conflict):
     opts = run_opts(files=['placeholder'])
     ret, printed = _do_run(cap_out, store, in_merge_conflict, opts)
@@ -554,6 +570,29 @@ def test_merge_conflict_modified(cap_out, store, in_merge_conflict):
     assert b'Unmerged files.  Resolve before committing.' in printed
 
 
+def test_merge_conflict_modified_post_checkout(
+    cap_out, store, in_merge_conflict
+):
+    # Touch another file so we have unstaged non-conflicting things
+    assert os.path.exists('placeholder')
+    with open('placeholder', 'w') as placeholder_file:
+        placeholder_file.write('bar\nbaz\n')
+
+    ret, printed = _do_run(
+        cap_out,
+        store,
+        in_merge_conflict,
+        run_opts(hook_stage='post-checkout'),
+    )
+    assert ret == 0
+    assert b'Unmerged files.  Resolve before committing.' not in printed
+    msg = (
+        b'Skipping `post-checkout` hooks since '
+        b'it\'s a file checkout or same head ref'
+    )
+    assert msg in printed
+
+
 def test_merge_conflict_resolved(cap_out, store, in_merge_conflict):
     cmd_output('git', 'add', '.')
     ret, printed = _do_run(cap_out, store, in_merge_conflict, run_opts())