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Getting Started with the Python SDK

Add prefab_cloud_python to your package dependencies

# pyproject.toml


prefab_cloud_python = "0.1.0"

Configure Options

from prefab_cloud_python import Options

options = Options(

To avoid passing your API key directly in code, we recommend setting it in your environment as PREFAB_API_KEY. Once you've done this you do not need to pass a value for that key to Options(...), as it will look in your ENV for a value for that key.

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_grpc_url - the gRPC endpoint (including port) you wish to connect to (i.e.
  • namespace - an optional namespace to define your client's scope when looking up config
  • 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.

Start the Client

from prefab_cloud_python import Client, Options

options = Options(...)
client = Client(options)

Unless your options are configured to run using only local data, the client will attempt to connect to the remote CDN and gRPC data sources and begin syncing data to a local store.

Basic Usage


It is a best practice to specify a default value for all configuration. This reduces the likelihood of exceptions due to None values.

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.

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`

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(
client = Client(options)
client.get("max-jobs-per-second") # => None

You can specify defaults for your application by creating a file .prefab.default.config.yaml

Add the following:

# .prefab.default.config.yaml
log-level.prefab: info
my-first-int-config: 30
my-first-feature-flag: false

Learn more about defaults.

Getting Started

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

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

Should output the following:

my-first-int-config 30
my-first-feature-flag false

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.

Run the code above again 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 lookup key and identity attributes.

The lookup key serves two purposes. First, it will be the unique key that helps us target a specific user using Lookup Key In when evaluating a flag.

Second, this key is used to make sure that percent rollout evaluations are consistently applied.

Attributes come into play when using the Property Is One Of and similar rule criteria.

lookup_key = "user-1234"
identity_attributes = {
"team_id": 432,
"user_id": 123,
"subscription_level": 'pro',
"email": ""

result = client.enabled("my-first-feature-flag", lookup_key=lookup_key, attributes=identity_attributes)
print("my-first-feature-flag is", result, "for", lookup_key)

How you choose the tracking_id for your user is up to you, but we have some suggestions and we would generally discourage unless you are positive you don't have anonymous users. See the section on Tracking IDs for our suggestion.

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

client.get("ff-with-string", default="default-string")
client.get("ff-with-int", default=5)


Namespaces allow you to share config between many applications while still allowing you to override when necessary.

For instance, let's assume that all our code shares an HTTP library. We can configure the HTTP library to get its retry count & timeout duration from our config store. We'll set http.connection.retries and http.connection.timeout in the default namespace.

All of our apps should initialize their config store in a namespace. For instance our User service can initialize with:

options = Options(

Prefab config will find the "closest" matching config when the UserService goes to look for a value of http.connection.timeout.

If our service namespace is services.user-service.web it will match overrides in namespaces services , services.user_service and services.user_service.web, but not services.user_service.cron or

Let's imagine that the UserService starts to go down because too many requests are timing out to a 3rd party service. We can quickly reduce the http.connection.timeout for our services.user_service namespace and solve the issue without pushing code or restarting.


Prefab's Python Client comes with a powerful upgrade to the default Python logging by building on top of structlog to provide dynamic log levels.

To use it, use the logging functions included in the Client public API


You can now control logging at any level of your stack. For convenience, we'll set these as local defaults in .prefab.default.config.yaml (learn more) but you can set and tweak these on-the-fly in the Prefab web app.

# .prefab.default.config.yaml
log-level.my_app.my_class: info
log-level.my_app.my_class.warn: warn
log-level.my_app.my_class.debug: debug
# my_app/
class MyClass:
def warn(self, client):
client.logger().info("never logs")
client.logger().debug("never logs")

def debug(self, client):

def default(self, client):
client.logger().debug("never logs")

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 Elixir log levels

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

Local Overrides

It can be very useful to modify your defaults locally without changing the defaults file or values in the API. To do this, add a file in your home directory or classpath called .prefab.default.config.yaml

# .prefab.default.config.yaml
mycorp.auth.api.url: ""
mycorp.auth.api.url: "localhost:9090"

Prefab will first load the defaults, then merge the remote API values over the top, and finally it will apply the overrides file on top of those values.


Prefab is designed to be extremely resilient. The client will try to pull live values from:

  1. A Fastly CDN backed by the API
  2. The Prefab API
  3. Prefab Streaming APIs

This strategy ensures the utmost reliability for your clients being able to pull live values, even in the case of a major outage of the Prefab APIs.

But wait, there's more.

In the terrible occurrence that the Prefab APIs are down for an extended period of time, your services should be able to bootstrap themselves and load from the CDN, but you would be unable to modify configuration.

In the event that one or all of these services become unavailable, the Python SDK stores a local copy of all data pulled from the remote endpoints, so it can continue to serve the last available live data, but would not be able to fetch or push new data until the APIs are restored.

Using Prefab For Rollouts

So you've built a new pipeline and are hoping to slowly dial up how much traffic uses it. You've got two great ways to do that with Prefab.

One approach is to simply use dynamic config. We can use a floating point number to specify the percent of traffic we want to rollout to and then evaluate that against a random number to determine whether to run the new code.

import random

if random.random() < client.get("percent-to-rollout"):

This approach works fine, but each evaluation of random.random() will get you a different result. Sometimes this is what you want, but if you'd like the rollout to keep server, requests, users in the new pipeline you may want to use a feature flag.

client.enabled("new-feature", lookup_key=any_consistent_id)


You can control the Prefab client's log level by changing the configuration value of log-level.prefab. In the rare case that you are trying to debug issues that occur before this config file has been read, set env var


By default this level is set to :warn


Specify LOCAL_ONLY and use your config.yaml file.

options = Options(data_sources="LOCAL_ONLY")
client = Client(options)