design patterns - Python: Try three times a function until all failed

2
votes

I am writing in Python 2.7 and encounter the following situation. I would like to try calling a function three times. If all three times raise errors, I will raise the last error I get. If any one of the calls succeed, I will quit trying and continue immediately.

Here is what I have right now:

output = None
error = None
for _e in range(3):
    error = None
    try:
        print 'trial %d!' % (_e + 1)
        output = trial_function()
    except Exception as e:
        error = e
    if error is None:
        break
if error is not None:
    raise error

Is there a better snippet that achieve the same use case?

3
pythondesign-patternserror-handling
Note that you are using return outside of a function so this code won't even run. Code that you post here should be a minimal working example, - kylieCatt
@lanAuld well then, updated question:) - Mai

3 Answers

5
votes

use decorator

from functools import wraps

def retry(times):

    def wrapper_fn(f):

        @wraps(f)
        def new_wrapper(*args,**kwargs):
            for i in range(times):
                try:
                    print 'try %s' % (i + 1)
                    return f(*args,**kwargs)
                except Exception as e:
                    error = e
            raise error

        return new_wrapper

    return wrapper_fn

@retry(3)
def foo():
    return 1/0;

print foo()
5
votes

Here is one possible approach:

def attempt(func, times=3):
    for _ in range(times):
        try:
            return func()
        except Exception as err:
            pass
    raise err

A demo with a print statement in:

>>> attempt(lambda: 1/0)
Attempt 1
Attempt 2
Attempt 3

Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    attempt(lambda: 1/0)
  File "<pyshell#17>", line 8, in attempt
    raise err
ZeroDivisionError: integer division or modulo by zero

If you're using Python 3.x and get an UnboundLocalError, you can adapt as follows:

def attempt(func, times=3):
    to_raise = None
    for _ in range(times):
        try:
            return func()
        except Exception as err:
            to_raise = err
    raise to_raise

This is because the err is cleared at the end of the try statement; per the docs:

When an exception has been assigned using as target, it is cleared at the end of the except clause.

2
votes

Ignoring the debug output and the ancient Python dialect, this looks good. The only thing I would change is to put it into a function, you could then simply return the result of trial_function(). Also, the error = None then becomes unnecessary, including the associated checks. If the loop terminates, error must have been set, so you can just throw it. If you don't want a function, consider using else in combination with the for loop and breaking after the first result.

for i in range(3):
    try:
        result = foo()
        break
    except Exception as error:
        pass
else:
    raise error
use_somehow(result)

Of course, the suggestion to use a decorator for the function still holds. You can also apply this locally, the decorator syntax is only syntactic sugar after all:

# retry from powerfj's answer below
rfoo = retry(3)(foo)
result = rfoo()