how to achieve - file write open on __del__?

Question

I m trying to do a some activity on class obj destruction. How do I achieve file open in __del__ function? (I m using Python 3.4)

class iam(object):

    def __init__(self):
        print("I m born")

    def __del__(self):
        f = open("memory_report.txt", "w")
        f.write("He gone safe")
        f.close()
    
if __name__ == '__main__':
    i = iam()
    print("Script Ends. Now to GC clean memory")


Output:

---

I m born
Script Ends. Now to GC clean memory
Exception ignored in: <bound method iam.__del__ of <__main__.iam object at 0x00000000022F1A58>>
Traceback (most recent call last):
  File "F:\Kumaresan\Code\Python\CommonLib\src\kmxPyQt\devConsole3\tet.py", line 14, in __del__
NameError: name 'open' is not defined    

Suksukarno

Why do you want to do that? Using __del__ at all is risky. Using it to do something like open a file is a sure road to trouble. – Tightlipped

@Jamie Bull: It might work for you but you can't rely on __del__ being called. – Galibi

You can't rely on other resources being available when __del__ is called if I'm reading the docs correctly. As in OP's case where __del__ is being called but can't find open. – Olcott

I m using Python 3.4 – Suksukarno

related: https://mcmap.net/q/294443/-how-to-force-deletion-of-a-python-object/674039 – Evolve

Answer 1

Below code is work fine.

class iam(object):

def __init__(self):
    print("I m born")

def __del__(self):
    #"open" function still in __builtins__ 
    f = open("memory_report.txt", "w")
    f.write("He gone safe")
    f.close()

def write_iam():
        i=iam()

if __name__ == '__main__':
    write_iam()
    print("Script Ends. Now to GC clean memory")

In this case:

class iam(object):

def __init__(self):
    print("I m born")

def __del__(self):
    #__builtins__.open  has remove 
    f = open("memory_report.txt", "w")
    f.write("He gone safe")
    f.close()

if __name__ == '__main__':
    i = iam()
    print("Script Ends. Now to GC clean memory")

When exit the __main__ function, before GC delete the "i" instance (execute i.__delete__) "open" function has remove from __builtins__.

Overshine

Answer 2

The problem is, as MuSheng tried to explain, that the __builtins__ are removed before your __del__ is called.

You can trigger the __del__ yourself by assigning None to the variable.

MuSheng's code could be this:

class iam():
    def __init__(self):
        print("I m born")

    def __del__(self):
        #"open" function still in __builtins__ 
        with open("memory_report.txt", "w") as f:
            f.write("He gone safe")

if __name__ == '__main__':
    i = iam()
    i = None # This triggers `__del__`
    print("Script Ends. Now to GC clean memory")

MuSheng deserves some upvotes, IMO

Insincerity

Answer 3

As others have mentioned, don't use the ____del___ method to perform such cleanup. Instead, use either contextmanagers (with-statement) or register atexit-handlers.

Mestee

atexit handlers helped me – Suksukarno

Answer 4

Below is an alternate I used - Using atexit handlers:

import atexit


class iam(object):

    def __init__(self):
        print("I m born")
        atexit.register(self.cleanup)

    def cleanup(self):
        f = open("memory_report.txt", "w")
        f.write("He gone safe")
        f.close()
        print ("Done")


if __name__ == '__main__':
    i = iam()
    print("Script Ends. Now to GC clean memory")

Output:

I m born
Script Ends. Now to GC clean memory
Done

Suksukarno