I have a loop starting with for i in range(0, 100). Normally it runs correctly, but sometimes it fails due to network conditions. Currently I have it set so that on failure, it will continue in the except clause (continue on to the next number for i).
Is it possible for me to reassign the same number to i and run through the failed iteration of the loop again?
Do a while True inside your for loop, put your try code inside, and break from that while loop only when your code succeeds.
for i in range(0,100):
while True:
try:
# do stuff
except SomeSpecificException:
continue
break
retrying: tenacity and backoff (2020 update)The retrying library was previously the way to go, but sadly it has some bugs and it hasn't got any updates since 2016. Other alternatives seem to be backoff and tenacity. During the time of writing this, the tenacity had more GItHub stars (2.3k vs 1.2k) and was updated more recently, hence I chose to use it. Here is an example:
from functools import partial
import random # producing random errors for this example
from tenacity import retry, stop_after_delay, wait_fixed, retry_if_exception_type
# Custom error type for this example
class CommunicationError(Exception):
pass
# Define shorthand decorator for the used settings.
retry_on_communication_error = partial(
retry,
stop=stop_after_delay(10), # max. 10 seconds wait.
wait=wait_fixed(0.4), # wait 400ms
retry=retry_if_exception_type(CommunicationError),
)()
@retry_on_communication_error
def do_something_unreliable(i):
if random.randint(1, 5) == 3:
print('Run#', i, 'Error occured. Retrying.')
raise CommunicationError()
for i in range(100):
do_something_unreliable(i)
The above code outputs something like:
Run# 3 Error occured. Retrying.
Run# 5 Error occured. Retrying.
Run# 6 Error occured. Retrying.
Run# 6 Error occured. Retrying.
Run# 10 Error occured. Retrying.
.
.
.
More settings for the tenacity.retry are listed on the tenacity GitHub page.
The clearest way would be to explicitly set i. For example:
i = 0
while i < 100:
i += 1
try:
# do stuff
except MyException:
continue
for _ in range(5):
try:
# replace this with something that may fail
raise ValueError("foo")
# replace Exception with a more specific exception
except Exception as e:
err = e
continue
# no exception, continue remainder of code
else:
break
# did not break the for loop, therefore all attempts
# raised an exception
else:
raise err
My version is similar to several of the above, but doesn't use a separate while loop, and re-raises the latest exception if all retries fail. Could explicitly set err = None at the top, but not strictly necessary as it should only execute the final else block if there was an error and therefore err is set.
Decorator is a good approach.
from functools import wraps
import time
class retry:
def __init__(self, success=lambda r:True, times=3, delay=1, raiseexception=True, echo=True):
self.success = success
self.times = times
self.raiseexception = raiseexception
self.echo = echo
self.delay = delay
def retry(fun, *args, success=lambda r:True, times=3, delay=1, raiseexception=True, echo=True, **kwargs):
ex = Exception(f"{fun} failed.")
r = None
for i in range(times):
if i > 0:
time.sleep(delay*2**(i-1))
try:
r = fun(*args, **kwargs)
s = success(r)
except Exception as e:
s = False
ex = e
# raise e
if not s:
continue
return r
else:
if echo:
print(f"{fun} failed.", "args:", args, kwargs, "\nresult: %s"%r)
if raiseexception:
raise ex
def __call__(self, fun):
@wraps(fun)
def wraper(*args, retry=0, **kwargs):
retry = retry if retry>0 else self.times
return self.__class__.retry(fun, *args,
success=self.success,
times=retry,
delay=self.delay,
raiseexception = self.raiseexception,
echo = self.echo,
**kwargs)
return wraper
some usage examples:
@retry(success=lambda x:x>3, times=4, delay=0.1)
def rf1(x=[]):
x.append(1)
print(x)
return len(x)
> rf1()
[1]
[1, 1]
[1, 1, 1]
[1, 1, 1, 1]
4
@retry(success=lambda x:x>3, times=4, delay=0.1)
def rf2(l=[], v=1):
l.append(v)
print(l)
assert len(l)>4
return len(l)
> rf2(v=2, retry=10) #overwite times=4
[2]
[2, 2]
[2, 2, 2]
[2, 2, 2, 2]
[2, 2, 2, 2, 2]
5
> retry.retry(lambda a,b:a+b, 1, 2, times=2)
3
> retry.retry(lambda a,b:a+b, 1, "2", times=2)
TypeError: unsupported operand type(s) for +: 'int' and 'str'
You can use Python retrying package. Retrying
It is written in Python to simplify the task of adding retry behavior to just about anything.
I use this, which can be used on any function:
def run_with_retry(func: callable, max_retries: int = 3, wait_seconds: int = 2, **func_params):
num_retries = 1
while True:
try:
return func(*func_params.values())
except Exception as e:
if num_retries > max_retries:
print('we have reached maximum errors and raising the exception')
raise e
else:
print(f'{num_retries}/{max_retries}')
print("Retrying error:", e)
num_retries += 1
sleep(wait_seconds)
Call like this:
def add(val1, val2):
return val1 + val2
run_with_retry(func=add, param1=10, param2=20)
I use following in my codes,
for i in range(0, 10):
try:
#things I need to do
except ValueError:
print("Try #{} failed with ValueError: Sleeping for 2 secs before next try:".format(i))
time.sleep(2)
continue
break
attempts = 3
while attempts:
try:
...
...
<status ok>
break
except:
attempts -=1
else: # executed only break was not raised
<status failed>Here is a quick decorator to handle this. 7 lines, no dependencies.
def retry(exception=Exception, retries=3, delay=0):
def wrap(func):
for i in range(retries):
try:
return func()
except exception as e:
print(f'Retrying {func.__name__}: {i}/{retries}')
time.sleep(delay)
raise e
return wrap
@retry()
def do_something():
...
@retry(HTTPError, retries=100, delay=3)
def download_something():
...
An addition that could be made is extending exception to handle multiple exceptions (splat a list).
If you want a solution without nested loops and invoking break on success you could developer a quick wrap retriable for any iterable. Here's an example of a networking issue that I run into often - saved authentication expires. The use of it would read like this:
client = get_client()
smart_loop = retriable(list_of_values):
for value in smart_loop:
try:
client.do_something_with(value)
except ClientAuthExpired:
client = get_client()
smart_loop.retry()
continue
except NetworkTimeout:
smart_loop.retry()
continue
Here is my take on this issue. The following retry function supports the following features:
import time
def retry(func, ex_type=Exception, limit=0, wait_ms=100, wait_increase_ratio=2, logger=None):
attempt = 1
while True:
try:
return func()
except Exception as ex:
if not isinstance(ex, ex_type):
raise ex
if 0 < limit <= attempt:
if logger:
logger.warning("no more attempts")
raise ex
if logger:
logger.error("failed execution attempt #%d", attempt, exc_info=ex)
attempt += 1
if logger:
logger.info("waiting %d ms before attempt #%d", wait_ms, attempt)
time.sleep(wait_ms / 1000)
wait_ms *= wait_increase_ratio
Usage:
def fail_randomly():
y = random.randint(0, 10)
if y < 10:
y = 0
return x / y
logger = logging.getLogger()
logger.setLevel(logging.INFO)
logger.addHandler(logging.StreamHandler(stream=sys.stdout))
logger.info("starting")
result = retry.retry(fail_randomly, ex_type=ZeroDivisionError, limit=20, logger=logger)
logger.info("result is: %s", result)
See my post for more info.
I like to use bool values for this, like so:
success = False
num_try = 0
while success is False:
if num_try >= 10: # or any number
# handle error how you please
try:
# code
success = True
except Exception as e:
# record or do something with exception if needed
num_try += 1
with this decorator, you can easily control errors
class catch:
def __init__(self, max=1, callback=None):
self.max = max
self.callback = callback
def set_max(self, max):
self.max = max
def handler(self, *args, **kwargs):
self.index = 0
while self.index < self.max:
self.index += 1
try:
self.func(self, *args, **kwargs)
except Exception as error:
if callable(self.callback):
self.callback(self, error, args, kwargs)
def __call__(self, func):
self.func = func
return self.handler
import time
def callback(cls, error, args, kwargs):
print('func args', args, 'func kwargs', kwargs)
print('error', repr(error), 'trying', cls.index)
if cls.index == 2:
cls.set_max(4)
else:
time.sleep(1)
@catch(max=2, callback=callback)
def test(cls, ok, **kwargs):
raise ValueError('ok')
test(1, message='hello')
You could have a dedicated function using return to short circuit the result. For example like this:
def my_function_with_retries(..., max_retries=100):
for attempt in range(max_retries):
try:
return my_function(...)
except SomeSpecificException as error:
logging.warning(f"Retrying after failed execution: {error}")
raise SomeOtherException()
If retrying a failed attempt x number of times is what you are looking for, a single for else loop is probably what you want. Consider this example with 3 attempts:
attempts = 3
for attempt in range(1, attempts+1):
try:
if attempt < 4:
raise TypeError(f"Error raised on attempt: {attempt}")
else:
print(f'Attempt {attempt} finally worked.')
except (TypeError) as error:
print(f'Attempt {attempt} hit the exception.')
continue
else:
break
else:
print(f'Exit after final attempt: {attempt}')
print(f'\nGo on to execute other code ...')
Gives the output:
Attempt 1 hit the exception.
Attempt 2 hit the exception.
Attempt 3 hit the exception.
Exit after final attempt: 3
Go on to execute other code ...
And with one more attempt it succeeds:
attempts = 4
Gives the output:
Attempt 1 hit the exception.
Attempt 2 hit the exception.
Attempt 3 hit the exception.
Attempt 4 finally worked.
Go on to execute other code ...
i recently worked with my python on a solution to this problem and i am happy to share it with stackoverflow visitors please give feedback if it is needed.
print("\nmonthly salary per day and year converter".title())
print('==' * 25)
def income_counter(day, salary, month):
global result2, result, is_ready, result3
result = salary / month
result2 = result * day
result3 = salary * 12
is_ready = True
return result, result2, result3, is_ready
i = 0
for i in range(5):
try:
month = int(input("\ntotal days of the current month: "))
salary = int(input("total salary per month: "))
day = int(input("Total Days to calculate> "))
income_counter(day=day, salary=salary, month=month)
if is_ready:
print(f'Your Salary per one day is: {round(result)}')
print(f'your income in {day} days will be: {round(result2)}')
print(f'your total income in one year will be: {round(result3)}')
break
else:
continue
except ZeroDivisionError:
is_ready = False
i += 1
print("a month does'nt have 0 days, please try again")
print(f'total chances left: {5 - i}')
except ValueError:
is_ready = False
i += 1
print("Invalid value, please type a number")
print(f'total chances left: {5 - i}')