Friday, June 20, 2014

A note on Python's __exit__() and errors

Python's context managers are a very neat way of handling code that needs a teardown once you are done. Python objects have do have a destructor method (__del__) called right before the last instance of the object is about to be destroyed. You can do a teardown there. However there is a lot of fine print to the __del__ method. A cleaner way of doing tear-downs is through Python's context manager, manifested as the with keyword.

class CrushMe:
  def __init__(self):
    self.f = open('test.txt', 'w')

  def foo(self, a, b):
    self.f.write(str(a - b))

  def __enter__(self):
    return self

  def __exit__(self, exc_type, exc_val, exc_tb):
    self.f.close()
    return True

with CrushMe() as c:
  c.foo(2, 3)

One thing that is important, and that got me just now, is error handling. I made the mistake of ignoring all those 'junk' arguments (exc_type, exc_val, exc_tb). I just skimmed the docs and what popped out is that you need to return True or False depending on whether there was an error. So I wrote my code to return False if there was a problem in saving the file (My actual code is a little more involved, but the same in spirit) and True otherwise.

So what happens now if you do
with CrushMe() as c:
  c.foo('2', '3')

Of course, YOU know that this code will error out - you can't subtract strings. But if you run this code, it will FAIL SILENTLY. This is because I was careless and did not consider what happens if there is an error SOMEWHERE ELSE.

The proper way to do this, as a minimum, is to change the code to

def __exit__(self, exc_type, exc_val, exc_tb):
    self.f.close()
    return True if exc_type is None else False

Another note: defining both a __del__ method and an __exit__ method can lead to tricky situations. The following code, for, instance, calls the close method twice.

class CrushMe:
  def __init__(self):
    self.f = open('test.txt', 'w')

  def foo(self, a, b):
    self.f.write(str(a - b))

  def __del__(self):
    self.close()

  def __enter__(self):
    return self

  def __exit__(self, exc_type, exc_val, exc_tb):
    self.close()
    return True if exc_type is None else False

  def close(self):
    print 'Called!'
    self.f.close()

with CrushMe() as c:
  c.foo(2, 3)

close gets called first by __exit__ when we exit the context and then when we exit the interpreter and the object is deleted.


Sunday, June 1, 2014

Feedburner and the fragmentation of google

Wordpress has a plugin for sending your post to twitter or linkedin (and linkedin has a builtin way of sending your updates to twitter) but I didn't see anything like that for blogger (except a G+ button for publicizing to that thriving community). Whenever I did a search I came up with some third party service. Finally I ran into Feedburner, which turns out to be Google's own service for publicizing things on social media. This just led me to an observation that google has so many services and they CAN talk to each other, but there are complicated layers between them.