Auto-detect type=UNPROCESSED when flag_value has non-basic types

Closes #2012

Author: kdeldycke

CHANGES.rst

@@ -40,6 +40,11 @@ Unreleased
 -   Change :class:`ParameterSource` to an :class:`~enum.IntEnum` and reorder
     its members from most to least explicit, so values can be compared to
     check whether a parameter was explicitly provided. :issue:`2879` :pr:`3248`
+-   Auto-detect ``type=UNPROCESSED`` for ``flag_value`` of non-basic types
+    (not ``str``, ``int``, ``float``, or ``bool``), so programmer-provided
+    Python objects like classes and enum members are passed through unchanged
+    instead of being stringified. Previously ``type=click.UNPROCESSED`` had
+    to be set explicitly. :issue:`2012`
 
 Version 8.3.2
 -------------

src/click/core.py

@@ -2714,6 +2714,11 @@ class Option(Parameter):
     :param hidden: hide this option from help outputs.
     :param attrs: Other command arguments described in :class:`Parameter`.
 
+    .. versionchanged:: 8.4
+        Non-basic ``flag_value`` types (not ``str``, ``int``, ``float``, or
+        ``bool``) are passed through unchanged instead of being stringified.
+        Previously, ``type=click.UNPROCESSED`` was required to preserve them.
+
     .. versionchanged:: 8.2
         ``envvar`` used with ``flag_value`` will always use the ``flag_value``,
         previously it would use the value of the environment variable.
@@ -2731,7 +2736,8 @@ class Option(Parameter):
         default value is ``False``.
 
     .. versionchanged:: 8.0.1
-        ``type`` is detected from ``flag_value`` if given.
+        ``type`` is detected from ``flag_value`` if given, for basic Python
+        types (``str``, ``int``, ``float``, ``bool``).
     """
 
     param_type_name = "option"
@@ -2831,7 +2837,20 @@ def __init__(
                     self.type = types.BoolParamType()
                 # Otherwise, guess the type from the flag value.
                 else:
-                    self.type = types.convert_type(None, flag_value)
+                    guessed = types.convert_type(None, flag_value)
+                    if (
+                        isinstance(guessed, types.StringParamType)
+                        and not isinstance(flag_value, str)
+                        and flag_value is not None
+                    ):
+                        # The flag_value type couldn't be auto-detected
+                        # (not str, int, float, or bool). Since flag_value
+                        # is a programmer-provided Python object, not CLI
+                        # input, pass it through unchanged instead of
+                        # stringifying it.
+                        self.type = types.UNPROCESSED
+                    else:
+                        self.type = guessed
 
         self.is_flag: bool = bool(is_flag)
         self.is_bool_flag: bool = bool(

tests/test_options.py

@@ -1406,6 +1406,11 @@ def test_type_from_flag_value():
     assert param.type is click.INT
     param = click.Option(["-b", "x"], flag_value=8)
     assert param.type is click.INT
+    # Non-basic types auto-detect as UNPROCESSED to avoid stringification.
+    param = click.Option(["-c", "x"], flag_value=EngineType.OSS)
+    assert param.type is click.UNPROCESSED
+    param = click.Option(["-d", "x"], flag_value=frozenset())
+    assert param.type is click.UNPROCESSED
 
 
 @pytest.mark.parametrize(
@@ -2127,13 +2132,13 @@ class Class2:
             [],
             EngineType.OSS,
         ),
-        # Type is not specified and default to string, so the default value is
-        # returned as a string, even if it is a boolean. Also, defaults to the
-        # flag_value instead of the default value to support legacy behavior.
+        # Type is not specified. For string flag_value, STRING type is used and
+        # the default value is converted to string. For non-basic types (like
+        # enums), UNPROCESSED is used and values pass through unchanged.
         ({"flag_value": "1", "default": True}, [], "1"),
         ({"flag_value": "1", "default": 42}, [], "42"),
-        ({"flag_value": EngineType.OSS, "default": True}, [], "EngineType.OSS"),
-        ({"flag_value": EngineType.OSS, "default": 42}, [], "42"),
+        ({"flag_value": EngineType.OSS, "default": True}, [], EngineType.OSS),
+        ({"flag_value": EngineType.OSS, "default": 42}, [], 42),
         # See: the result is the same if we force the type to be str.
         ({"type": str, "flag_value": 1, "default": True}, [], "1"),
         ({"type": str, "flag_value": 1, "default": 42}, [], "42"),
@@ -2199,28 +2204,29 @@ def scan(pro):
             ["--opt2"],
             EngineType.PRO,
         ),
-        # Check that passing exotic flag values like classes is supported, but are
-        # rendered to strings when the type is not specified.
+        # Exotic flag values like classes are passed through unchanged when no
+        # explicit type is given (UNPROCESSED is auto-detected).
+        # https://github.com/pallets/click/issues/2012
         # https://github.com/pallets/click/issues/3121
         (
             {"flag_value": Class1, "default": True},
             {"flag_value": Class2},
             [],
-            "<class 'test_options.Class1'>",
+            Class1,
         ),
         (
             {"flag_value": Class1, "default": True},
             {"flag_value": Class2},
             ["--opt1"],
-            "<class 'test_options.Class1'>",
+            Class1,
         ),
         (
             {"flag_value": Class1, "default": True},
             {"flag_value": Class2},
             ["--opt2"],
-            "<class 'test_options.Class2'>",
+            Class2,
         ),
-        # Even the default is processed as a string.
+        # String and None defaults pass through unchanged.
         ({"flag_value": Class1, "default": "True"}, {"flag_value": Class2}, [], "True"),
         ({"flag_value": Class1, "default": None}, {"flag_value": Class2}, [], None),
         # To get the classes as-is, we need to specify the type as UNPROCESSED.
@@ -2245,18 +2251,18 @@ def scan(pro):
         ),
         # Setting the default to a class, an instance of the class is returned instead
         # of the class itself, because the default is allowed to be callable (and
-        # consummd). And this happens whatever the type is.
+        # consumed). And this happens whatever the type is.
         (
             {"flag_value": Class1, "default": Class1},
             {"flag_value": Class2},
             [],
-            re.compile(r"'<test_options.Class1 object at 0x[0-9A-Fa-f]+>'"),
+            re.compile(r"<test_options.Class1 object at 0x[0-9A-Fa-f]+>"),
         ),
         (
             {"flag_value": Class1, "default": Class2},
             {"flag_value": Class2},
             [],
-            re.compile(r"'<test_options.Class2 object at 0x[0-9A-Fa-f]+>'"),
+            re.compile(r"<test_options.Class2 object at 0x[0-9A-Fa-f]+>"),
         ),
         (
             {"flag_value": Class1, "type": UNPROCESSED, "default": Class1},
@@ -2322,12 +2328,13 @@ def cli(dual_option):
             ["--opt"],
             Class1,
         ),
-        # Without UNPROCESSED, the class is str()-ified by the default STRING type.
-        ({"flag_value": Class1, "default": True}, [], "<class 'test_options.Class1'>"),
+        # Without explicit UNPROCESSED, the class still passes through unchanged
+        # because UNPROCESSED is auto-detected for non-basic flag_value types.
+        ({"flag_value": Class1, "default": True}, [], Class1),
         (
             {"flag_value": Class1, "default": True},
             ["--opt"],
-            "<class 'test_options.Class1'>",
+            Class1,
         ),
         # Explicit default=Class1 (not via default=True alignment): callable IS invoked,
         # because the user explicitly set a callable as the default.
@@ -2473,6 +2480,48 @@ def cli(value):
     assert opt.get_default(ctx, call=True) is expected_get_default
 
 
+def test_flag_value_not_stringified_for_custom_types(runner):
+    """Non-basic flag_value types are passed through unchanged without
+    requiring ``type=click.UNPROCESSED``.
+
+    Regression test for https://github.com/pallets/click/issues/2012
+    """
+
+    @click.command()
+    @click.option("--cls1", "config_cls", flag_value=Class1, default=True)
+    @click.option("--cls2", "config_cls", flag_value=Class2)
+    def cli(config_cls):
+        click.echo(repr(config_cls), nl=False)
+
+    # Default activates --cls1 (default=True resolves to flag_value).
+    result = runner.invoke(cli, [])
+    assert result.exit_code == 0
+    assert result.output == repr(Class1)
+
+    result = runner.invoke(cli, ["--cls1"])
+    assert result.exit_code == 0
+    assert result.output == repr(Class1)
+
+    result = runner.invoke(cli, ["--cls2"])
+    assert result.exit_code == 0
+    assert result.output == repr(Class2)
+
+    # Enum flag_value without explicit type is also preserved.
+    @click.command()
+    @click.option("--oss", "engine", flag_value=EngineType.OSS, default=True)
+    @click.option("--pro", "engine", flag_value=EngineType.PRO)
+    def cli2(engine):
+        click.echo(repr(engine), nl=False)
+
+    result = runner.invoke(cli2, [])
+    assert result.exit_code == 0
+    assert result.output == repr(EngineType.OSS)
+
+    result = runner.invoke(cli2, ["--pro"])
+    assert result.exit_code == 0
+    assert result.output == repr(EngineType.PRO)
+
+
 def test_custom_type_frozenset_flag_value(runner):
     """Check that frozenset is correctly handled as a type, a flag value and a default.