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Terry: You forgot to use a raw string for your timeit.repeat check, which is why it blew up. It was evaluating the 0円 when you defined the statement string itself, not the contents. If you use r'b"0円" * 7' it works just fine by deferring backslash escape processing until the string is actually eval-ed, rather than when you create the string.
For example, on my (admittedly underpowered) laptop (Win7 x64, Py 3.3.0 64-bit):
>>> min(timeit.repeat(r'b"0円" * 7'))
0.07514287752866267
>>> min(timeit.repeat(r'bytes(7)'))
0.7210309422814021
>>> min(timeit.repeat(r'b"0円" * 7000'))
0.8994351749659302
>>> min(timeit.repeat(r'bytes(7000)'))
2.06750710129117
For a short bytes, the difference is enormous (as I suspected, the lookup of bytes dominates the runtime). For much longer bytes, it's still winning by a lot, because the cost of having the short literal first, then multiplying it, is still trivial next to the lookup cost.
P.S. I made a mistake: str does accept an int argument (obviously), but it has completely different meaning.