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Getting Started with the Python SDK

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Add prefab_cloud_python to your package dependencies

# pyproject.toml
prefab-cloud-python = "^0.10.3"

Initialize Client

If you set PREFAB_API_KEY as an environment variable, initializing the client is as easy as

import prefab_cloud_python

prefab_cloud_python.set_options(prefab_cloud_python.Options()) # reads PREFAB_API_KEY env var

Unless your options are configured to run using only local data, the client will attempt to connect to the remote CDN.

Special Considerations with Forking servers like Gunicorn that use workers

Webservers like gunicorn can be configured to either use threads or fork child process workers. When forking, the prefab client must be re-created in order to continue to fetch updated configuration.

# gunicorn configuration hook
def post_worker_init(worker):

You may also do something like using uWSGI decorators

def post_fork():

This clears the package-singleton client and on the next prefab_cloud_python.get_client() it will be recreated with the options previously set with set_options()

Basic Usage


Here we ask for the value of a config named max-jobs-per-second, and we specify 10 as a default value if no value is available.

prefab_cloud_python.get_client().get("max-jobs-per-second", default=10) # => 10

If no default is provided, the default behavior is to raise a MissingDefaultException.

# raises a `MissingDefaultException`
Handling Undefined Configs

If you would prefer your application return None instead of raising an error, you can set on_no_default="RETURN_NONE" when creating your Options object.

options = Options(
prefab_cloud_python.get_client().get("max-jobs-per-second") # => None

Getting Started

Now create a config named my-first-int-config in the Prefab UI. Set a default value to 50 and sync your change to the API.

Add a feature flag named my-first-feature-flag in the Prefab UI. Add boolean variants of true and false. Set the inactive variant to false, make the flag active and add a rule of type ALWAYS_TRUE with the variant to serve as true. Remember to sync your change to the API.

config_key = "my-first-int-config"
print(config_key, prefab_cloud_python.get_client().get(config_key))

ff_key = "my-first-feature-flag"
print(config_key, prefab_cloud_python.get_client().enabled(ff_key))

Run the code above and you should see:

my-first-int-config 50
my-first-feature-flag true

Congrats! You're ready to rock!

Feature Flags

Feature flags become more powerful when we give the flag evaluation rules more information to work with.

We do this by providing a context for the current user (and/or team, request, etc)

context = {
"user": {
"key": 123,
"subscription_level": "pro",
"email": ""
"team": {
"key": 432,
"device": {
"key": "abcdef",
"mobile": False

result = prefab_cloud_python.get_client().enabled("my-first-feature-flag", context=context)

Feature flags don't have to return just true or false. You can get other data types using get:

prefab_cloud_python.get_client().get("ff-with-string", default="default-string", context=context)
prefab_cloud_python.get_client().get("ff-with-int", default=5)

Thread-local context

To avoid having to pass a context explicitly to every call to get or enabled, it is possible to set a thread-local context that will be evaluated as the default argument to context= if none is given.

from prefab_cloud_python import Context
context = {
"user": {
"key": 123,
"subscription_level": "pro",
"email": ""
"team": {
"key": 432,
"device": {
"key": "abcdef",
"mobile": False

shared_context = Context(context)


# with this set, the following two client calls are equivalent

result = prefab_cloud_python.get_client().enabled("my-first-feature-flag")
result = prefab_cloud_python.get_client().enabled("my-first-feature-flag", context=context)

Scoped context

It is also possible to scope a context for a particular block of code, without needing to set and unset the thread-local context

import prefab_cloud_python
from prefab_cloud_python import Client

context = {
"user": {
"key": 123,
"subscription_level": "pro",
"email": ""
"team": {
"key": 432,
"device": {
"key": "abcdef",
"mobile": False

with Client.scoped_context(context):
result1 = prefab_cloud_python.get_client().enabled("my-first-feature-flag")

result2 = prefab_cloud_python.get_client().enabled("my-first-feature-flag", context=context)

result1 == result2 #=> True


Prefab's Python Client upgrades provides a logging filter that can be plugged into logging or structlogger to provide dynamic log levels. The client assumes your loggers are initialized with the name of each module, ie logger = logging.get_logger(__name__)

Targeted Log Levels

You can use Targeting to change your log levels based on the current user/request/device context using our rules engine.

Log levels

To be as language agnostic as possible, Prefab provides a standardized subset of log levels that can be mapped to language-specific log levels. The language-agnostic levels are the levels that should be set in the Prefab UI or in your local overrides. The list below shows the mappings from Prefab log levels to Python log levels

Prefab => Python
debug => :debug
info => :info
warn => :warn
error => :error
fatal => :critical

Configuration for Standard Logging

In standard logging there are two steps 1) Create an instance of LoggerFilter and configure it as a filter on the logging streamhandler 2) Set the root logger's loglevel to logging.DEBUG so that the LoggerFilter will see all the log records

import logging

# basic logging setup - yours may vary

root_logger = logging.getLogger()
ch = logging.StreamHandler(sys.stdout)
logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")

# key step

Now your normal logging use cases eg

logger = logging.getLogger(__name__)

can be controlled dynamically from Prefab

See example

Note the LoggerFilter has an overrideable logger_name method so you can subclass and determine your own logger name as needed

Configuration for Structlogger

The configuration below is for a structlogger setup backed by the standard logger.

Using the structlog.stdlib.LoggerFactory() ensures the logger instances have names. The initial two processors ensure the log name and level are available on the event dictionary.

import structlog

structlog.processors.TimeStamper(fmt="%Y-%m-%d %H:%M:%S", utc=False),,
logger_factory=structlog.stdlib.LoggerFactory(), # Use Python's logging factory

See example

Note the LoggerProcessor has an overrideable logger_name method so you can subclass and determine your own logger name from elements on the logger record as needed.

Please contact us for help with your configuration if it varies from one of these standard cases.


At this time, it's not possible to dynamically control the loglevel of the prefab client itself. Instead control the Prefab client's log level by changing the prefab_client_log_level in the Options class at start up.

By default this level is set to Logging.WARNING


Specify LOCAL_ONLY and use your config.yaml file.



Available Option parameters

  • api_key - your SDK API key
  • prefab_api_url - the API endpoint your API key has been created for (i.e.
  • prefab_datasources - one of "ALL" (default) or "LOCAL_ONLY", determines whether to fetch data from remote sources or use only local data
  • prefab_config_classpath_dir - the directory from which to load locally defined configuration. This data will be overwritten by data pulled from remote sources. This value defaults to the root of your project (i.e. ".")
  • prefab_config_override_dir - the directory from which to load local override data. Any data found will be loaded overtop of data pulled from remote sources. This value defaults to your $HOME directory.
  • prefab_envs - one or more environment names from which to load local configuration and overrides. See Local config and overrides below for additional information.
  • on_no_default - one of "RAISE" (default) or "RETURN_NONE". This determines how the client behaves when a request for a config cannot find a value, and no default is supplied. These settings will, respectively, raise a MissingDefaultException, or return None.
  • on_connection_failure - one of "RETURN" (default) or "RAISE". This determines what should happen if the connection to a remote datasource times out. These settings will, respectively, return whatever is in the local cache from the latest sync from the remote source, or else raise an InitializationTimeoutException.
  • collect_sync_interval - how often to send telemetry to Prefab (seconds, defaults to 30)
  • collect_evaluation_summaries - send aggregate data about config and feaure flag evaluations, results (defaults to True) Evaluation Summary telemetry Implemented in v0.10+
  • collect_logs - send aggregate logger volume data to Prefab (defaults to True)
  • context_upload_mode - send context information to prefab. Values (from the Options.ContextUploadMode enum) are NONE (don't send any context data), SHAPE_ONLY to only send the schema of the contexts to prefab (field name, data types), PERIODIC_EXAMPLE to send the data types AND the actual contexts being used to Prefab Context telemetry Implemented in v0.10+
  • global_context - an immutable global context to be used in all lookups. Use this for things like availability zone, machine type...
  • on_ready_callback - register a single method to be called when the client has loaded its first configuration and is ready for use