Decorator preserve signature

[ankitpatel96]

Hi,
We've been using your caching framework for some time now and we've discovered an issue where the decorators @cache_on_arguments (and @cache_multi_on_arguments) don't preserve the signatures of the decorated functions. An example:

In [1]: from dogpile.cache import make_region

In [2]: region = make_region().configure(
   ...:     'dogpile.cache.memory'
   ...: )

In [3]: import inspect

In [4]: def func(a, b, c=True, *args, **kwargs):
   ...:     return None

In [5]: inspect.getargspec(func)
Out[5]: ArgSpec(args=['a', 'b', 'c'], varargs='args', keywords='kwargs', defaults=(True,))

In [6]: cached_func = region.cache_on_arguments()(func)

In [7]: inspect.getargspec(cached_func)
Out[7]: ArgSpec(args=[], varargs='arg', keywords='kw', defaults=None)

To remedy this, I'm submitting a PR that uses the decorator module to preserve the signature of functions decorated by cache_on_arguments.

I've also added tests to check this behavior.

I wasn't sure where to put a new dependency, so I put it in setup.py under install_requires. Let me know if there's a better place for it

Thanks!

[zzzeek]

would rather remove the <5 here, I wouldn't worry that some major breaking change is coming in decorator 5

[zzzeek]

issue created at #137

[zzzeek]

Dear contributor -

This pull request is being moved to Gerrit, at https://gerrit.sqlalchemy.org/#/c/sqlalchemy/dogpile.cache/+/996, where it may be tested and reviewed more closely. As such, the pull request itself is being marked "closed" or "declined", however your contribution is merely being moved to our central review system. Please register at https://gerrit.sqlalchemy.org#/register/ to send and receive comments regarding this item.