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Trying out a Retry decorator in Python

Date: 2009-11-17  |   Tags: python

The Python wiki has a Retry decorator example which retries calling a failure-prone function using an exponential backoff algorithm. I modified it slightly to check for exceptions instead of a False return value to indicate failure. Each time the decorated function throws an exception, the decorator will wait a period of time and retry calling the function until the maximum number of tries is used up. If the decorated function fails on the last try, the exception will occur unhandled. 

import time

def retry(ExceptionToCheck, tries=4, delay=3, backoff=2):
    """Retry decorator
    original from http://wiki.python.org/moin/PythonDecoratorLibrary#Retry
    """
    def deco_retry(f):
        def f_retry(*args, **kwargs):
            mtries, mdelay = tries, delay
            try_one_last_time = True
            while mtries > 1:
                try:
                    f(*args, **kwargs)
                    try_one_last_time = False
                    break
                except ExceptionToCheck, e:
                    print "%s, Retrying in %d seconds..." % (str(e), mdelay)
                    time.sleep(mdelay)
                    mtries -= 1
                    mdelay *= backoff
            if try_one_last_time:
                f(*args, **kwargs)
            return
        return f_retry # true decorator
    return deco_retry

Try an "always fail" case:

@retry(Exception, tries=4)
def test_fail(text):
    raise Exception("Fail")

test_fail("it works!")

Results:

Fail, Retrying in 3 seconds...
Fail, Retrying in 6 seconds...
Fail, Retrying in 12 seconds...
Traceback (most recent call last):
  File "retry_decorator.py", line 47, in 
    test_fail("it works!")
  File "retry_decorator.py", line 26, in f_retry
    f(*args, **kwargs)
  File "retry_decorator.py", line 33, in test_fail
    raise Exception("Fail")
Exception: Fail

Try a "success" case:

@retry(Exception, tries=4)
def test_success(text):
    print "Success: ", text

test_success("it works!")

Results:

Success:  it works!

Try a "random fail" case:

import random

@retry(Exception, tries=4)
def test_random(text):
    x = random.random()
    if x < 0.5:
        raise Exception("Fail")
    else:
        print "Success: ", text

test_random("it works!")

Example results:

Fail, Retrying in 3 seconds...
Success:  it works!

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