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Python: Try three times a function until all failed [duplicate]
(4 answers)
Closed 2 years ago.
I'm having an hard time figuring out how to implement a simple retry function in Python, where my code should have to try to send a message to telegram for a fixed amount of times, and if after tot times it will still fail, print an error. Instead, if it succeeds, break and go on.
So i have the following array:
Retries = [1, 2, 3]
Which means that, if it fails, wait one second, and if it fails again wait 2 seconds and then 3 seconds.
Then i have the following code:
for x in Retries:
try:
updater.bot.send_message(chat_id='myid', text='TEST')
except Exception as e:
time.sleep(x)
This code will try three times and wait 1, 2, 3, seconds between each try, but the problem is that:
Even if the message will be sent, it will still keep trying, while it needs to break
If it will fail for three times, it should print an error, i don't know how to do that
I found this library, but i think it's overkill for such a simple task. Can anyone help me out on this?
I recommend using pythons arcane but useful else clause for for loops:
for x in Retries:
try:
updater.bot.send_message(chat_id='myid', text='TEST')
except Exception as e:
time.sleep(x)
else:
break
else:
# raise the real error here
If the function call succeeds, your code will break out of the loop and not trigger the else clause. The else clause is only triggered if the loop completes without a break.
Use a break in the try block after sending the message like this -
for x in Retries:
try:
print('Hi there')
break #Add a break here
except Exception as e:
time.sleep(x)
Hi there
I have a Python program that connects to a server to send it some commands, but occasionally I get this error:
TimeoutError: [WinError 10060] A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond
In previous situations like this, I would use something like this:
try:
Do something
except KeyError:
do something else
Could I do the same thing in this same situation? I.e.,
try:
Do something
except TimeoutError:
Do something again
And if so, what would I do after the except TimeoutError? Would I just do the same command again?
Could I do the same thing in this same situtation
Yes! You can use try/except for any exception, and TimeoutError is nothing special.
and if so what would I do after the except TimeoutError? would I just do the same command again?
If you only want to retry once, and let a second timeout count as a real error, yes:
try:
do something
except TimeoutError:
do something
(If "do something" is more than a simple statement, you probably want to factor out the code so you don't repeat yourself.)
However, if you want to retry multiple times, you probably want a loop:
for _ in range(max_retries):
try:
do something
break
except TimeoutError:
pass
You may want to add an else clause to the for to distinguish the two cases (succeeded, and did a break, vs. never succeeded and just ran out of tries).
Since the idea here is usually to deal with possibly-transient errors, there are additional things you might want to add, such as:
Re-raising the error, or raising a different one, after max_retries.
Logging at progressively higher levels (e.g., a debug message for a single failure, but a warning for `max_retries).
Retrying with exponential backoff (wait 1 second, then 2, 4, 8, …).
Pushing the URL to the end of the work queue instead of retrying immediately. You can use (URL, retry_count) pairs if you also want max_retries logic, (URL, timestamp) pairs if you also want exponential backoff, or both if you want both. (Of course this only works if you don't care about the order of responses, or can reorder them at the end.)
Different rules for different exceptions (e.g., 500, 502, 503, and 504 errors can all be caused by overload on a server or proxy, but the best retry logic may be different—and the best heuristics for 2018 may be different from 2010 or 2025).
For complicated combinations, a retry decorator, like the one linked in jterrace's helpful answer, is a great way to wrap up the behavior.
You can catch the TimeoutError like you mentioned:
import socket
import sys
try:
dosomething()
except socket.TimeoutError:
print >> sys.stderr, 'Retrying after TimeoutError'
dosomething()
You could also use the retry decorator pattern on a function:
#retry(socket.TimeoutError)
def dosomething():
# code that causes a TimeoutError
...
def f():
pass #insert code here
To repeat once after the error:
try:
f()
except TimeoutError:
f()
Or to loop until success:
while True:
try:
f()
break
except TimeoutError:
pass
Or with a limited number:
attempts = 3
while attempts:
try:
f()
break
except TimeoutError:
attempts -= 1
Please run each example. They are ready!
Example 1
import sys
try:
incorrect.syntaxThatIJustMadeUP()
except:
print((sys.exc_info()[0])) #Now you know what to except and CATCH
else:
print("You will never see this message")
Example 2
import sys
try:
incorrect.syntaxThatIJustMadeUP()
except NameError:
print("There is a problem with your SYNTAX, Dude!")
except:
print((sys.exc_info()[0])) # In case another uncontrollable network problem occurs, User rages-Snaps IJ45
else:
print("You will never see this message unless TRY succeeds")
print("Why not put this try in a loop?")
Will the following code:
while True:
try:
print("waiting for 10 seconds...")
continue
print("never show this")
finally:
time.sleep(10)
Always print the message "waiting for 10 seconds...", sleep for 10 seconds, and do it again? In other words, do statements in finally clauses run even when the loop is continue-ed?
From the python docs:
When a return, break or continue statement is executed in the try suite of a try...finally statement, the finally clause is also executed ‘on the way out.’ A continue statement is illegal in the finally clause. (The reason is a problem with the current implementation — this restriction may be lifted in the future).
The documentation uses slightly unclear language ("on the way out") to explain how this scenario plays out. If a continue statement is executed inside of an exception clause, the code in the finally clause will be executed and then the loop will continue on to the next iteration.
Here's a very clear example that demonstrates the behavior.
Code:
i=0
while i<5:
try:
assert(i!=3) #Raises an AssertionError if i==3
print("i={0}".format(i))
except:
continue
finally:
i+= 1; #Increment i
'''
Output:
i=0
i=1
i=2
i=4
'''
Now from the latest version of python (3.8.4) , 'continue' can be used inside 'finally' blocks.enter image description here
What is the intended use of the optional else clause of the try statement?
The statements in the else block are executed if execution falls off the bottom of the try - if there was no exception. Honestly, I've never found a need.
However, Handling Exceptions notes:
The use of the else clause is better
than adding additional code to the try
clause because it avoids accidentally
catching an exception that wasn’t
raised by the code being protected by
the try ... except statement.
So, if you have a method that could, for example, throw an IOError, and you want to catch exceptions it raises, but there's something else you want to do if the first operation succeeds, and you don't want to catch an IOError from that operation, you might write something like this:
try:
operation_that_can_throw_ioerror()
except IOError:
handle_the_exception_somehow()
else:
# we don't want to catch the IOError if it's raised
another_operation_that_can_throw_ioerror()
finally:
something_we_always_need_to_do()
If you just put another_operation_that_can_throw_ioerror() after operation_that_can_throw_ioerror, the except would catch the second call's errors. And if you put it after the whole try block, it'll always be run, and not until after the finally. The else lets you make sure
the second operation's only run if there's no exception,
it's run before the finally block, and
any IOErrors it raises aren't caught here
There is one big reason to use else - style and readability. It's generally a good idea to keep code that can cause exceptions near the code that deals with them. For example, compare these:
try:
from EasyDialogs import AskPassword
# 20 other lines
getpass = AskPassword
except ImportError:
getpass = default_getpass
and
try:
from EasyDialogs import AskPassword
except ImportError:
getpass = default_getpass
else:
# 20 other lines
getpass = AskPassword
The second one is good when the except can't return early, or re-throw the exception. If possible, I would have written:
try:
from EasyDialogs import AskPassword
except ImportError:
getpass = default_getpass
return False # or throw Exception('something more descriptive')
# 20 other lines
getpass = AskPassword
Note: Answer copied from recently-posted duplicate here, hence all this "AskPassword" stuff.
Python try-else
What is the intended use of the optional else clause of the try statement?
The intended use is to have a context for more code to run if there were no exceptions where it was expected to be handled.
This context avoids accidentally handling errors you did not expect.
But it's important to understand the precise conditions that cause the else clause to run, because return, continue, and break can interrupt the control flow to else.
In Summary
The else statement runs if there are no exceptions and if not interrupted by a return, continue, or break statement.
The other answers miss that last part.
From the docs:
The optional else clause is executed if and when control flows off the
end of the try clause.*
(Bolding added.) And the footnote reads:
*Currently, control “flows off the end” except in the case of an
exception or the execution of a return, continue, or break statement.
It does require at least one preceding except clause (see the grammar). So it really isn't "try-else," it's "try-except-else(-finally)," with the else (and finally) being optional.
The Python Tutorial elaborates on the intended usage:
The try ... except statement has an optional else clause, which, when
present, must follow all except clauses. It is useful for code that
must be executed if the try clause does not raise an exception. For
example:
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except OSError:
print('cannot open', arg)
else:
print(arg, 'has', len(f.readlines()), 'lines')
f.close()
The use of the else clause is better than adding additional code to
the try clause because it avoids accidentally catching an exception
that wasn’t raised by the code being protected by the try ... except
statement.
Example differentiating else versus code following the try block
If you handle an error, the else block will not run. For example:
def handle_error():
try:
raise RuntimeError('oops!')
except RuntimeError as error:
print('handled a RuntimeError, no big deal.')
else:
print('if this prints, we had no error!') # won't print!
print('And now we have left the try block!') # will print!
And now,
>>> handle_error()
handled a RuntimeError, no big deal.
And now we have left the try block!
One use: test some code that should raise an exception.
try:
this_should_raise_TypeError()
except TypeError:
pass
except:
assert False, "Raised the wrong exception type"
else:
assert False, "Didn't raise any exception"
(This code should be abstracted into a more generic test in practice.)
Try-except-else is great for combining the EAFP pattern with duck-typing:
try:
cs = x.cleanupSet
except AttributeError:
pass
else:
for v in cs:
v.cleanup()
You might think this naïve code is fine:
try:
for v in x.cleanupSet:
v.clenaup()
except AttributeError:
pass
This is a great way of accidentally hiding severe bugs in your code. I typo-ed cleanup there, but the AttributeError that would let me know is being swallowed. Worse, what if I'd written it correctly, but the cleanup method was occasionally being passed a user type that had a misnamed attribute, causing it to silently fail half-way through and leave a file unclosed? Good luck debugging that one.
I find it really useful when you've got cleanup to do that has to be done even if there's an exception:
try:
data = something_that_can_go_wrong()
except Exception as e: # yes, I know that's a bad way to do it...
handle_exception(e)
else:
do_stuff(data)
finally:
clean_up()
Even though you can't think of a use of it right now, you can bet there has to be a use for it. Here is an unimaginative sample:
With else:
a = [1,2,3]
try:
something = a[2]
except IndexError:
print("out of bounds")
else:
print(something)
Without else:
try:
something = a[2]
except IndexError:
print("out of bounds")
if "something" in locals():
print(something)
Here you have the variable something defined if no error is thrown. You can remove this outside the try block, but then it requires some messy detection if a variable is defined.
There's a nice example of try-else in PEP 380. Basically, it comes down to doing different exception handling in different parts of the algorithm.
It's something like this:
try:
do_init_stuff()
except:
handle_init_suff_execption()
else:
try:
do_middle_stuff()
except:
handle_middle_stuff_exception()
This allows you to write the exception handling code nearer to where the exception occurs.
From Errors and Exceptions # Handling exceptions - docs.python.org
The try ... except statement has an optional else clause, which,
when present, must follow all except clauses. It is useful for code
that must be executed if the try clause does not raise an exception.
For example:
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except IOError:
print 'cannot open', arg
else:
print arg, 'has', len(f.readlines()), 'lines'
f.close()
The use of the else clause is better than adding additional code to
the try clause because it avoids accidentally catching an exception
that wasn’t raised by the code being protected by the try ... except
statement.
try:
statements # statements that can raise exceptions
except:
statements # statements that will be executed to handle exceptions
else:
statements # statements that will be executed if there is no exception
Example :
try:
age=int(input('Enter your age: '))
except:
print ('You have entered an invalid value.')
else:
if age <= 21:
print('You are not allowed to enter, you are too young.')
else:
print('Welcome, you are old enough.')
The Output :
>>>
Enter your age: a
You have entered an invalid value.
>>> RESTART
>>>
Enter your age: 25
Welcome, you are old enough.
>>> RESTART
>>>
Enter your age: 13
You are not allowed to enter, you are too young.
>>>
Copied from : https://geek-university.com/python/the-try-except-else-statements/
Looking at Python reference it seems that else is executed after try when there's no exception.
The optional else clause is executed if and when control flows off the end of the try clause. 2 Exceptions in the else clause are not handled by the preceding except clauses.
Dive into python has an example where, if I understand correctly, in try block they try to import a module, when that fails you get exception and bind default but when it works you have an option to go into else block and bind what is required (see link for the example and explanation).
If you tried to do work in catch block it might throw another exception - I guess that's where the else block comes handy.
That's it. The 'else' block of a try-except clause exists for code that runs when (and only when) the tried operation succeeds. It can be used, and it can be abused.
try:
fp= open("configuration_file", "rb")
except EnvironmentError:
confdata= '' # it's ok if the file can't be opened
else:
confdata= fp.read()
fp.close()
# your code continues here
# working with (possibly empty) confdata
Personally, I like it and use it when appropriate. It semantically groups statements.
I would add another use case that seems straight forward when handling DB sessions:
# getting a DB connection
conn = db.engine.connect()
# and binding to a DB session
session = db.get_session(bind=conn)
try:
# we build the query to DB
q = session.query(MyTable).filter(MyTable.col1 == 'query_val')
# i.e retrieve one row
data_set = q.one_or_none()
# return results
return [{'col1': data_set.col1, 'col2': data_set.col2, ...}]
except:
# here we make sure to rollback the transaction,
# handy when we update stuff into DB
session.rollback()
raise
else:
# when no errors then we can commit DB changes
session.commit()
finally:
# and finally we can close the session
session.close()
Perhaps a use might be:
#debug = []
def debuglog(text, obj=None):
" Simple little logger. "
try:
debug # does global exist?
except NameError:
pass # if not, don't even bother displaying
except:
print('Unknown cause. Debug debuglog().')
else:
# debug does exist.
# Now test if you want to log this debug message
# from caller "obj"
try:
if obj in debug:
print(text) # stdout
except TypeError:
print('The global "debug" flag should be an iterable.')
except:
print('Unknown cause. Debug debuglog().')
def myfunc():
debuglog('Made it to myfunc()', myfunc)
debug = [myfunc,]
myfunc()
Maybe this will lead you too a use.
I have found the try: ... else: construct useful in the situation where you are running database queries and logging the results of those queries to a separate database of the same flavour/type. Let's say I have lots of worker threads all handling database queries submitted to a queue
#in a long running loop
try:
query = queue.get()
conn = connect_to_db(<main db>)
curs = conn.cursor()
try:
curs.execute("<some query on user input that may fail even if sanitized">)
except DBError:
logconn = connect_to_db(<logging db>)
logcurs = logconn.cursor()
logcurs.execute("<update in DB log with record of failed query")
logcurs.close()
logconn.close()
else:
#we can't put this in main try block because an error connecting
#to the logging DB would be indistinguishable from an error in
#the mainquery
#We can't put this after the whole try: except: finally: block
#because then we don't know if the query was successful or not
logconn = connect_to_db(<logging db>)
logcurs = logconn.cursor()
logcurs.execute("<update in DB log with record of successful query")
logcurs.close()
logconn.close()
#do something in response to successful query
except DBError:
#This DBError is because of a problem with the logging database, but
#we can't let that crash the whole thread over what might be a
#temporary network glitch
finally:
curs.close()
conn.close()
#other cleanup if necessary like telling the queue the task is finished
Of course if you can distinguish between the possible exceptions that might be thrown, you don't have to use this, but if code reacting to a successful piece of code might throw the same exception as the successful piece, and you can't just let the second possible exception go, or return immediately on success (which would kill the thread in my case), then this does come in handy.
Here is another place where I like to use this pattern:
while data in items:
try
data = json.loads(data)
except ValueError as e:
log error
else:
# work on the `data`
An else block can often exist to complement functionality that occurs in every except block.
try:
test_consistency(valuable_data)
except Except1:
inconsistency_type = 1
except Except2:
inconsistency_type = 2
except:
# Something else is wrong
raise
else:
inconsistency_type = 0
"""
Process each individual inconsistency down here instead of
inside the except blocks. Use 0 to mean no inconsistency.
"""
In this case, inconsistency_type is set in each except block, so that behaviour is complemented in the no-error case in else.
Of course, I'm describing this as a pattern that may turn up in your own code someday. In this specific case, you just set inconsistency_type to 0 before the try block anyway.
One of the use scenarios I can think of is unpredictable exceptions, which can be circumvented if you try again. For instance, when the operations in try block involves random numbers:
while True:
try:
r = random.random()
some_operation_that_fails_for_specific_r(r)
except Exception:
continue
else:
break
But if the exception can be predicted, you should always choose validation beforehand over an exception. However, not everything can be predicted, so this code pattern has its place.
I have found else useful for dealing with a possibly incorrect config file:
try:
value, unit = cfg['lock'].split()
except ValueError:
msg = 'lock monitoring config must consist of two words separated by white space'
self.log('warn', msg)
else:
# get on with lock monitoring if config is ok
An exception reading the lock config disables lock monitoring and ValueErrors log a helpful warning message.
Suppose your programming logic depends on whether a dictionary has an entry with a given key. You can test the result of dict.get(key) using if... else... construct, or you can do:
try:
val = dic[key]
except KeyError:
do_some_stuff()
else:
do_some_stuff_with_val(val)
One could use this construct for handling exceptions in a common way within the finally clause, while doing something else when there's no exception:
class TooManyRetries(RuntimeError):
pass
n_tries = 0
max_retries = 2
while True:
try:
n_tries += 1
if n_tries >= max_retries:
raise TooManyRetries
fail_prone_operation()
except Exception1 as ex:
# handle1
except Exception2 as ex:
# handle2
except Exception3 as ex:
# handle3
except TooManyRetries as ex:
raise
else: # No exception
n_tries = 0
finally:
common_restore_state()
continue
I may have missed it in the dozens of answers, but I prefer to minimize the LOC in a try: block, but I like how finally: can clean up a code segment. The else: block provides a clean way to accommodate that for file I/O, database work, etc., lots of examples above.
The else: block is confusing and (nearly) useless. It's also part of the for and while statements.
Actually, even on an if-statement, the else: can be abused in truly terrible ways creating bugs that are very hard to find.
Consider this.
if a < 10:
# condition stated explicitly
elif a > 10 and b < 10:
# condition confusing but at least explicit
else:
# Exactly what is true here?
# Can be hard to reason out what condition is true
Think twice about else:. It is generally a problem. Avoid it except in an if-statement and even then consider documenting the else- condition to make it explicit.