MemoryError in Python leads to empty pickle file

Question

I'm working on a library-type checkout/checkin system. When the user clicks exit , the program calls a close_window function which dumps the current dictionary objects into pickle files before the window is destroyed.

def close_window(self):
    if messagebox.askokcancel("Quit", "You want to close the program now?"):
        patrons.dump_data()

        self.master.destroy()

When the program is started again, it calls a load_data function which loads the pickled files. Somehow I ran into MemoryError when exiting the system and one of the pickled files was overwritten with an empty file. From the documentation , I gather that MemoryError occurs when the program creates too many objects and runs out of memory. I am not sure why this happened in my case since I am not dealing with large data. The pickled file that got overwritten was only 1 KB.

How can I ensure my pickled file is not overwritten with an empty file when MemoryError occurs? This can lead to serious data loss. I am new to programming and am using this project to learn. It is possible I've done something seriously wrong to lead to the memory error or maybe I just need more computer memory. In any case, it doesn't make sense to overwrite a saved file with an empty file whether memory error or not.

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Python34\lib\tkinter\__init__.py", line 1487, in __call__
    return self.func(*args)
  File "C:/Python34/Lib/site-packages/toolkit/main.py", line 435, in close_window
    patrons.dump_data()
  File "C:\Python34\Lib\site-packages\toolkit\patrons.py", line 20, in dump_data
    pickle.dump(patronslist, f)
MemoryError

This error is partially discussed in MemoryError while pickling data in python . Here though, I got an empty file, not even a partial file. And I want to know if there is a workaround for this problem. Perhaps saving the pickled data to a temporary file. If no memory error occurred during the save, then the temp file can be used to overwrite the permanent file (but this may yet trigger another MemoryError right?).

I run Win7 x86, 3 GB RAM, Python 3.4.1

Answer 1

Based on Gerrat's comment above, I wonder if the following is a good way to go about this:

patrons.py
def dump_data():
    with open("./pickled_dicts/temp_patrons.pkl", 'wb') as f:
        global patronslist
        pickle.dump(patronslist, f)


main.py
def close_window(self):
    if messagebox.askokcancel("Quit", "You want to close the program now?"):
        try:
            patrons.dump_data()
            os.remove("./pickled_dicts/patrons.pkl")
            os.rename("./pickled_dicts/temp_patrons.pkl", "./pickled_dicts/patrons.pkl")
        except MemoryError:
            messagebox.showerror("Memory Problem", "Your computer experienced memory problem. Your last session was not saved.")     
        self.master.destroy()

Essentially, I am first saving the dictionary object to a temporary file ( temp_patrons.pkl ) which is renamed to my permanent file ( patrons.pkl ) assuming no MemoryError . If MemoryError , then the original patrons.pkl remains.