Add retry decorator and function wrapper

[taldcroft]

Description

Being able to flexibly retry certain function calls is a useful feature. I found this retry package (https://github.com/invl/retry), which is basically what we want, except:

• It appears to be abandoned and hasn't had any releases in 4 years.
• The details weren't quite right, specifically the default message for a failure.

So I copied the whole thing into ska_helpers/retry in the first commit, then added provenance and license information, then made it to my liking.

Testing

• Passes unit tests on MacOS
• Functional testing

Functional testing

from ska_helpers.retry import retry

@retry(tries=3, delay=0.5, backoff=4)
def func(a, b=1):
    print(f'hey {a} {b}')
    raise Exception('hello')

>>> func(3, b=5)                                                                                                   
hey 3 5
WARNING: func(3, b=5) exception: hello, retrying in 0.5 seconds...
hey 3 5
WARNING: func(3, b=5) exception: hello, retrying in 2.0 seconds...
hey 3 5
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
<ipython-input-2-faa7e5b4124a> in <module>
----> 1 func(3, b=5)

~/git/ska_helpers/ska_helpers/retry/api.py in wrapper(*args, **kwargs)
     77         @functools.wraps(f)
     78         def wrapper(*args, **kwargs):
---> 79             return __retry_internal(f, exceptions, tries, delay, max_delay,
     80                                     backoff, jitter, logger, args=args, kwargs=kwargs)
     81         return wrapper

~/git/ska_helpers/ska_helpers/retry/api.py in __retry_internal(f, exceptions, tries, delay, max_delay, backoff, jitter, logger, args, kwargs)
     30     while _tries:
     31         try:
---> 32             return f(*args, **kwargs)
     33         except exceptions as e:
     34             _tries -= 1

~/git/ska_helpers/go.py in func(a, b)
      4 def func(a, b=1):
      5     print(f'hey {a} {b}')
----> 6     raise Exception('hello')

Exception: hello