Question

There’s a file that I would like to make sure does not grow larger than 2 GB (as it must run on a system that uses ext 2). What’s a good way to check a file’s size bearing in mind that I will be writing to this file in between checks? In particular, do I need to worry about buffered, unflushed changes that haven’t been written to disk yet?

Answer 1

You could start with something like this:

class TrackedFile(file):
    def __init__(self, filename, mode):
        self.size = 0
        super(TrackedFile, self).__init__(filename, mode)
    def write(self, s):
        self.size += len(s)
        super(TrackedFile, self).write(s)

Then you could use it like this:

>>> f = TrackedFile('palindrome.txt', 'w')
>>> f.size
0
>>> f.write('A man a plan a canal ')
>>> f.size
21
>>> f.write('Panama')
27

Obviously, this implementation doesn’t work if you aren’t writing the file from scratch, but you could adapt your __init__ method to handle initial data. You might also need to override some other methods: writelines, for instance.

This works regardless of encoding, as strings are just sequences of bytes.

>>> f2 = TrackedFile('palindrome-latin1.txt', 'w')
>>> f2.write(u'A man a plan a canál '.encode('latin1')
>>> f3 = TrackedFile('palindrome-utf8.txt', 'w')
>>> f3.write(u'A man a plan a canál '.encode('utf-8'))
>>> f2.size
21
>>> f3.size
22