pallets/flask · error · NoAppException

Failed to parse as an attribute name or function call.

Error message

Failed to parse {app_name!r} as an attribute name or function call.

What it means

Raised by find_app_by_string when FLASK_APP's app-name portion cannot be parsed as a Python expression at all (ast.parse raises SyntaxError). This is the parsing fallback before attribute/call dispatch.

Solutions

Use a valid Python identifier or call expression after the colon: FLASK_APP='module:app'.
Quote the whole value in the shell to avoid splitting.
Double-check balanced parentheses for call expressions.

Example fix

// before
# export FLASK_APP='module:app('  # raises
// after
# export FLASK_APP='module:app()'

Defensive patterns

Strategy: validation

Validate before calling

import ast
try:
    ast.parse(app_name.strip(), mode='eval')
except SyntaxError:
    raise ValueError('FLASK_APP value is not a valid expression')

Type guard

import ast
def is_valid_app_expr(s) -> bool:
    try:
        ast.parse(s.strip(), mode='eval')
        return True
    except SyntaxError:
        return False

Try / catch

try:
    app = find_app_by_string(module, app_name)
except NoAppException:
    # fix FLASK_APP and retry
    raise

Prevention

Use a valid identifier or call expression after the colon.
Quote the FLASK_APP value in the shell.
Avoid stray whitespace/punctuation.

When it happens

Trigger: export FLASK_APP='module:app 123' or any value whose post-colon part is not a valid Python identifier or call expression (unbalanced parens, stray operators).

Common situations: Typos in FLASK_APP; shell quoting that mangles the value; stray whitespace/punctuation from copy-paste.

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

AI-assisted analysis of pallets/flask@3596b1ab61 (2026-08-11). Data as JSON: /api/errors/97fb03a4ece6b66b. Report an issue: GitHub.

Appendix: source

Thrown at src/flask/cli.py:131

        return True
    finally:
        # Delete tb to break a circular reference.
        # https://docs.python.org/2/library/sys.html#sys.exc_info
        del tb


def find_app_by_string(module: ModuleType, app_name: str) -> Flask:
    """Check if the given string is a variable name or a function. Call
    a function to get the app instance, or return the variable directly.
    """
    from . import Flask

    # Parse app_name as a single expression to determine if it's a valid
    # attribute name or function call.
    try:
        expr = ast.parse(app_name.strip(), mode="eval").body
    except SyntaxError:
        raise NoAppException(
            f"Failed to parse {app_name!r} as an attribute name or function call."
        ) from None

    if isinstance(expr, ast.Name):
        name = expr.id
        args = []
        kwargs = {}
    elif isinstance(expr, ast.Call):
        # Ensure the function name is an attribute name only.
        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:

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