Fix show_default string not displayed in prompts

src/click/core.py

@@ -3145,10 +3145,10 @@ def prompt_for_value(self, ctx: Context) -> t.Any:
                 default = bool(default)
             return confirm(self.prompt, default)
 
-        # If show_default is set to True/False, provide this to `prompt` as well. For
-        # non-bool values of `show_default`, we use `prompt`'s default behavior
+        # Pass show_default to prompt. When it's a string, the prompt will
+        # display that string instead of the actual default value.
         prompt_kwargs: t.Any = {}
-        if isinstance(self.show_default, bool):
+        if self.show_default is not None:
             prompt_kwargs["show_default"] = self.show_default
 
         return prompt(

src/click/termui.py

@@ -60,7 +60,7 @@ def hidden_prompt_func(prompt: str) -> str:
 def _build_prompt(
     text: str,
     suffix: str,
-    show_default: bool = False,
+    show_default: bool | str = False,
     default: t.Any | None = None,
     show_choices: bool = True,
     type: ParamType | None = None,
@@ -69,7 +69,10 @@ def _build_prompt(
     if type is not None and show_choices and isinstance(type, Choice):
         prompt += f" ({', '.join(map(str, type.choices))})"
     if default is not None and show_default:
-        prompt = f"{prompt} [{_format_default(default)}]"
+        if isinstance(show_default, str):
+            prompt = f"{prompt} [{show_default}]"
+        else:
+            prompt = f"{prompt} [{_format_default(default)}]"
     return f"{prompt}{suffix}"
 
 
@@ -88,7 +91,7 @@ def prompt(
     type: ParamType | t.Any | None = None,
     value_proc: t.Callable[[str], t.Any] | None = None,
     prompt_suffix: str = ": ",
-    show_default: bool = True,
+    show_default: bool | str = True,
     err: bool = False,
     show_choices: bool = True,
 ) -> t.Any:

tests/test_termui.py

@@ -710,3 +710,22 @@ def cli(flag):
         assert result.output == expected_output
         assert not result.stderr
         assert result.exit_code == 0 if expected not in (REPEAT, INVALID) else 1
+
+
+def test_string_show_default_in_prompt(runner):
+    """When show_default is a string, the prompt should display that string
+    instead of the actual default value. See pallets/click#2836."""
+
+    @click.command()
+    @click.option(
+        "--name",
+        default="actual_default",
+        show_default="custom label",
+        prompt=True,
+    )
+    def cmd(name):
+        click.echo(name)
+
+    result = runner.invoke(cmd, input="\n", standalone_mode=False)
+    assert "custom label" in result.output
+    assert "actual_default" not in result.output.split("\n")[0]