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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 signature(f, s=None, block_size=RS_DEFAULT_BLOCK_LEN):
 """ Generate a signature for the file `f`. The signature will be written to `s`. If `s` is omitted, a temporary file will be used. This function returns the signature file `s`. You can specify the size of the blocks using the optional `block_size` parameter. """

    if s is None:
        s = tempfile.SpooledTemporaryFile(max_size=MAX_SPOOL, mode='wb+')
    job = _librsync.rs_sig_begin(block_size, RS_DEFAULT_STRONG_LEN)
    try:
        _execute(job, f, s)
    finally:
        _librsync.rs_job_free(job)
    return s