Retry mechanism

If your code depends on external factors (e.g. the availability of web services or the network connection) you may want to retry in case of failures.

Demo scenario

The code on this page uses the following demo scenario: A list of numbers is iterated and used as denominators. If the number is 0 (and a ZeroDivisionError is raised) the code retries up to three times (in reality you would try to fix the root cause).

For loop

import time

attempts_max = 3
wait_between = 2.0
denominator = [1, 2, 0, 3]
for d in denominator:
    for _ in range(attempts_max):
        try:
            # Execute logic
            print(f'1/{d} = {1 / d}')
        except ZeroDivisionError:
            # Logic failed (take neccesary steps before retry)
            time.sleep(wait_between)
            print('retry')
        else:
            # Logic successful
            break
    else:
        # Max. number of attempts failed
        raise RuntimeError('max number of attempts failed')

While loop

import time

attempts_max = 3
wait_between = 2.0
denominator = [1, 2, 0, 3]
def retry_while_loop():
i = 0
attempt_count = 0
while i < len(denominator):
    d = denominator[i]
    try:
        # Execute logic
        print(f'1/{d} = {1 / d}')
    except ZeroDivisionError:
        if attempt_count >= attempts_max:
            # Max. number of attempts failed
            raise RuntimeError('max number of attempts failed')
        attempt_count += 1
        # Logic failed (take neccesary steps before retry)
        time.sleep(wait_between)
        print('retry')
        continue
    i += 1

Function decorator

import functools
import time

def retry(num_tries: int, wait_between: float):
    def decorator_retry(func):
        @functools.wraps(func)
        def wrapper_retry(*args, **kwargs):
            for _ in range(num_tries-1):
                try:
                    return func(*args, **kwargs, retry_error=RetryError)
                except ZeroDivisionError:
                    # Logic failed (take neccesary steps before retry)
                    time.sleep(wait_between)
            # Last try.
            return func(*args, **kwargs)
        return wrapper_retry
    return decorator_retry

@retry(num_tries=3, wait_between=2.0)
def retry_decorated():
    denominator = [1, 2, 0, 3]
    for d in denominator:
        print(f'1/{d} = {1 / d}')