Affected: Python SDK
Symptoms
During SDK initialization, the following warning appears:
Initialization timeout exceeded for LaunchDarkly Client or an error occurred. Feature Flags may not yet be available.
Flag evaluations performed before initialization completes may also produce:
Feature Flag evaluation attempted before client has initialized!
Evaluations use last-known values from a configured persistent feature store when one is available and already populated. Otherwise, they return the fallback value supplied to variation().
Cause
The Python SDK logs this warning from the LDClient constructor when it cannot download the initial feature flag data within its startup wait period, or when initialization encounters an error. The wait period is controlled by the start_wait parameter on the constructor and defaults to five seconds.
This single message covers both the timeout case and the error case, so it does not identify the underlying problem. Earlier entries in the SDK log almost always contain the specific cause.
Common causes include:
An incorrect, expired, or malformed SDK key.
Running under uWSGI without threads enabled. The SDK requires background threads to fetch flag data.
DNS, firewall, proxy, or allowlist restrictions.
TLS inspection or an untrusted corporate certificate authority.
A Relay Proxy that has not completed its own initialization.
An unavailable persistent store when the Python SDK is running in Daemon mode.
Creating and closing an SDK client for every request or function invocation.
Incorrect initialization in a forking or worker-based server.
A large environment payload caused by extensive targeting or large lists of individual targets.
Resource or outbound-connection exhaustion.
Temporary network or LaunchDarkly service disruption.
If a timeout exception occurs, it will not permanently close the connection. For retryable failures, the SDK continues attempting to connect in the background.
Solution
1. Enable debug logging before initializing the client
Configure logging before calling ldclient.get(), then check initialization state:
import logging import os import ldclient from ldclient.config import Config logging.basicConfig(level=logging.DEBUG) logging.getLogger("ldclient").setLevel(logging.DEBUG) ldclient.set_config(Config(os.environ["LAUNCHDARKLY_SDK_KEY"])) client = ldclient.get() if not client.is_initialized(): logging.warning("LaunchDarkly has not initialized; fallback values will be used.")
Review the messages immediately before the initialization warning.
Debug logs may contain sensitive context data. Review and redact logs before sharing them.
2. Read the underlying error programmatically
Because the warning itself does not name the failure, use the data source status provider to retrieve the error kind and HTTP status code directly:
status = client.data_source_status_provider.status print(status.state) if status.error is not None: print(status.error.kind, status.error.status_code, status.error.message)
The state value is INITIALIZING, VALID, INTERRUPTED, or OFF. A state of OFF means the SDK hit an unrecoverable error, such as a rejected SDK key, and stopped retrying.
3. Address the error shown in the logs or status provider
HTTP 401, 403, or 404: The SDK treats all three as unrecoverable and gives up permanently rather than retrying. For 401 and 403 the SDK explicitly labels the failure as an invalid SDK key. Confirm the application uses the correct server-side SDK key for the intended environment, and check for stray whitespace or newline characters. A 403 in this context usually indicates a key problem rather than a firewall problem.
HTTP 400, 408, 429, or any 5xx: These are retryable, and the SDK backs off and reconnects. Persistent failures here point to a proxy, gateway, or upstream service issue.
Connection timeout or no response at all: Ensure the runtime can reach the required LaunchDarkly domains and that firewalls or proxies permit long-lived HTTPS streaming connections.
Certificate verification error: Add the certificate authority used by your corporate proxy to the application's trusted certificate store. Do not disable certificate verification in production.
Relay Proxy errors: Confirm the Relay Proxy is healthy and has completed initialization. The Relay has its own
initTimeout, which defaults to 10 seconds, and its behavior on timeout depends on theignoreConnectionErrorssetting.DNS or socket error: Investigate DNS, routing, NAT, proxy, and outbound-connection limits from the affected runtime.
Run the LaunchDarkly connectivity tests and the hello-python application from the same host, container, Lambda, or VPC as the affected application. If the sample also fails, the problem is likely related to credentials or network connectivity. If it succeeds, compare its initialization and lifecycle with your application.
4. Enable threads when running under uWSGI
The SDK relies on background threads and cannot initialize under uWSGI unless threading is enabled. The SDK detects this case and logs an error at startup. Pass either enable-threads or threads with a value greater than 1:
[uwsgi] enable-threads = true
5. Reuse a single client
Create one shared client per process and LaunchDarkly project, and do not initialize or close the SDK for each request. Using the client created in step 1:
def evaluate_flag(context): return client.variation("example-flag", context, False)
Close the client only when the process terminates. For AWS Lambda, initialize the client outside the handler so warm invocations can reuse it.
For worker-based servers that fork processes, create the client before the fork and reinitialize it in the child by calling postfork(), which is available in Python SDK 9.11 and later:
import uwsgidecorators @uwsgidecorators.postfork def post_fork_client_initialization(): ldclient.get().postfork()
6. Check the environment payload
If only one LaunchDarkly environment initializes slowly, compare its flag and segment data with working environments. Large payloads commonly result from:
Large numbers of individually targeted contexts.
Extensive targeting rules.
Long value lists.
Excessive or oversized segments.
Unused flags that have not been archived.
Use context attributes, targeting rules, or Big Segments instead of very large individual-target lists.
7. Avoid masking the problem by only increasing start_wait
The startup wait is the start_wait argument to the LDClient constructor, and defaults to five seconds:
client = ldclient.get() # uses the default start_wait of 5 seconds
Increasing this value may reduce warnings when initialization is consistently slow, but it also delays application startup and does not resolve the underlying cause. The SDK logs a warning if start_wait exceeds 60 seconds and recommends blocking no longer than that. Prefer fixing the root cause and proceeding with safe fallback values.
8. Design for failed initialization
Always supply a fallback value that keeps the application in a safe, working state. The final argument to variation() is that fallback:
enabled = client.variation("example-flag", context, False)
For additional cold-start resilience, consider using the Relay Proxy together with a persistent data store. A persistent store alone does not resolve connection or initialization problems, and a store that has never been populated provides nothing to fall back on.
If the problem continues
Contact LaunchDarkly Support and provide:
Python and SDK versions.
UTC timestamps and frequency.
Project and environment names, but not the SDK key.
Redacted debug logs from before and after initialization.
The
stateanderrorvalues from the data source status provider.Runtime and network details, including whether uWSGI, Gunicorn, or another worker-based server is in use.
Connectivity-test results from the affected runtime.
Whether the issue affects every environment or only one.