WIP: Fix HelpFormatter.write_usage producing spurious characters

CHANGES.rst

@@ -59,6 +59,11 @@ Unreleased
 -   :class:`~click.formatting.TextWrapper` and
     :func:`~click.formatting.wrap_text` now measure line width in visible
     characters, ignoring ANSI escape sequences. :pr:`3420`
+-   Fix :meth:`HelpFormatter.write_usage` emitting only a blank line when
+    called without ``args``. The usage prefix and program name are now
+    written even when no arguments follow, and the trailing separator
+    space is stripped so the line ends at the program name.
+    :issue:`3360` :pr:`3434`
 -   Show custom error messages from types when :func:`prompt` with
     ``hide_input=True`` fails validation, instead of always showing a
     generic message. Built-in type messages mask the input value.

src/click/formatting.py

@@ -164,6 +164,14 @@ def write_usage(self, prog: str, args: str = "", prefix: str | None = None) -> N
         usage_prefix = f"{prefix:>{self.current_indent}}{prog} "
         text_width = self.width - self.current_indent
 
+        if not args:
+            # Without args, the prefix's trailing space and the wrap_text
+            # call that would normally place args on the line are both
+            # unnecessary. Emit just the prefix line.
+            self.write(usage_prefix.rstrip(" "))
+            self.write("\n")
+            return
+
         if text_width >= (term_len(usage_prefix) + 20):
             # The arguments will fit to the right of the prefix.
             indent = " " * term_len(usage_prefix)

tests/test_formatting.py

@@ -499,3 +499,118 @@ def test_write_usage_styled_prefix_keeps_options_on_one_line():
 
     visible = strip_ansi(rendered)
     assert visible == "Usage: cli [OPTIONS]\n"
+
+
+@pytest.mark.parametrize(
+    ("formatter_kwargs", "current_indent", "prog", "args", "prefix", "expected"),
+    [
+        # Issue #3360: the default prefix used to emit only
+        # a blank line because ``wrap_text("", initial_indent=usage_prefix)``
+        # returned ``""`` and discarded the prefix.
+        pytest.param(
+            {},
+            0,
+            "Program",
+            "",
+            None,
+            "Usage: Program\n",
+            id="empty-args-default-prefix",
+        ),
+        # A caller-supplied prefix is preserved verbatim.
+        pytest.param(
+            {},
+            0,
+            "Program",
+            "",
+            "Run: ",
+            "Run: Program\n",
+            id="empty-args-custom-prefix",
+        ),
+        # ``current_indent`` is preserved even with no args to render.
+        pytest.param(
+            {},
+            4,
+            "Program",
+            "",
+            None,
+            "Usage: Program\n",
+            id="empty-args-indented",
+        ),
+        # Prog too long to share a line with args: the wrap branch must not
+        # emit a second line.
+        pytest.param(
+            {"width": 20},
+            0,
+            "VeryLongProgramName",
+            "",
+            None,
+            "Usage: VeryLongProgramName\n",
+            id="empty-args-long-prog",
+        ),
+        # With non-empty args, the separator space between prog and args is preserved.
+        pytest.param(
+            {},
+            0,
+            "Program",
+            "[OPTIONS]",
+            None,
+            "Usage: Program [OPTIONS]\n",
+            id="with-args-default-prefix",
+        ),
+    ],
+)
+def test_help_formatter_write_usage(
+    formatter_kwargs, current_indent, prog, args, prefix, expected
+):
+    """``HelpFormatter.write_usage`` renders a single usage line whose
+    trailing separator tracks whether ``args`` is non-empty.
+    """
+    f = click.HelpFormatter(**formatter_kwargs)
+    f.current_indent = current_indent
+    if prefix is None:
+        f.write_usage(prog, args)
+    else:
+        f.write_usage(prog, args, prefix=prefix)
+    assert f.getvalue() == expected
+
+
+def test_help_formatter_write_usage_without_args_styled_prefix():
+    """A downstream-styled prefix is preserved when ``args`` is empty:
+    the ANSI escape sequences survive, only the trailing separator is
+    removed.
+    """
+    styled_prefix = "\x1b[38;2;38;139;210m\x1b[1mUsage:\x1b[0m "
+    f = click.HelpFormatter()
+    f.write_usage("cli", prefix=styled_prefix)
+    rendered = f.getvalue()
+    assert strip_ansi(rendered) == "Usage: cli\n"
+    assert "\x1b[" in rendered
+
+
+@pytest.mark.parametrize(
+    ("command_kwargs", "expected_usage_line"),
+    [
+        # End-to-end regression for #3360: an empty ``options_metavar`` with
+        # no parameters used to render a blank usage line.
+        pytest.param(
+            {"options_metavar": ""},
+            "Usage: cli",
+            id="empty-options-metavar-no-params",
+        ),
+        # End-to-end regression: ``options_metavar=None`` is the documented
+        # way to suppress the ``[OPTIONS]`` slot entirely.
+        pytest.param(
+            {"options_metavar": None},
+            "Usage: cli",
+            id="none-options-metavar-no-params",
+        ),
+    ],
+)
+def test_command_write_usage_no_args(runner, command_kwargs, expected_usage_line):
+    """End-to-end: a command with no parameters and an empty or absent
+    ``options_metavar`` renders a usage line with just the program name,
+    no trailing space.
+    """
+    cli = click.Command("cli", **command_kwargs)
+    result = runner.invoke(cli, ["--help"])
+    assert result.output.splitlines()[0] == expected_usage_line