Paradoxically, a UnicodeDecodeError happens when _encoding_. The cause of it seems to be the encoding-specific encode() functions that normally expect a parameter of type unicode. It appears that on seeing an str parameter, the encode() functions "up-convert" it into unicode before applying their own encoding. It also appears that the "up-conversion" makes no assumption of str parameter's encoding, assuming it to be ascii. Hence a decoding failure inside an encoder.

   1 >>> u"\u0411".encode("utf-8")
   2 '\xd0\x91'
   3 >>> "a".encode("utf-8")
   4 'a'
   5 >>> "\xd0\x91".encode("utf-8")
   6 Traceback (most recent call last):
   7   File "<stdin>", line 1, in <module>
   8 UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 0: ordinal not in range(128)

CategoryUnicode