python3.0 base64 error

[yang michael]

I use base64 module on python3.0
like:

import base64
b="hello world"
a=base64.b64encode(b)
print(a)

but when i run it,it catch a error:
Traceback (most recent call last):
  File "/home/jackie-yang/yd5m19/pythonstudy/test.py", line 4, in <module>
    a=base64.b64encode(b)
  File "/usr/local/lib/python3.0/base64.py", line 56, in b64encode
    raise TypeError("expected bytes, not %s" % s.__class__.__name__)
TypeError: expected bytes, not str



and also the document's example:

>>> import base64
>>> encoded = base64.b64encode('data to be encoded')
>>> encoded
'ZGF0YSB0byBiZSBlbmNvZGVk'
>>> data = base64.b64decode(encoded)
>>> data
'data to be encoded'

can not run too.

what happen?
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