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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def filename_sanitized(self, extension, default_filename='file'):
 """Returns a filename that is safer to use on the filesystem. The filename will not contain a slash (nor the path separator for the current platform, if different), it will not start with a dot, and it will have the expected extension. No guarantees that makes it "safe enough". No effort is made to remove special characters; using this value blindly might overwrite existing files, etc. """


        assert extension
        assert extension[0] != '.'
        assert default_filename
        assert '.' not in default_filename
        extension = '.' + extension

        fname = self.filename_unsafe
        if fname is None:
            fname = default_filename
        fname = posixpath.basename(fname)
        fname = os.path.basename(fname)
        fname = fname.lstrip('.')
        if not fname:
            fname = default_filename
        if not fname.endswith(extension):
            fname = fname + extension
        return fname