Python SDK reference

note

This SDK is not currently supported on Harness Self-Managed Enterprise Edition (on premises).

This topic describes how to use the Harness Feature Flags Java SDK for your Java application.

For getting started quickly, you can use our sample code from the Python SDK README. You can also clone and run a sample application from the Python SDK GitHub Repository.

Before You Begin

You should read and understand the following:

Version

The current version of this SDK is 1.2.2.

Requirements

To use this SDK, make sure you:

Install Python 3.7 or newer
Install pip
Download the SDK from our GitHub repository
Create a Java application, or clone our sample application.
Create a Feature Flag on the Harness Platform. If you are following along with the SDK README sample code, make sure your flag is called harnessappdemodarkmode
Create an SDK key and make a copy of it

Install the SDK

Install the python SDK using pip

python -m pip install harness-featureflags

Initialize the SDK

To initialize the Python SDK, you need to:

Add your Server SDK Key to connect to your Harness Environment.
Add a Target that you want to Evaluate against a Feature Flag.
(Optional) Configure the SDK options. For more details on what features you can configure for this SDK, go to Configure the SDK.

note

If the SDK fails to authenticate with the Feature Flags service and cannot initialise, the SDK will operate in a "limited" mode and will only serve the default values you provide in variation calls.

Add the Server SDK Key

To connect to the correct Environment that you set up on the Harness Platform, you need to add the Server SDK Key from that Environment. Input the Server SDK Key into the api_key parameter. For example:

"""  
Put the API Key here from your environment  
"""  
api_key = "YOUR_API_KEY";  
  
cf = CfClient(api_key);

Add a Target

What is a Target?

Targets are used to control which users see which Variation of a Feature Flag, for example, if you want to do internal testing, you can enable the Flag for some users and not others. When creating a Target, you give it a name and a unique identifier. Often Targets are users but you can create a Target from anything that can be uniquely identified, such as an app or a machine.

For more information about Targets, go to Targeting Users With Flags.

To add a Target, build it and pass in arguments for the following:


Parameter	Description	Required?	Example
`identifier`	Unique ID for the Target.Read Regex requirements for Target names and identifiers below for accepted characters.	Required	`identifier='HT_1'`
`name`	Name for this Target. This does not have to be unique. Note: If you don’t provide a value, the name will be the same as the identifier.Read Regex requirements for Target names and identifiers below for accepted characters.	OptionalNote: If you don't want to send a name, don't send the parameter. Sending an empty argument will cause an error.	`name="Harness_Target_1"`
`attributes`	Additional data you can store for a Target, such as email addresses or location.	Optional	`attributes={"email": "demo@harness.io"}`

Regex requirements for Target names and identifiers

Identifier

Regex: ^[A-Za-z0-9.@_-]*$
Must consist of only alphabetical characters, numbers, and the following symbols:
. (period)
@ (at sign)
-(dash)
_ (underscore)

The characters can be lowercase or uppercase but cannot include accented letters, for example Cafe_789.

Name Regex: ^[\\p{L}\\d .@_-]*$

Must consist of only alphabetical characters, numbers, and the following symbols:
. (period)
@ (at sign)
-(dash)
_ (underscore)
(space)

The characters can be lowercase or uppercase and can include accented letters, for example Café_123.

For example:

 target = Target(identifier='HT_1', name="Harness_Target_1", attributes={"email": "demo@harness.io"})

Configure the SDK

You can configure the following features of the SDK:


Name	Example	Description	Default Value
baseUrl	`with_base_url("https://config.ff.harness.io/api/1.0")`	The URL used to fetch Feature Flag Evaluations. When using the Relay Proxy, change this to: `http://localhost:7000`	`https://config.ff.harness.io/api/1.0`
eventUrl	`with_events_url("https://events.ff.harness.io/api/1.0")`	The URL for posting metrics data to the Feature Flag service. When using the Relay Proxy, change this to: `http://localhost:7000`	`https://events.ff.harness.io/api/1.0`
pollInterval	`Config(pull_interval=60)`	The interval in seconds that we poll for changes when you are using stream mode.	`60` (seconds)
streamEnabled	`with_stream_enabled(True)`	Set to `True` to enable streaming mode. Set to `False` to disable streaming mode.	`True`
analyticsEnabled	`with_analytics_enabled(True)`	Set to `True` to enable analytics. Set to `False` to disable analytics. Note: When enabled, analytics data is posted every 60 seconds.	`True`
maxAuthRetries	with_max_auth_retries(10)	The number of retry attempts to make if client authentication fails on a retryable HTTP error	10

For example:

# Create a Feature Flag Client  
    client = CfClient(apiKey,  
                      with_base_url("https://config.ff.harness.io/api/1.0"),  
                      with_events_url("https://events.ff.harness.io/api/1.0"),  
                      with_stream_enabled(True),  
                      with_analytics_enabled(True),  
                      Config(pull_interval=60))

Evaluate a Flag

Evaluating a Flag is when the SDK processes all Flag rules and returns the correct Variation of that Flag for the Target you provide.

If a matching Flag can’t be found, or the SDK can’t remotely fetch flags, the default value is returned, and an error is logged.

There are different methods for the different Variation types and for each method you need to pass in:

Identifier of the Flag you want to evaluate
The Target object you want to evaluate against
The default Variation

For example:

Evaluate a boolean Variation

result = cf.bool_variation("identifier_of_your_bool_flag", target, False);

Evaluate a string Variation

result = client.string_variation('identifier_of_your_string_flag', target, "")

Evaluate a number Variation

result = client.number_variation('identifier_of_your_number_flag', target, -1)

Evaluate a JSON Variation

client.json_variation('identifier_of_your_json_flag', target, {})

Test your app is connected to Harness

When you receive a response showing the current status of your Feature Flag, go to the Harness Platform and toggle the Flag on and off. Then, check your app to verify if the Flag Variation displayed is updated with the Variation you toggled.

note

The SDK must run for at least 60 seconds before it sends metrics. Please ensure metrics have not been disabled in the SDK.

Close the SDK Client

In most applications, you won't need to close the SDK client.

However, you should close the SDK client if:

Your application is about to terminate. Closing the client ensures that all associated resources are released.
You have determined that you do not need to evaluate flags again in your application lifecycle.

important

The SDK does not evaluate flags after the client is closed.

To close the SDK client:

Assuming you have initialized an SDK client instance named client, call the following function:
```
client.close()
```

Additional options

Configure your logger

The SDK provides a logger that wraps the standard Python logging package. You can import and use it with the following:

from featureflags.util import log  
log.info("Hello, World!")

To change the default log level, you can use the standard logging levels

from featureflags.util import log  
import logging  
  
log.setLevel(logging.WARN)

Use the Relay Proxy

When using your Feature Flag SDKs with a Harness Relay Proxy you need to change the default URL and events URL to http://localhost:7000 when initializing the SDK. To do this:

Import the URL helper functions, for example:

from featureflags.config import with_base_url  
from featureflags.config import with_events_url

Pass the new URLs in when initializing the SDK, for example:

    client = CfClient(api_key,  
                      with_base_url("https://config.feature-flags.uat.harness.io/api/1.0"),  
                      with_events_url("https://event.feature-flags.uat.harness.io/api/1.0"))

Sample code for a Python application

Here is a sample code for integrating with the Python SDK:

import time  
  
from featureflags.evaluations.auth_target import Target  
from featureflags.client import CfClient  
from featureflags.util import log  
from featureflags.config import with_base_url  
from featureflags.config import with_events_url  
  
  
def main():  
    log.debug("Starting example")  
    api_key = "Your API key"  
    client = CfClient(api_key,  
                      with_base_url("https://config.ff.harness.io/api/1.0"),  
                      with_events_url("https://events.ff.harness.io/api/1.0"))  
  
    target = Target(identifier='HT_1', name="Harness_Target_1", attributes={"location": "emea"})
  
    while True:  
        result = client.bool_variation('harnessappdemodarkmode', target, False)  
        log.debug("Result %s", result)  
        time.sleep(10)  
  
if __name__ == "__main__":  
    main()

Troubleshooting

The SDK logs the following codes for certain lifecycle events, for example authentication, which can aid troubleshooting.

Code	Description
1000	Successfully initialized
1001	Failed to initialize due to authentication error
1002	Failed to initialize due to a missing or empty API key
1003	`wait_for_initialzation` was called and the SDK is waiting for initialzation to complete
2000	Successfully authenticated
2001	Authentication failed with a non recoverable error
2002	Authentication failed and is retrying
2003	Authentication failed and max retries have been exceeded
3000	SDK Closing
3001	SDK closed successfully
4000	Polling service started
4001	Polling service stopped
5000	Streaming service started
5001	Streaming service stopped
5002	Streaming event received
5003	Streaming disconnected and is retrying to connect
5004	Streaming stopped
5005	Stream is still retrying to connect after 4 attempts
6000	Evaluation was successfully
6001	Evaluation failed and the default value was returned
7000	Metrics service has started
7001	Metrics service has stopped
7002	Metrics posting failed
7003	Metrics posting success
7004	Metrics max target size exceeded
7005	Metrics batch targets sending success
7006	Metrics batch targets sending failed

Before You Begin​

Version​

Requirements​

Install the SDK​

Initialize the SDK​

Add the Server SDK Key​

Add a Target​

Configure the SDK​

Evaluate a Flag​

Evaluate a boolean Variation​

Evaluate a string Variation​

Evaluate a number Variation​

Evaluate a JSON Variation​

Test your app is connected to Harness​

Close the SDK Client​

Additional options​

Configure your logger​

Use the Relay Proxy​

Sample code for a Python application​

Troubleshooting​

Before You Begin

Version

Requirements

Install the SDK

Initialize the SDK

Add the Server SDK Key

Add a Target

Configure the SDK

Evaluate a Flag

Evaluate a boolean Variation

Evaluate a string Variation

Evaluate a number Variation

Evaluate a JSON Variation

Test your app is connected to Harness

Close the SDK Client

Additional options

Configure your logger

Use the Relay Proxy

Sample code for a Python application

Troubleshooting