问题
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?
回答1:
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
回答2:
I prefer to limit the number of retries, so that if there's a problem with that specific item you will eventually continue onto the next one, thus:
for i in range(100):
for attempt in range(10):
try:
# do thing
except:
# perhaps reconnect, etc.
else:
break
else:
# we failed all the attempts - deal with the consequences.
回答3:
The retrying package is a nice way to retry a block of code on failure.
For example:
@retry(wait_random_min=1000, wait_random_max=2000)
def wait_random_1_to_2_s():
print("Randomly wait 1 to 2 seconds between retries")
回答4:
Here is a solution similar to others, but it will raise the exception if it doesn't succeed in the prescribed number or retries.
tries = 3
for i in range(tries):
try:
do_the_thing()
except KeyError as e:
if i < tries - 1: # i is zero indexed
continue
else:
raise
break
回答5:
The more "functional" approach without using those ugly while loops:
def tryAgain(retries=0):
if retries > 10: return
try:
# Do stuff
except:
retries+=1
tryAgain(retries)
tryAgain()
回答6:
The clearest way would be to explicitly set i. For example:
i = 0
while i < 100:
i += 1
try:
# do stuff
except MyException:
continue
回答7:
Using recursion
for i in range(100):
def do():
try:
## Network related scripts
except SpecificException as ex:
do()
do() ## invoke do() whenever required inside this loop
回答8:
A generic solution with a timeout:
import time
def onerror_retry(exception, callback, timeout=2, timedelta=.1):
end_time = time.time() + timeout
while True:
try:
yield callback()
break
except exception:
if time.time() > end_time:
raise
elif timedelta > 0:
time.sleep(timedelta)
Usage:
for retry in onerror_retry(SomeSpecificException, do_stuff):
retry()
回答9:
There is something similar in the Python Decorator Library.
Please bear in mind that it does not test for exceptions, but the return value. It retries until the decorated function returns True.
A slightly modified version should do the trick.
回答10:
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.
回答11:
Using while and a counter:
count = 1
while count <= 3: # try 3 times
try:
# do_the_logic()
break
except SomeSpecificException as e:
# If trying 3rd time and still error??
# Just throw the error- we don't have anything to hide :)
if count == 3:
raise
count += 1
回答12:
You can use Python retrying package. Retrying
It is written in Python to simplify the task of adding retry behavior to just about anything.
回答13:
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
回答14:
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
回答15:
Here's my idea on how to fix this:
j = 19
def calc(y):
global j
try:
j = j + 8 - y
x = int(y/j) # this will eventually raise DIV/0 when j=0
print("i = ", str(y), " j = ", str(j), " x = ", str(x))
except:
j = j + 1 # when the exception happens, increment "j" and retry
calc(y)
for i in range(50):
calc(i)
回答16:
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}')
回答17:
increment your loop variable only when the try clause succeeds
来源：https://stackoverflow.com/questions/2083987/how-to-retry-after-exception