I have a piece of code in Python that seems to cause an error probabilistically because it is accessing a server and sometimes that server has a 500 internal server error. I want to keep trying until I do not get the error. My solution was:
while True:
try:
#code with possible error
except:
continue
else:
#the rest of the code
break
This seems like a hack to me. Is there a more Pythonic way to do this?
This question is related to
python
error-handling
Maybe decorator based? You can pass as decorator arguments list of exceptions on which we want to retry and/or number of tries.
def retry(exceptions=None, tries=None):
if exceptions:
exceptions = tuple(exceptions)
def wrapper(fun):
def retry_calls(*args, **kwargs):
if tries:
for _ in xrange(tries):
try:
fun(*args, **kwargs)
except exceptions:
pass
else:
break
else:
while True:
try:
fun(*args, **kwargs)
except exceptions:
pass
else:
break
return retry_calls
return wrapper
from random import randint
@retry([NameError, ValueError])
def foo():
if randint(0, 1):
raise NameError('FAIL!')
print 'Success'
@retry([ValueError], 2)
def bar():
if randint(0, 1):
raise ValueError('FAIL!')
print 'Success'
@retry([ValueError], 2)
def baz():
while True:
raise ValueError('FAIL!')
foo()
bar()
baz()
of course the 'try' part should be moved to another funcion becouse we using it in both loops but it's just example;)
e = ''
while e == '':
try:
response = ur.urlopen('https://https://raw.githubusercontent.com/MrMe42/Joe-Bot-Home-Assistant/mac/Joe.py')
e = ' '
except:
print('Connection refused. Retrying...')
time.sleep(1)
This should work. It sets e to '' and the while loop checks to see if it is still ''. If there is an error caught be the try statement, it prints that the connection was refused, waits 1 second and then starts over. It will keep going until there is no error in try, which then sets e to ' ', which kills the while loop.
When retrying due to error, you should always:
A simple generic way to solve this problem while covering those concerns would be to use the backoff library. A basic example:
import backoff
@backoff.on_exception(
backoff.expo,
MyException,
max_tries=5
)
def make_request(self, data):
# do the request
This code wraps make_request with a decorator which implements the retry logic. We retry whenever our specific error MyException occurs, with a limit of 5 retries. Exponential backoff is a good idea in this context to help minimize the additional burden our retries place on the remote server.
Here is a short piece of code I use to capture the error as a string. Will retry till it succeeds. This catches all exceptions but you can change this as you wish.
start = 0
str_error = "Not executed yet."
while str_error:
try:
# replace line below with your logic , i.e. time out, max attempts
start = raw_input("enter a number, 0 for fail, last was {0}: ".format(start))
new_val = 5/int(start)
str_error=None
except Exception as str_error:
pass
WARNING: This code will be stuck in a forever loop until no exception occurs. This is just a simple example and MIGHT require you to break out of the loop sooner or sleep between retries.
Im attempting this now, this is what i came up with;
placeholder = 1
while placeholder is not None:
try:
#Code
placeholder = None
except Exception as e:
print(str(datetime.time(datetime.now()))[:8] + str(e)) #To log the errors
placeholder = e
time.sleep(0.5)
continue
Like most of the others, I'd recommend trying a finite number of times and sleeping between attempts. This way, you don't find yourself in an infinite loop in case something were to actually happen to the remote server.
I'd also recommend continuing only when you get the specific exception you're expecting. This way, you can still handle exceptions you might not expect.
from urllib.error import HTTPError
import traceback
from time import sleep
attempts = 10
while attempts > 0:
try:
#code with possible error
except HTTPError:
attempts -= 1
sleep(1)
continue
except:
print(traceback.format_exc())
#the rest of the code
break
Also, you don't need an else block. Because of the continue in the except block, you skip the rest of the loop until the try block works, the while condition gets satisfied, or an exception other than HTTPError comes up.
Here is one that hard fails after 4 attempts, and waits 2 seconds between attempts. Change as you wish to get what you want form this one:
from time import sleep
for x in range(0, 4): # try 4 times
try:
# msg.send()
# put your logic here
str_error = None
except Exception as str_error:
pass
if str_error:
sleep(2) # wait for 2 seconds before trying to fetch the data again
else:
break
Here is an example with backoff:
from time import sleep
sleep_time = 2
num_retries = 4
for x in range(0, num_retries):
try:
# put your logic here
str_error = None
except Exception as str_error:
pass
if str_error:
sleep(sleep_time) # wait before trying to fetch the data again
sleep_time *= 2 # Implement your backoff algorithm here i.e. exponential backoff
else:
break
The itertools.iter_except recipes encapsulates this idea of "calling a function repeatedly until an exception is raised". It is similar to the accepted answer, but the recipe gives an iterator instead.
From the recipes:
def iter_except(func, exception, first=None):
""" Call a function repeatedly until an exception is raised."""
try:
if first is not None:
yield first() # For database APIs needing an initial cast to db.first()
while True:
yield func()
except exception:
pass
You can certainly implement the latter code directly. For convenience, I use a separate library, more_itertools, that implements this recipe for us (optional).
Code
import more_itertools as mit
list(mit.iter_except([0, 1, 2].pop, IndexError))
# [2, 1, 0]
Details
Here the pop method (or given function) is called for every iteration of the list object until an IndexError is raised.
For your case, given some connect_function and expected error, you can make an iterator that calls the function repeatedly until an exception is raised, e.g.
mit.iter_except(connect_function, ConnectionError)
At this point, treat it as any other iterator by looping over it or calling next().
what about the retrying library on pypi? I have been using it for a while and it does exactly what I want and more (retry on error, retry when None, retry with timeout). Below is example from their website:
import random
from retrying import retry
@retry
def do_something_unreliable():
if random.randint(0, 10) > 1:
raise IOError("Broken sauce, everything is hosed!!!111one")
else:
return "Awesome sauce!"
print do_something_unreliable()
Here's an utility function that I wrote to wrap the retry until success into a neater package. It uses the same basic structure, but prevents repetition. It could be modified to catch and rethrow the exception on the final try relatively easily.
def try_until(func, max_tries, sleep_time):
for _ in range(0,max_tries):
try:
return func()
except:
sleep(sleep_time)
raise WellNamedException()
#could be 'return sensibleDefaultValue'
Can then be called like this
result = try_until(my_function, 100, 1000)
If you need to pass arguments to my_function, you can either do this by having try_until forward the arguments, or by wrapping it in a no argument lambda:
result = try_until(lambda : my_function(x,y,z), 100, 1000)
Many different answers here but I used a combination of a few of the brilliant answers and threw in my own not-so-secret sauce to come up with the following recipe:
def actioin_one(self):
action = # define attempted action here
failure = bool(action.status_code != 200) # example ... replace this.
self.attempt_action_repeatedly(action, 5, failure)
def action_two(self):
action = # define attempted action here
failure = bool(action.status_code != 200) # example ... replace this.
self.attempt_action_repeatedly(action, 5, failure)
def attempt_action_repeatedly(self, action, authorized_attempts, failure):
attempts = 0
sleep_time = 2 # seconds
more_attempts_left = bool(attempts < authorized_attempts)
while failure and more_attempts_left:
action
attempts += 1
sleep(sleep_time)
if not more_attempts_left:
print("Max number of attempts exceeded. Attempts failed because:")
data = action.json() # customize this
print(data['detail']) # customize this
Maybe something like this:
connected = False
while not connected:
try:
try_connect()
connected = True
except ...:
pass
Source: Stackoverflow.com