Fixes #3084 — Restore optional value behavior for options with flag_value

CHANGES.rst

@@ -5,6 +5,8 @@ Version 8.3.x
 
 Unreleased
 
+-   If ``flag_value`` is set or no default is provided the flag can be accepted
+    without an argument.:issue:`3084` :pr:`#3104`
 -   Don't discard pager arguments by correctly using ``subprocess.Popen``. :issue:`3039`
     :pr:`3055`
 -   Replace ``Sentinel.UNSET`` default values by ``None`` as they're passed through

src/click/core.py

@@ -2654,6 +2654,10 @@ class Option(Parameter):
     :param hidden: hide this option from help outputs.
     :param attrs: Other command arguments described in :class:`Parameter`.
 
+    .. versionchanged:: 8.3.dev
+         If ``flag_value`` is set or no default is provided, the flag can be
+         accepted without an argument.
+
     .. versionchanged:: 8.2
         ``envvar`` used with ``flag_value`` will always use the ``flag_value``,
         previously it would use the value of the environment variable.
@@ -2747,10 +2751,13 @@ def __init__(
             # Implicitly a flag because secondary options names were given.
             elif self.secondary_opts:
                 is_flag = True
-        # The option is explicitly not a flag. But we do not know yet if it needs a
-        # value or not. So we look at the default value to determine it.
+
+        # Handle options that are not flags but provide a flag_value.
+        # If flag_value is set or no default is provided the flag can be accepted
+        # without an argument.
+        # https://github.com/pallets/click/issues/3084
         elif is_flag is False and not self._flag_needs_value:
-            self._flag_needs_value = self.default is UNSET
+            self._flag_needs_value = flag_value is not UNSET or self.default is UNSET
 
         if is_flag:
             # Set missing default for flags if not explicitly required or prompted.

tests/test_options.py

@@ -15,6 +15,7 @@
 from click import Option
 from click import UNPROCESSED
 from click._utils import UNSET
+from click.testing import CliRunner
 
 
 def test_prefixes(runner):
@@ -2275,3 +2276,36 @@ def rcli(scm_ignore_files):
     result = runner.invoke(rcli, ["--without-scm-ignore-files"])
     assert result.stdout == "frozenset()"
     assert result.exit_code == 0
+
+
+def test_flag_value_optional_behavior():
+    """Test that options with flag_value and is_flag=False use flag_value
+    when only flag is provided
+
+    Reproduces https://github.com/pallets/click/issues/3084
+    """
+
+    @click.command()
+    @click.option("--name", is_flag=False, flag_value="Flag", default="Default")
+    def hello(name):
+        click.echo(f"Hello, {name}!")
+
+    runner = CliRunner()
+    result = runner.invoke(hello, ["--name"])
+    assert result.exit_code == 0
+    assert result.output == "Hello, Flag!\n"
+
+
+def test_flag_value_with_type_conversion():
+    """Test that flag_value is correctly type-converted when used as an option value."""
+
+    @click.command()
+    @click.option("--count", is_flag=False, flag_value="1", type=int, default=0)
+    def repeat(count):
+        for i in range(count):
+            click.echo(f"Line {i + 1}")
+
+    runner = CliRunner()
+    result = runner.invoke(repeat, ["--count"])
+    assert result.exit_code == 0
+    assert result.output == "Line 1\n"