is there a pythonic way to try something up to a maximum number of times? [duplicate]

Answer 1

Building on Dana's answer, you might want to do this as a decorator:

def retry(howmany):
    def tryIt(func):
        def f():
            attempts = 0
            while attempts < howmany:
                try:
                    return func()
                except:
                    attempts += 1
        return f
    return tryIt

Then...

@retry(5)
def the_db_func():
    # [...]

Enhanced version that uses the decorator module

import decorator, time

def retry(howmany, *exception_types, **kwargs):
    timeout = kwargs.get('timeout', 0.0) # seconds
    @decorator.decorator
    def tryIt(func, *fargs, **fkwargs):
        for _ in xrange(howmany):
            try: return func(*fargs, **fkwargs)
            except exception_types or Exception:
                if timeout is not None: time.sleep(timeout)
    return tryIt

Then...

@retry(5, MySQLdb.Error, timeout=0.5)
def the_db_func():
    # [...]

To install the decorator module:

$ easy_install decorator

Answer 2

This is my generic solution:

class TryTimes(object):
    ''' A context-managed coroutine that returns True until a number of tries have been reached. '''

    def __init__(self, times):
        ''' times: Number of retries before failing. '''
        self.times = times
        self.count = 0

    def __next__(self):
        ''' A generator expression that counts up to times. '''
        while self.count < self.times:
            self.count += 1
        yield False

    def __call__(self, *args, **kwargs):
        ''' This allows "o() calls for "o = TryTimes(3)". '''
        return self.__next__().next()

    def __enter__(self):
        ''' Context manager entry, bound to t in "with TryTimes(3) as t" '''
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        ''' Context manager exit. '''
        return False # don't suppress exception

This allows code like the following:

with TryTimes(3) as t:
    while t():
        print "Your code to try several times"

Also possible:

t = TryTimes(3)
while t():
    print "Your code to try several times"

This can be improved by handling exceptions in a more intuitive way, I hope. Open to suggestions.

Answer 3

1.Definition:

def try_three_times(express):
    att = 0
    while att < 3:
        try: return express()
        except: att += 1
    else: return u"FAILED"

2.Usage:

try_three_times(lambda: do_some_function_or_express())

I use it for parse html context.

Answer 4

UPDATE: there is a better maintained fork of the retrying library called tenacity, which supports more features and is in general more flexible.

---

Yes, there is the retrying library, which has a decorator that implements several kinds of retrying logic that you can combine:

Some examples:

@retry(stop_max_attempt_number=7)
def stop_after_7_attempts():
    print "Stopping after 7 attempts"

@retry(wait_fixed=2000)
def wait_2_s():
    print "Wait 2 second between retries"

@retry(wait_exponential_multiplier=1000, wait_exponential_max=10000)
def wait_exponential_1000():
    print "Wait 2^x * 1000 milliseconds between each retry,"
    print "up to 10 seconds, then 10 seconds afterwards"