如果抛出异常，如何再次调用try块中的代码？

Question

I am new to python and I am writing code that uses OAuth to authenticate and when the token expires after 60 minutes, it needs to get a new one.

try:
    if uploadedContent is not None:
       thing.action(uploadedContent)

except LoginOrScopeRequired:
    print("Logging in...")
    set_access_credentials({"identity", "submit"}, get_access_token())

I currently have this code to handle getting a new token if it expires, but the problem is that if there was an exception it skips over the action it needed to do. I understand that I could take what was inside the try block and append it to end of the except block, but it there a more elegant way to do this?

Some of my research led to the with statement, but I didn't understand with well enough to know if it would solve my problem. So is appending it to the end the best solution or is there something better?

Answer 1

It is considered Idiomatic Python to do this with a function decorator/wrapper:

Example:

#!/usr/bin/env python


from functools import wraps


def retry_on_error(ntries=1):
    """
    A decorator that returns a wrapper function that calls
    the wrapped function repeatedly up to ntries if an
    exception is encountered.
    """

    def decorator(f):  # wrapping the original function
        @wraps(f)  # make the wrapped function look like the original
        def wrapper(*args, **kwargs):  # our function wrapped that calls the original
            for i in xrange(ntries):
                try:
                    return f(*args, **kwargs)
                except Exception as e:
                    print("Error executing {0:s} retrying {1:d}/{2:d}".format(f.__name__, i, ntries))
                    print("Error was {0:s}".format(e))

        return wrapper

    return decorator  # returning the new wrapped function


@retry_on_error()
def f():
    n = getattr(f, "n", 0)
    try:
        if not n:
            raise ValueError("n < 0")
    finally:
        setattr(f, "n", n + 1)

Output:

$ python -i foo.py
>>> f()
Error executing f retrying 0/1
Error was n < 0
>>> f()
>>> 

See: Python Decorators for other examples.

Update: There is also a nice library that implements this functionality with a few more features: retrying as well as several other related/similar questions How to retry after exception in python? and Pythonic way of retry running a function

Update #2: I've commented the decorator a bit so hopefully you can understand what's going on in each step of the process. Admittedly decorators aren't that easy to understand at first so I recommend you read Understanding Python Decorators in 12 easy step

Answer 2

Some languages like ruby let you put a retry statement in the exception catch block that makes this incredibly simple. Unfortunately in Python you will need to wrap this in a while statement:

success = False
while not success
    try:
        if uploadedContent is not None:
           thing.action(uploadedContent)
        success = True
    except LoginOrScopeRequired:
        print("Logging in...")
        set_access_credentials({"identity", "submit"}, get_access_token())

Note that the line success = True will only be reached if no exception occurs.

EDIT

You will also want to keep track of the number of attempts in a counter to ensure this doesn't loop forever and exits after 3 retries for example.