python-poetry/poetry · error · PoetryConsoleError

Invalid template string in 'virtualenvs.prompt' setting: {e}

Error message

Invalid template string in 'virtualenvs.prompt' setting: {e}

What it means

Raised as PoetryConsoleError in EnvManager.create_venv when formatting the virtualenvs.prompt config value raises a ValueError - i.e. the template string is structurally malformed for str.format, most commonly unbalanced curly braces.

Source

Thrown at src/poetry/utils/env/env_manager.py:459

            venv = venv_path
        else:
            name = self.generate_env_name(name, str(cwd))
            name = f"{name}-py{python.minor_version.to_string()}"
            venv = venv_path / name

        if venv_prompt is not None:
            try:
                venv_prompt = venv_prompt.format(
                    project_name=self._poetry.package.name or "virtualenv",
                    python_version=python.minor_version.to_string(),
                )
            except KeyError as e:
                raise PoetryConsoleError(
                    f"Invalid template variable '{e.args[0]}' in 'virtualenvs.prompt' setting.\n"
                    f"Valid variables are: {{project_name}}, {{python_version}}"
                ) from e
            except ValueError as e:
                raise PoetryConsoleError(
                    f"Invalid template string in 'virtualenvs.prompt' setting: {e}"
                ) from e

        if not venv.is_dir():
            if create_venv is False:
                self._io.write_error_line(
                    "<fg=black;bg=yellow>"
                    "Skipping virtualenv creation, "
                    "as specified in config file."
                    "</>"
                )

                return self.get_system_env()

            if venv.is_file():
                self._io.write_error_line(
                    f"<warning>{venv} is not a virtual environment but a file. Removing it.</warning>"
                )

View on GitHub (pinned to 92b74dcfe3)

Solutions

  1. Balance the braces or escape literal ones: use {{ for a literal '{' and }} for '}'.
  2. Reconfigure with a well-formed template: 'poetry config virtualenvs.prompt "({project_name})"'.
  3. Reset to default: 'poetry config --unset virtualenvs.prompt'.
  4. Validate the string with Python first: '"...".format(project_name="x", python_version="3.11")'.

Example fix

// before
poetry config virtualenvs.prompt "{project_name"
# ValueError: unmatched brace -> PoetryConsoleError

// after
poetry config virtualenvs.prompt "{project_name}-py{python_version}"
Defensive patterns

Strategy: validation

Validate before calling

def prompt_template_parses(tpl: str) -> bool:
    try:
        tpl.format(project_name="x", python_version="3.11")
        return True
    except (KeyError, ValueError):
        return False

# before setting config:
assert prompt_template_parses(new_prompt), "malformed template string"

Type guard

from poetry.console.exceptions import PoetryConsoleError

def is_invalid_prompt_string(e: Exception) -> bool:
    return isinstance(e, PoetryConsoleError) and "Invalid template string" in str(e)

Try / catch

from poetry.console.exceptions import PoetryConsoleError

try:
    manager.create_venv()
except PoetryConsoleError as e:
    if "Invalid template string" in str(e):
        run_cli(["poetry", "config", "--unset", "virtualenvs.prompt"])
        manager.create_venv()
    else:
        raise

Prevention

When it happens

Trigger: Configuring 'virtualenvs.prompt' with a string containing an odd number of braces, e.g. "{project_name" (missing close) or a lone '}' - anything str.format rejects with ValueError. Caught at env_manager.py:458-461.

Common situations: User wants a literal brace in the prompt but forgets to escape it as {{ or }}; truncation/copy-paste drops a brace; a config file edit leaves an unterminated placeholder.

Related errors

AI-assisted analysis of python-poetry/poetry@92b74dcfe3 (2026-08-04). Data as JSON: /data/errors/03ba3ecfbde5820f.json. Report an issue: GitHub.