gistfile1.py

import rasterio

with rasterio.open('input.tif') as src:
    source = src.read_band(1)

def window_transform(transform, offset):
    result = transform[:]
    result[3] = transform[3] + offset[0]*transform[5]
    result[0] = transform[0] + offset[1]*transform[1]
    return result

# This is the idiom for copying parameters from a file to use
# when writing a new file.
kwargs = src.meta
kwargs.update(
    width=512,
    height=512,
    # calculates a new affine transformation matrix, offset 256 rows and 0 columns
    # from the one in src.
    transform=window_transform(src.transform, (256, 0))

with rasterio.open('output.tif', 'w', **kwargs) as dst:
    dst.write_band(1, source[:512,:512])