insert-files: support fixed value columns via -c colname:text:value

Extends the -c/--column spec so a column can be populated with a
literal value for every inserted file, e.g. -c file_type:text:gif,
without a separate update pass. Existing colname:coldef shorthands
and compound --pk behavior are unchanged.

Closes #140

docs/cli.rst

@@ -1747,6 +1747,13 @@ You can change the name of one of these columns using a ``-c colname:coldef`` pa
     sqlite-utils insert-files gifs.db images *.gif \
         -c path -c md5 -c last_modified:mtime --pk=path
 
+You can also populate a column with a fixed value for every file using a ``-c colname:text:value`` parameter. This is useful for tagging every row from a given import with the same value, without a separate update pass:
+
+.. code-block:: bash
+
+    sqlite-utils insert-files gifs.db images *.gif \
+        -c path -c content -c file_type:text:gif --pk=path
+
 You can pass ``--replace`` or ``--upsert`` to indicate what should happen if you try to insert a file with an existing primary key. Pass ``--alter`` to cause any missing columns to be added to the table.
 
 The full list of column definitions you can use is as follows:

sqlite_utils/cli.py

@@ -2978,8 +2978,19 @@ def insert_files(
                         "size": lambda p, data=stdin_data: len(data),
                     }
                 for coldef in column:
-                    if ":" in coldef:
-                        colname, coltype = coldef.rsplit(":", 1)
+                    parts = coldef.split(":", 2)
+                    if len(parts) == 3:
+                        # Fixed value, e.g. -c file_type:text:gif
+                        colname, coltype, fixed_value = parts
+                        if coltype != "text":
+                            raise click.ClickException(
+                                "Fixed value column definitions must use "
+                                "'colname:text:value', not '{}'".format(coldef)
+                            )
+                        row[colname] = fixed_value
+                        continue
+                    elif len(parts) == 2:
+                        colname, coltype = parts
                     else:
                         colname, coltype = coldef, coldef
                     try:

tests/test_insert_files.py

@@ -117,6 +117,60 @@ def test_insert_files(silent, pk_args, expected_pks):
         assert set(db["files"].pks) == set(expected_pks)
 
 
+def test_insert_files_fixed_value_column():
+    runner = CliRunner()
+    with runner.isolated_filesystem():
+        tmpdir = pathlib.Path(".")
+        db_path = str(tmpdir / "files.db")
+        (tmpdir / "one.gif").write_text("gif content", "utf-8")
+        result = runner.invoke(
+            cli.cli,
+            [
+                "insert-files",
+                db_path,
+                "files",
+                str(tmpdir / "one.gif"),
+                "-c",
+                "path",
+                "-c",
+                "content",
+                "-c",
+                "file_type:text:gif",
+                "--pk",
+                "path",
+                "--pk",
+                "file_type",
+            ],
+            catch_exceptions=False,
+        )
+        assert result.exit_code == 0, result.output
+        db = Database(db_path)
+        row = next(iter(db["files"].rows))
+        assert row["file_type"] == "gif"
+        assert set(db["files"].pks) == {"path", "file_type"}
+
+
+def test_insert_files_fixed_value_column_bad_type():
+    runner = CliRunner()
+    with runner.isolated_filesystem():
+        tmpdir = pathlib.Path(".")
+        db_path = str(tmpdir / "files.db")
+        (tmpdir / "one.gif").write_text("gif content", "utf-8")
+        result = runner.invoke(
+            cli.cli,
+            [
+                "insert-files",
+                db_path,
+                "files",
+                str(tmpdir / "one.gif"),
+                "-c",
+                "file_type:mtime:gif",
+            ],
+        )
+        assert result.exit_code == 1
+        assert "colname:text:value" in result.output
+
+
 @pytest.mark.parametrize(
     "use_text,encoding,input,expected",
     (