Can I pass an exception as an argument to a function in python?

Question

I am new to python. I am trying to create a retry decorator that, when applied to a function, will keep retrying until some criteria is met (for simplicity, say retry 10 times).

def retry():
    def wrapper(func):
        for i in range(0,10):
            try:
                func()
                break
            except:
               continue
    return wrapper

Now that will retry on any exception. How can I change it such that it retries on specific exceptions. e.g, I want to use it like:

@retry(ValueError, AbcError)
def myfunc():
    //do something

I want myfunc to be retried only of it throws ValueError or AbcError.

Added a break now. Thanks!. Edit: I think it was wrong logic for retry. I just need the for loop I think — Hari Menon
– Hari Menon, Commented Aug 7, 2013 at 11:17

Martijn Pieters · Accepted Answer · 2013-08-07 11:53:08Z

11

You can supply a tuple of exceptions to the except .. block to catch:

from functools import wraps

def retry(*exceptions, **params):
    if not exceptions:
        exceptions = (Exception,)
    tries = params.get('tries', 10)

    def decorator(func):
        @wraps(func)
        def wrapper(*args, **kw):
            for i in range(tries):
                try:
                    return func(*args, **kw)
                except exceptions:
                    pass
        return wrapper
    return decorator

The catch-all *exceptions parameter will always result in a tuple. I've added a tries keyword as well, so you can configure the number of retries too:

@retry(ValueError, TypeError, tries=20)
def foo():
    pass

Demo:

>>> @retry(NameError, tries=3)
... def foo():
...     print 'Futzing the foo!'
...     bar
... 
>>> foo()
Futzing the foo!
Futzing the foo!
Futzing the foo!

edited Aug 7, 2013 at 11:53

answered Aug 7, 2013 at 11:18

Martijn Pieters

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5 Comments

Duncan Over a year ago

Actually you can catch a "variable exception or a tuple of things you can catch", but that tends to make my head hurt if I think about it too long.

Martijn Pieters Over a year ago

@glglgl: it needed a little more than just return wrapper to make this a proper decorator factory.

glglgl Over a year ago

I wonder if it would be useful to even parametrize the 10 with def retry(*exceptions, **config): and for i in range(0, config.get('retries', 10)):. It was not requested by the OP, but it is maybe quite useful...

Hari Menon Over a year ago

Related question - What if I want to catch and rethrow the exception if it is still failing after 10 tries?

Martijn Pieters Over a year ago

@Raze2dust: Call raise at the end of the wrapper. It'll raise the last caught exception; you don't have to do that in the except clause.

Zaur Nasibov · Accepted Answer · 2013-08-07 11:40:52Z

2

from functools import wraps

class retry(object):
    def __init__(self, *exceptions):
        self.exceptions = exceptions

    def __call__(self, f):
        @wraps(f) # required to save the original context of the wrapped function
        def wrapped(*args, **kwargs):
            for i in range(0,10):
                try:
                    f(*args, **kwargs)
                except self.exceptions:
                    continue
        return wrapped

Usage:

@retry(ValueError, Exception)
def f():
    print('In f')
    raise ValueError


>>> f()
In f
In f
In f
In f
In f
In f
In f
In f
In f
In f

edited Aug 7, 2013 at 11:40

answered Aug 7, 2013 at 11:22

Zaur Nasibov

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Comments

Paolo Casciello · Accepted Answer · 2013-08-07 11:18:35Z

1

You can check the error class:

except Exception as e:
    for et in error_types: #(or args)
        if isinstance(e, et):
            continue
    raise e #re-raise

answered Aug 7, 2013 at 11:18

Paolo Casciello

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1 Comment

Zaur Nasibov Over a year ago

You can except an iterable object (e.g. tuple, list) with exceptions, so no need to iterate later.

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