Closing a file in Class exit or del method?

Question

I want to write a class that is able to write an html file. I now have the following skeleton:

class ColorWheel(object): def __init__(self, params): self.params = params def __enter__(self): self.f = open('color_wheel.html', 'w') self._write_header() return self def __exit__(self, type_unused, value_unused, traceback_unused): self._write_footer() self.f.close() def wheel(self): # Code here to write the body of the html file self.f.write('BODY HERE')

I use this class with:

with ColorWheel(params) as cw: cw.wheel()

The file is exactly written as I expect it to be. However, when I run this, I get the following error:

Exception ValueError: 'I/O operation on closed file' in <bound method ColorWheel.__del__ of ColorWheel.ColorWheel object at 0x0456A330>> ignored

I assume it is trying to close the file while it has already been closed. Is this correct? If so, what would be the proper way to close the file?

i seen this error when open a file with and then dont write the other part of the code inside with — Kasravnd
– Kasravnd, Commented Aug 31, 2014 at 9:39

Martijn Pieters · Accepted Answer · 2014-08-31 09:35:44Z

7

You also have a __del__ method trying to write to the file after closing it. When cw goes out of scope and is cleaned up, the __del__ hook is called and you appear to try and write to the file at that point.

You can test if a file is closed already with:

if not self.f.closed: # do something with file

answered Aug 31, 2014 at 9:35

Martijn Pieters

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6 Comments

Alessandro Suglia Over a year ago

The answer is correct. I want to add a reference to the official documentation that you should read in order to understand the with statement: docs.python.org/3.4/reference/…

physicalattraction Over a year ago

I thought I did not have a del method, but it turns out I do, since I inherited from another class. I didn't show that above, I thought it would be irrelevant. It all makes sense now.

Sylvain Leroux Over a year ago

Is it guaranteed that __del__ will be called as soon as the object goes out of scope ? Or is it implementation dependent ?

Martijn Pieters Over a year ago

@Sylvain: that's dependent on the implementation. CPython uses reference counting and there the __del__ is called instantly. But IronPython and Jython use a garbage collection scheme and the call will be delayed.

Sylvain Leroux Over a year ago

@Martijn Concerning Jython: as the JVM does not guaranteed finalize() to even be called on program termination, does that imply __del__ might not be even be called too? I can't find a clear answer for recent version of Jython by googling around...

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