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

0 votes
asked Feb 19, 2009 by ben

I have a python script which is querying a MySQL server on a shared linux host. For some reason, queries to MySQL often return a "server has gone away" error:

_mysql_exceptions.OperationalError: (2006, 'MySQL server has gone away')

If you try the query again immediately afterwards, it usually succeeds. So, I'd like to know if there's a sensible way in python to try to execute a query, and if it fails, to try again, up to a fixed number of tries. Probably I'd want it to try 5 times before giving up altogether.

Here's the kind of code I have:

conn = MySQLdb.connect(host, user, password, database)
cursor = conn.cursor()

try:
    cursor.execute(query)
    rows = cursor.fetchall()
    for row in rows:
        # do something with the data
except MySQLdb.Error, e:
    print "MySQL Error %d: %s" % (e.args[0], e.args[1])

Clearly I could do it by having another attempt in the except clause, but that's incredibly ugly, and I have a feeling there must be a decent way to achieve this.

9 Answers

0 votes
answered Jan 19, 2009 by cdleary

I'd refactor it like so:

def callee(cursor):
    cursor.execute(query)
    rows = cursor.fetchall()
    for row in rows:
        # do something with the data

def caller(attempt_count=3, wait_interval=20):
    """:param wait_interval: In seconds."""
    conn = MySQLdb.connect(host, user, password, database)
    cursor = conn.cursor()
    for attempt_number in range(attempt_count):
        try:
            callee(cursor)
        except MySQLdb.Error, e:
            logging.warn("MySQL Error %d: %s", e.args[0], e.args[1])
            time.sleep(wait_interval)
        else:
            break

Factoring out the callee function seems to break up the functionality so that it's easy to see the business logic without getting bogged down in the retry code.

0 votes
answered Jan 19, 2009 by dwc

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
0 votes
answered Jan 19, 2009 by webjunkie
conn = MySQLdb.connect(host, user, password, database)
cursor = conn.cursor()

for i in range(3):
    try:
        cursor.execute(query)
        rows = cursor.fetchall()
        for row in rows:
            # do something with the data
        break
    except MySQLdb.Error, e:
        print "MySQL Error %d: %s" % (e.args[0], e.args[1])
0 votes
answered Jan 22, 2009 by kiv

Like S.Lott, I like a flag to check if we're done:

conn = MySQLdb.connect(host, user, password, database)
cursor = conn.cursor()

success = False
attempts = 0

while attempts < 3 and not success:
    try:
        cursor.execute(query)
        rows = cursor.fetchall()
        for row in rows:
            # do something with the data
        success = True 
    except MySQLdb.Error, e:
        print "MySQL Error %d: %s" % (e.args[0], e.args[1])
        attempts += 1
0 votes
answered Feb 19, 2009 by dana

How about:

conn = MySQLdb.connect(host, user, password, database)
cursor = conn.cursor()
attempts = 0

while attempts < 3:
    try:
        cursor.execute(query)
        rows = cursor.fetchall()
        for row in rows:
            # do something with the data
        break
    except MySQLdb.Error, e:
        attempts += 1
        print "MySQL Error %d: %s" % (e.args[0], e.args[1])
0 votes
answered Jan 11, 2013 by user1970198

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.

0 votes
answered Jan 4, 2015 by user5637641

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.

0 votes
answered Jan 23, 2015 by elias

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"
0 votes
answered Jan 27, 2015 by peter-wood
def successful_transaction(transaction):
    try:
        transaction()
        return True
    except SQL...:
        return False

succeeded = any(successful_transaction(transaction)
                for transaction in repeat(transaction, 3))
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