Thursday, July 19, 2012

PyFlakes-clean import of local_settings.py

Any project have own bunch of settings for different environments: for dev, production, staging. Everyone use it on daily basis. But annoying thing is that PyFlakes, great code analysis tool, warn about that. And it's reasonable.

So, to have this functionality, but without warning I use this pattern:

try:
    import local_settings
    for setting in dir(local_settings):
        if setting.startswith('_'):
            continue
        locals()[setting] = getattr(local_settings, setting)
except ImportError:
    pass

Simple and useful!


Update: I've added ignore of attributes which starting with underscore. Thanks to Igor Davydenko

Sunday, July 8, 2012

Unstable HTTP services: what we can do to easily handle that?

The story: I have several HTTP service providers which works quite unstable. Yes, I had to have in mind this during development. But we're all thought that issues are "temporary" and will gone when we going to production. We accurately added logging.error in every place and move on with other stuff.

But our expectation about temporary nature of service behavior will never happen. Service sometimes work slowly, sometimes return HTTP errors and so on. We receive tons of exceptions every day. We had to do something with that.

The solution: Here is safe_exec decorator which help solve this problem. You can specify how many times you want to try execute function, what's timeout between them and what exceptions are expected during execution decorated function. For example, urllib2.urlopen may generate urllib2.URLError or  urllib2.HTTPError.

import logging
import time

from functools import wraps

__all__ = ("safe_exec",)


def safe_exec(exceptions, shakes=3, timeout=1, title="", **kwargs):
    """
    Decorator to safely execute function or method
    within `shakes` trying.

    In case provide argument `default` exception will not
    be raised and will return provided value.
    """
    def wrap(func):
        if not isinstance(exceptions, tuple):
            raise TypeError(
                "First argument of safe_exec should be tuple of exceptions"
            )

        @wraps(func)
        def wrapped(*args, **kwargs):
            result = None
            orig_exception = None
            for shake in range(shakes):
                try:
                    result = func(*args, **kwargs)
                    break
                except exceptions, orig_exception:
                    logging.warn("%s: Sorry, can't execute %s, shake #%d",
                        title,
                        func.__name__,
                        shake,
                        exc_info=True
                    )
                    time.sleep(timeout)
            else:
                logging.error(
                    "%s: Can't execute `%s` after %d shakes",
                    title,
                    func.__name__,
                    shakes
                )

                if "default" in kwargs:
                    return kwargs.get("default")

                raise orig_exception

            return result
        return wrapped
    return wrap


Sample usage:

import urllib2

@safe_exec((urllib2.URLError, urllib2.HTTPError), shakes=2)
def download(url):
    return urllib2.urlopen(url).read()

download("http://slow-resource.com/")