Python SDK reference

Overview

This topic documents how to get started with the Python SDK, and links to reference information on all of the supported features.

Get started

After you complete the Getting Started process, follow these instructions to start using the LaunchDarkly SDK in your Python application.

Install the SDK

First, install the LaunchDarkly SDK as a dependency in your application using your application’s dependency manager. If you want to depend on a specific version, refer to the SDK releases page to identify the latest version.

Here’s how:

$
pip install launchdarkly-server-sdk

Next, import the LaunchDarkly client in your application code:

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import ldclient
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from ldclient.config import Config

Initialize the client

After you install and import the SDK, create a single, shared instance of ldclient. Specify your SDK key here to authorize your application to connect to a particular environment within LaunchDarkly.

The get() function enforces the singleton pattern. You should only have one instance of the client in your application.

Only create one instance of client.

Here’s how:

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ldclient.set_config(Config("sdk-key-123abc"))
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client = ldclient.get()

Evaluate a context

You can use client to check which variation a particular context will receive for a given feature flag. To learn more, read Evaluating flags and Flag evaluation reasons. For more information about how contexts are specified, read Context configuration.

In the v8.0 example, the context key is the string “context-key-123abc”. In the v7.x example, the user key is the string “user-key-123abc”:

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from ldclient import Context
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context = Context.builder("context-key-123abc").name("Sandy").build()
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flag_value = client.variation("flag-key-123abc", context, False)
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if flag_value:
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    # application code to show the feature
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else:
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    # the code to run if the feature is off

Considerations with worker-based servers

The LaunchDarkly SDK relies on multiple threads to operate correctly. These threads provide essential functionality, including delivering flag updates and sending event data.

If the main process which instantiated the SDK is itself forked, the SDK will still evaluate flags, but it will be unable to receive changes to those flags in that child process. This is because threads do not survive the forking process in Python.

The good news is the LaunchDarkly SDK is compatible with worker-based servers. However, there are a few considerations:

• You should initialize a new client before the forking process.
• After you have forked the worker process, you can call postfork() to reinitialize the client. This way, your client will accurately reflect flag changes in the forked thread.

Here’s how:

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# 1. Create the client before forking.
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ldclient.set_config(Config("sdk-key-123abc"))
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client = ldclient.get()
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# 2. From the newly forked process, reinitialize the client by calling `postfork`.
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# Examples for specific servers are shown below.
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client.postfork()

Any configuration that you provide to the SDK must survive the forking process independently. We recommend that you add any listeners or hooks after the postfork() call, unless you are certain they can survive the forking process.

If you are using the Relay Proxy, you must use Relay Proxy v8.11 or later to use postfork().

To learn more about the specific configuration options available in this SDK, read ldclient.config. To learn more about the postfork() reinitialization, read ldclient.postfork.

Example: Configure uWSGI

In uWSGI environments, you must set the enable-threads option. After uwsgi has forked the worker process, you can call postfork() to reinitialize the client. You can do this using uwsgidecorators.

Here’s how:

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import uwsgidecorators
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# Instantiate the client before the fork
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ldclient.set_config(LDConfig("sdk-key-123abc"))
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client = ldclient.get()
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@uwsgidecorators.postfork
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def post_fork_client_initialization():
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    # Reinitialize the client after the fork
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    ldclient.get().postfork()

HTTPS proxy

Python’s standard HTTP library provides a built-in HTTPS proxy. If the HTTPS_PROXY environment variable is present, then the SDK will proxy all network requests through the URL provided.

Here’s how to set the HTTPS_PROXY environment variable on Mac/Linux systems:

$
export HTTPS_PROXY=https://web-proxy.domain.com:8080

Here’s how to set the HTTPS_PROXY environment variable on Windows systems:

$
set HTTPS_PROXY=https://web-proxy.domain.com:8080

Here’s how to set the HTTPS_PROXY environment variable from within Python:

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os.environ["HTTPS_PROXY"] = "https://web-proxy.domain.com:8080"

Shut down the client

Shut down the client when your application terminates. To learn more, read Shutting down.

Supported features

This SDK supports the following features:

• Anonymous contexts and users
• Big segments
• Configuration, including
  • Application metadata configuration
  • Migration configuration
  • Service endpoint configuration
• Context configuration
• Evaluating flags
• Flag evaluation reasons
• Flushing events
• Getting all flags
• Hooks
• Identifying and changing contexts
• Logging configuration
• Migrations
• Monitoring SDK status
• Offline mode
• OpenTelemetry
• Private attributes
• Reading flags from a file
• Relay Proxy configuration
  • Using proxy mode
  • Using daemon mode
• Secure mode
• Sending custom events
• Shutting down
• Storing data
• Subscribing to flag changes
• Test data sources
• Web proxy configuration