pallets/flask · error · NoAppException

Failed to parse arguments as literal values

Error message

Failed to parse arguments as literal values: {app_name!r}.

What it means

Raised by find_app_by_string when the call expression parses but its positional or keyword arguments are not Python literals (ast.literal_eval raises ValueError). Only literal values can be supplied to a factory via FLASK_APP.

Solutions

1. Pass only literal arguments: export FLASK_APP='module:create_app("production")'.
2. Read environment variables inside the factory instead of passing them via FLASK_APP.
3. Use a wrapper function that computes args and then calls the factory.

Example fix

// before
# export FLASK_APP='module:create_app(os.environ["CFG"])'  # raises
// after
# export FLASK_APP='module:create_app("production")'
# and read os.environ inside the factory

import ast
expr = ast.parse(app_name.strip(), mode='eval').body
for arg in getattr(expr, 'args', []):
    ast.literal_eval(arg)
for kw in getattr(expr, 'keywords', []):
    ast.literal_eval(kw.value)

import ast
def args_are_literals(s) -> bool:
    try:
        expr = ast.parse(s.strip(), mode='eval').body
        for arg in getattr(expr, 'args', []):
            ast.literal_eval(arg)
        return True
    except (ValueError, SyntaxError):
        return False

try:
    app = find_app_by_string(module, app_name)
except NoAppException:
    # replace non-literal args with literal strings
    raise

When it happens

Trigger: export FLASK_APP='module:create_app(os.environ["CONFIG"])' — arguments reference names/calls, not literals. Only values like strings, numbers, tuples are allowed.

Common situations: Trying to inject runtime/env values directly into the FLASK_APP factory call; using variable names instead of literal strings.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Failed to parse as an attribute name or function call.
• Function reference must be a simple name
• Detected factory ' ' in module ' ', but could not call it…
• Detected multiple Flask applications in module
• Failed to find attribute

AI-assisted analysis of pallets/flask@3596b1ab61 (2026-08-11). Data as JSON: /api/errors/6bb6ca401b913dcb. Report an issue: GitHub.

Appendix: source

Thrown at src/flask/cli.py:159

        if not isinstance(expr.func, ast.Name):
            raise NoAppException(
                f"Function reference must be a simple name: {app_name!r}."
            )

        name = expr.func.id

        # Parse the positional and keyword arguments as literals.
        try:
            args = [ast.literal_eval(arg) for arg in expr.args]
            kwargs = {
                kw.arg: ast.literal_eval(kw.value)
                for kw in expr.keywords
                if kw.arg is not None
            }
        except ValueError:
            # literal_eval gives cryptic error messages, show a generic
            # message with the full expression instead.
            raise NoAppException(
                f"Failed to parse arguments as literal values: {app_name!r}."
            ) from None
    else:
        raise NoAppException(
            f"Failed to parse {app_name!r} as an attribute name or function call."
        )

    try:
        attr = getattr(module, name)
    except AttributeError as e:
        raise NoAppException(
            f"Failed to find attribute {name!r} in {module.__name__!r}."
        ) from e

    # If the attribute is a function, call it with any args and kwargs
    # to get the real application.
    if inspect.isfunction(attr):
        try:

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