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The UnicodeDecodeError normally happens when decoding an str string from a certain coding. Since codings map only a limited number of str strings to unicode characters, an illegal sequence of str characters will cause the coding-specific decode() to fail.

Decoding from str to unicode.

>>> "a".decode("utf-8")
>>> "\x81".decode("utf-8")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "encodings/", line 16, in decode
UnicodeDecodeError: 'utf8' codec can't decode byte 0x81 in position 0: unexpected code byte

>>> "a\x81b".decode("utf-8", "replace")

Paradoxically, a UnicodeDecodeError may happen when _encoding_. The cause of it seems to be the coding-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 converting to their own coding. It also appears that such "up-conversion" makes no assumption of str parameter's coding, choosing a default ascii decoder. Hence a decoding failure inside an encoder.

Unlike a similar case with UnicodeEncodeError, such a failure cannot be always avoided. This is because the str result of encode() must be a legal coding-specific sequence. However, a more flexible treatment of the unexpected str argument type might first validate the str argument by decoding it, then return it unmodified if the validation was successful. As of Python2.5, this is not implemented.

Alternatively, a TypeError exception could always be thrown on receiving an str argument in encode() functions. (This would require StreamWriter.write() to accept only unicode. The underlying stream's .write() will receive only str's).

Encoding from unicode to str.

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

Python 3000 will prohibit encoding of bytes, according to PEP 3137: "encoding always takes a Unicode string and returns a bytes sequence, and decoding always takes a bytes sequence and returns a Unicode string".


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