Retry decorator in Python

In this post , we will be discussing about Retry decorator and how to use it in python.

Generally , we might have encountered many scenarios where we want to execute a piece of code until we get expected result or we may need to wait for the some operation should complete before proceeding further.

Before directly going to the implementation , I will explain a simple use case where I had to use retry decorator.

The use case is , there is an API with following endpoints:-

1.api/getData
2.api/status
3.api/data

api/getData , returns “id” instead of directly returning data for every request made(has some input parameters).

api/status , takes the parameter “id” returned from api/getData , and checks if the status of that “id” is success or not (returns true/false).

Until api/status returns true we cannot make a request api/data because data is not yet parsed. So, api/status indicates if we can make a request to api/data to get the data which accepts “id” as parameter.

So , in this case we have make continuous requests to api/status endpoint to check the status.

Lets see how we can achieve this , with and without using retry decorator.

Without Retry decorator

def check_status(id):
     status = requests.get(status_url)
     if status == True:
          return True
     else:
         return False
def get_data(id):
     
     status = None
     while status is None or status == False:
           time.sleep(5)
           status = check_status(id)
     response = requests.get(url)
     return response;

In above pseudo code , I am making a call to “check_status” method in while loop and for every request I am delaying it for 5 seconds and it has to wait until status is “True” , only then we can fetch the data using “api/data”
endpoint.

So , its a kind of retry pattern where we are repeatedly checking the status of id by delaying it by 5 seconds.

The same can be implemented using “Retry decorator”.

With Retry decorator

To install retry decorator , use below command.

pip install retry

To use it in your code , include below statement in your code.

from retry import retry

To make any function as a retry logic , just add @retry() statement above your function definition as below.

@retry()
def check_status:

Retry decorator accepts some parameters ,

exceptions - you can specify your custom exception class , which indicates that you are expecting the function to retry when specified exception occurs.
tries - specifies the maximum number of times function can be retried. Default value is "-1".
delay - specifies the amount of time the function waits before next retry , similar to time.sleep() we have used above. Default value is "0".

Above are some of the parameters that retry decorator accepts , for more information refer documentation.

Implementation:

Instead of using time.sleep(5) , I have used retry decorator declared above “wait_for_success_status” function with 3 parameters Exception type which indicates that on that specific type of exception function has to retry , delay with 5 seconds which delays the function execution by 5 seconds , tries with value 6 which indicates that function can be retried maximum of 6 times.

We can add multiple exception types as well.

@retry((ValueError,TypeError,InvalidStatus), delay=5, tries=6)

In this case function will be re-tried if any one of exception occurs.