def METHOD_NAME(self):
    """Generate more realistic snap data.
    Most tests don't need realistic data, and can use the default dummy
    data that's created by setUp. However, tests that actually run xdelta3
    could use more accurate data. This file generates larger
    binary files. The files will be similar with roughly 0.1 variance along
    1KB block boundaries.
    """
    mean_size = 2**20
    size_stddev = 2**15
    snap_size = int(random.normalvariate(mean_size, size_stddev))
    scratchdir = self.useFixture(fixtures.TempDir()).path
    self.source_file = os.path.join(scratchdir, "source-snap")
    self.target_file = os.path.join(scratchdir, "target-snap")
    # source snap is completely random:
    with open(self.source_file, "wb") as source, open(
        self.target_file, "wb"
    ) as target:
        for i in range(0, snap_size, 1024):
            block = os.urandom(1024)
            source.write(block)
            if random.randint(0, 9) == 0:
                target.write(os.urandom(1024))
            else:
                target.write(block)