Question

I’ve found something very strange. See this short code below.

import os

class Logger(object):
    def __init__(self):
        self.pid = os.getpid()
        print "os: %s." %os

    def __del__(self):
        print "os: %s." %os

def temp_test_path():
    return "./[%d].log" %(os.getpid())

logger = Logger()

This is intended for illustrative purposes. It just prints the imported module os, on the construstion and destruction of a class (never mind the name Logger). However, when I run this, the module os seems to “disappear” to None in the class destructor. The following is the output.

os: <module 'os' from 'C:\Python27\lib\os.pyc'>.
os: None.

Where is said os: None. is my problem. It should be identical to the first output line. However, look back at the python code above, at the function temp_test_path(). If I alter the name of this function slightly, to say temp_test_pat(), and keep all of the rest of the code exactly the same, and run it, I get the expected output (below).

os: <module 'os' from 'C:\Python27\lib\os.pyc'>.
os: <module 'os' from 'C:\Python27\lib\os.pyc'>.

I can’t find any explanation for this except that it’s a bug. Can you? By the way I’m using Windows 7 64 bit.

Answer 1

If you are relying on the interpreter shutdown to call your __del__ it could very well be that the os module has already been deleted before your __del__ gets called. Try explicitly doing a del logger in your code and sleep for a bit. This should show it clearly that the code functions as you expect.

I also want to link you to this note in the official documentation that __del__ is not guaranteed to be called in the CPython implementation.