reflex-dev/reflex · error · ValueError

Missing value for environment variable {field.name} and no d

Error message

Missing value for environment variable {field.name} and no default value found

What it means

Thrown when building a dataclass-driven config/state from environment variables and a required field has neither an environment value nor a declared default or default_factory. The fallback lookup in get_default_value_for_field finds dataclasses.MISSING for both, so it raises ValueError naming the field.

Source

Thrown at packages/reflex-base/src/reflex_base/environment.py:51

def get_default_value_for_field(field: dataclasses.Field) -> Any:
    """Get the default value for a field.

    Args:
        field: The field.

    Returns:
        The default value.

    Raises:
        ValueError: If no default value is found.
    """
    if field.default != dataclasses.MISSING:
        return field.default
    if field.default_factory != dataclasses.MISSING:
        return field.default_factory()
    msg = f"Missing value for environment variable {field.name} and no default value found"
    raise ValueError(msg)


# TODO: Change all interpret_.* signatures to value: str, field: dataclasses.Field once we migrate rx.Config to dataclasses
def interpret_boolean_env(value: str, field_name: str) -> bool:
    """Interpret a boolean environment variable value.

    Args:
        value: The environment variable value.
        field_name: The field name.

    Returns:
        The interpreted value.

    Raises:
        EnvironmentVarValueError: If the value is invalid.
    """
    true_values = ["true", "1", "yes", "y"]
    false_values = ["false", "0", "no", "n"]

View on GitHub (pinned to 45b8ed5ab7)

Solutions

Set the missing environment variable named in the message (export it or add it to .env)
Give the dataclass field a default or default_factory if it's optional
Add a startup check listing required env vars before the app boots

Example fix

# before
@dataclasses.dataclass
class Settings:
    api_key: str  # no default
# shell: API_KEY unset -> ValueError
# after (option 1)
export API_KEY=...
# after (option 2)
@dataclasses.dataclass
class Settings:
    api_key: str = ""

Defensive patterns

Strategy: validation

Validate before calling

import os
required = [f.name for f in dataclasses.fields(Settings)
            if f.default is dataclasses.MISSING and f.default_factory is dataclasses.MISSING]
missing = [n for n in required if os.environ.get(n) is None]
if missing:
    raise SystemExit(f"Missing env vars: {missing}")

Type guard

null

Try / catch

try:
    Settings.from_env()
except ValueError as e:
    if "Missing value for environment variable" in str(e):
        # prompt or apply defaults
        ...

Prevention

Fail fast at boot with an env checklist
Use .env files locally and a secrets manager in prod; never rely on memory for required vars

When it happens

Trigger: A dataclass field without a default whose environment variable is unset, passed to the env-parsing machinery (e.g. update_from_env / get_default_value_for_field on fields like a required API key).

Common situations: Deploying without setting required env vars (DATABASE_URL, API keys), renaming env vars between versions, or missing .env files in CI/production.

Understand the failure class

Background: "environment variable is not set" and "Missing keys in environment" errors: what missing required env var messages mean and how to fix them — this error's family across 28 libraries.

Related errors

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/031fe3bdd60e3a53. Report an issue: GitHub.