Metadata-Version: 2.1 Name: backoff Version: 1.10.0 Summary: Function decoration for backoff and retry Home-page: https://github.com/litl/backoff License: MIT Keywords: retry,backoff,decorators Author: Bob Green Author-email: rgreen@goscoutgo.com Requires-Python: >=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.* Classifier: Development Status :: 5 - Production/Stable Classifier: Intended Audience :: Developers Classifier: License :: OSI Approved :: MIT License Classifier: Natural Language :: English Classifier: Operating System :: OS Independent Classifier: Programming Language :: Python Classifier: Programming Language :: Python :: 2 Classifier: Programming Language :: Python :: 2.7 Classifier: Programming Language :: Python :: 3 Classifier: Programming Language :: Python :: 3.5 Classifier: Programming Language :: Python :: 3.6 Classifier: Programming Language :: Python :: 3.7 Classifier: Programming Language :: Python :: 3.8 Classifier: Topic :: Internet :: WWW/HTTP Classifier: Topic :: Software Development :: Libraries :: Python Modules Classifier: Topic :: Utilities Project-URL: Repository, https://github.com/litl/backoff Description-Content-Type: text/x-rst backoff ======= .. image:: https://travis-ci.org/litl/backoff.svg?branch=master :target: https://travis-ci.org/litl/backoff?branch=master .. image:: https://coveralls.io/repos/litl/backoff/badge.svg?branch=master :target: https://coveralls.io/r/litl/backoff?branch=master .. image:: https://img.shields.io/pypi/v/backoff.svg :target: https://pypi.python.org/pypi/backoff **Function decoration for backoff and retry** This module provides function decorators which can be used to wrap a function such that it will be retried until some condition is met. It is meant to be of use when accessing unreliable resources with the potential for intermittent failures i.e. network resources and external APIs. Somewhat more generally, it may also be of use for dynamically polling resources for externally generated content. Decorators support both regular functions for synchronous code and `asyncio `_'s coroutines for asynchronous code. Examples ======== Since Kenneth Reitz's `requests `_ module has become a defacto standard for synchronous HTTP clients in Python, networking examples below are written using it, but it is in no way required by the backoff module. @backoff.on_exception --------------------- The ``on_exception`` decorator is used to retry when a specified exception is raised. Here's an example using exponential backoff when any ``requests`` exception is raised: .. code-block:: python @backoff.on_exception(backoff.expo, requests.exceptions.RequestException) def get_url(url): return requests.get(url) The decorator will also accept a tuple of exceptions for cases where the same backoff behavior is desired for more than one exception type: .. code-block:: python @backoff.on_exception(backoff.expo, (requests.exceptions.Timeout, requests.exceptions.ConnectionError)) def get_url(url): return requests.get(url) **Give Up Conditions** Optional keyword arguments can specify conditions under which to give up. The keyword argument ``max_time`` specifies the maximum amount of total time in seconds that can elapse before giving up. .. code-block:: python @backoff.on_exception(backoff.expo, requests.exceptions.RequestException, max_time=60) def get_url(url): return requests.get(url) Keyword argument ``max_tries`` specifies the maximum number of calls to make to the target function before giving up. .. code-block:: python @backoff.on_exception(backoff.expo, requests.exceptions.RequestException, max_tries=8, jitter=None) def get_url(url): return requests.get(url) In some cases the raised exception instance itself may need to be inspected in order to determine if it is a retryable condition. The ``giveup`` keyword arg can be used to specify a function which accepts the exception and returns a truthy value if the exception should not be retried: .. code-block:: python def fatal_code(e): return 400 <= e.response.status_code < 500 @backoff.on_exception(backoff.expo, requests.exceptions.RequestException, max_time=300, giveup=fatal_code) def get_url(url): return requests.get(url) When a give up event occurs, the exception in question is reraised and so code calling an `on_exception`-decorated function may still need to do exception handling. @backoff.on_predicate --------------------- The ``on_predicate`` decorator is used to retry when a particular condition is true of the return value of the target function. This may be useful when polling a resource for externally generated content. Here's an example which uses a fibonacci sequence backoff when the return value of the target function is the empty list: .. code-block:: python @backoff.on_predicate(backoff.fibo, lambda x: x == [], max_value=13) def poll_for_messages(queue): return queue.get() Extra keyword arguments are passed when initializing the wait generator, so the ``max_value`` param above is passed as a keyword arg when initializing the fibo generator. When not specified, the predicate param defaults to the falsey test, so the above can more concisely be written: .. code-block:: python @backoff.on_predicate(backoff.fibo, max_value=13) def poll_for_message(queue) return queue.get() More simply, a function which continues polling every second until it gets a non-falsey result could be defined like like this: .. code-block:: python @backoff.on_predicate(backoff.constant, interval=1) def poll_for_message(queue) return queue.get() Jitter ------ A jitter algorithm can be supplied with the ``jitter`` keyword arg to either of the backoff decorators. This argument should be a function accepting the original unadulterated backoff value and returning it's jittered counterpart. As of version 1.2, the default jitter function ``backoff.full_jitter`` implements the 'Full Jitter' algorithm as defined in the AWS Architecture Blog's `Exponential Backoff And Jitter `_ post. Note that with this algorithm, the time yielded by the wait generator is actually the *maximum* amount of time to wait. Previous versions of backoff defaulted to adding some random number of milliseconds (up to 1s) to the raw sleep value. If desired, this behavior is now available as ``backoff.random_jitter``. Using multiple decorators ------------------------- The backoff decorators may also be combined to specify different backoff behavior for different cases: .. code-block:: python @backoff.on_predicate(backoff.fibo, max_value=13) @backoff.on_exception(backoff.expo, requests.exceptions.HTTPError, max_time=60) @backoff.on_exception(backoff.expo, requests.exceptions.Timeout, max_time=300) def poll_for_message(queue): return queue.get() Runtime Configuration --------------------- The decorator functions ``on_exception`` and ``on_predicate`` are generally evaluated at import time. This is fine when the keyword args are passed as constant values, but suppose we want to consult a dictionary with configuration options that only become available at runtime. The relevant values are not available at import time. Instead, decorator functions can be passed callables which are evaluated at runtime to obtain the value: .. code-block:: python def lookup_max_time(): # pretend we have a global reference to 'app' here # and that it has a dictionary-like 'config' property return app.config["BACKOFF_MAX_TIME"] @backoff.on_exception(backoff.expo, ValueError, max_time=lookup_max_time) Event handlers -------------- Both backoff decorators optionally accept event handler functions using the keyword arguments ``on_success``, ``on_backoff``, and ``on_giveup``. This may be useful in reporting statistics or performing other custom logging. Handlers must be callables with a unary signature accepting a dict argument. This dict contains the details of the invocation. Valid keys include: * *target*: reference to the function or method being invoked * *args*: positional arguments to func * *kwargs*: keyword arguments to func * *tries*: number of invocation tries so far * *elapsed*: elapsed time in seconds so far * *wait*: seconds to wait (``on_backoff`` handler only) * *value*: value triggering backoff (``on_predicate`` decorator only) A handler which prints the details of the backoff event could be implemented like so: .. code-block:: python def backoff_hdlr(details): print ("Backing off {wait:0.1f} seconds afters {tries} tries " "calling function {target} with args {args} and kwargs " "{kwargs}".format(**details)) @backoff.on_exception(backoff.expo, requests.exceptions.RequestException, on_backoff=backoff_hdlr) def get_url(url): return requests.get(url) **Multiple handlers per event type** In all cases, iterables of handler functions are also accepted, which are called in turn. For example, you might provide a simple list of handler functions as the value of the ``on_backoff`` keyword arg: .. code-block:: python @backoff.on_exception(backoff.expo, requests.exceptions.RequestException, on_backoff=[backoff_hdlr1, backoff_hdlr2]) def get_url(url): return requests.get(url) **Getting exception info** In the case of the ``on_exception`` decorator, all ``on_backoff`` and ``on_giveup`` handlers are called from within the except block for the exception being handled. Therefore exception info is available to the handler functions via the python standard library, specifically ``sys.exc_info()`` or the ``traceback`` module. Asynchronous code ----------------- Backoff supports asynchronous execution in Python 3.5 and above. To use backoff in asynchronous code based on `asyncio `_ you simply need to apply ``backoff.on_exception`` or ``backoff.on_predicate`` to coroutines. You can also use coroutines for the ``on_success``, ``on_backoff``, and ``on_giveup`` event handlers, with the interface otherwise being identical. The following examples use `aiohttp `_ asynchronous HTTP client/server library. .. code-block:: python @backoff.on_exception(backoff.expo, aiohttp.ClientError, max_time=60) async def get_url(url): async with aiohttp.ClientSession() as session: async with session.get(url) as response: return await response.text() Logging configuration --------------------- By default, backoff and retry attempts are logged to the 'backoff' logger. By default, this logger is configured with a NullHandler, so there will be nothing output unless you configure a handler. Programmatically, this might be accomplished with something as simple as: .. code-block:: python logging.getLogger('backoff').addHandler(logging.StreamHandler()) The default logging level is INFO, which corresponds to logging anytime a retry event occurs. If you would instead like to log only when a giveup event occurs, set the logger level to ERROR. .. code-block:: python logging.getLogger('backoff').setLevel(logging.ERROR) It is also possible to specify an alternate logger with the ``logger`` keyword argument. If a string value is specified the logger will be looked up by name. .. code-block:: python @backoff.on_exception(backoff.expo, requests.exception.RequestException, logger='my_logger') # ... It is also supported to specify a Logger (or LoggerAdapter) object directly. .. code-block:: python my_logger = logging.getLogger('my_logger') my_handler = logging.StreamHandler() my_logger.add_handler(my_handler) my_logger.setLevel(logging.ERROR) @backoff.on_exception(backoff.expo, requests.exception.RequestException, logger=my_logger) # ... Default logging can be disabled all together by specifying ``logger=None``. In this case, if desired alternative logging behavior could be defined by using custom event handlers.