[Python] Why am I getting "NameError: global name 'open' is not defined" in __del__?

I'm getting a NameError in the __del__ function of a class. I do not understand why 'open' is not accessible inside the function __del__. I am using Python 3.4.0

Python Code:

class Contoller:

...

def __del__(self):
store = {}
...
pickle.dump(store, open('data.p', 'wb'))

class MyWindow(Gtk.Window):

def __init__(self):
...
self.controller = Contoller(self)
...
self.connect("delete-event", self.quit)
...

...

def quit(self, widget, event):
del self.controller
Gtk.main_quit()


Error Message:

Traceback (most recent call last):
File "main.py", line 69, in __del__
NameError: name 'open' is not defined

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