Handling exceptions inside context managers

Question

I have some code where I try to reach a resource but sometimes it is unavailable, and results in an exception. I tried to implement a retry engine using context managers, but I can't handle the exception raised by the caller inside the __enter__ context for my context manager.

class retry(object):     def __init__(self, retries=0):         self.retries = retries         self.attempts = 0     def __enter__(self):         for _ in range(self.retries):             try:                 self.attempts += 1                 return self             except Exception as e:                 err = e     def __exit__(self, exc_type, exc_val, traceback):         print 'Attempts', self.attempts 

These are some examples which just raise an exception (which I expected to handle)

>>> with retry(retries=3): ...     print ok ...  Attempts 1 Traceback (most recent call last):   File "<stdin>", line 2, in <module> NameError: name 'ok' is not defined >>>  >>> with retry(retries=3): ...     open('/file') ...  Attempts 1 Traceback (most recent call last):   File "<stdin>", line 2, in <module> IOError: [Errno 2] No such file or directory: '/file' 

Is there any way to intercept these exception(s) and handle them inside the context manager?

Answer 1

Quoting __exit__,

If an exception is supplied, and the method wishes to suppress the exception (i.e., prevent it from being propagated), it should return a true value. Otherwise, the exception will be processed normally upon exit from this method.

By default, if you don't return a value explicitly from a function, Python will return None, which is a falsy value. In your case, __exit__ returns None and that is why the exeception is allowed to flow past the __exit__.

So, return a truthy value, like this

class retry(object):      def __init__(self, retries=0):         ...       def __enter__(self):         ...      def __exit__(self, exc_type, exc_val, traceback):         print 'Attempts', self.attempts         print exc_type, exc_val         return True                                   # or any truthy value  with retry(retries=3):     print ok 

the output will be

Attempts 1 <type 'exceptions.NameError'> name 'ok' is not defined 

---

If you want to have the retry functionality, you can implement that with a decorator, like this

def retry(retries=3):     left = {'retries': retries}      def decorator(f):         def inner(*args, **kwargs):             while left['retries']:                 try:                     return f(*args, **kwargs)                 except NameError as e:                     print e                     left['retries'] -= 1                     print "Retries Left", left['retries']             raise Exception("Retried {} times".format(retries))         return inner     return decorator   @retry(retries=3) def func():     print ok  func()