The UnicodeEncodeError normally happens when encoding a unicode string into a certain coding. Since codings map only a limited number of unicode characters to str strings, a non-presented character will cause the coding-specific encode() to fail.

Encoding from unicode to str.

>>> u"a".encode("iso-8859-15")
'a'
>>> u"\u0411".encode("iso-8859-15")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "encodings/iso8859_15.py", line 12, in encode
UnicodeEncodeError: 'charmap' codec can't encode character u'\u0411' in position 0: character maps to <undefined>

>>> u"a\u0411b".encode("iso-8859-15", "replace")
'a?b'
>>> u"a\u0411b".encode("iso-8859-15", "backslashreplace")
'a\\u0411b'
>>> u"a\u0411b".encode("iso-8859-15", "xmlcharrefreplace")
'a&#1041;b'

Paradoxically, a UnicodeEncodeError may happen when _decoding_. The cause of it seems to be the coding-specific decode() functions that normally expect a parameter of type str. It appears that on seeing a unicode parameter, the decode() functions "down-convert" it into str, then decode the result assuming it to be of their own coding. It also appears that the "down-conversion" is performed using the ASCII encoder. Hence a nencoding failure inside a decoder.

The choice of the ASCII encoder for "down-conversion" might be considered wise because it is an intersection of all codings. The subsequent decoding may only accept a coding-specific str.

However, unlike a similar issue with UnicodeDecodeError while encoding, there would be not ambiguity if decode() simply returned the unicode argument unmodified. There seems to be not such a shortcut in decode() functions as of Python2.5.

Alternatively, a TypeError exception could always be thrown on receiving a unicode argument in decode() functions. (This would require stream.read() to produce only str for StreamReader.read(). The latter would only produce unicode).

Decoding from str to unicode.

>>> "a".decode("utf-8")
u'a'
>>> "\xd0\x91".decode("utf-8")
u'\u0411'
>>> u"a".decode("utf-8")      # Unexpected argument type.
u'a'
>>> u"\u0411".decode("utf-8") # Unexpected argument type.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "encodings/utf_8.py", line 16, in decode
UnicodeEncodeError: 'ascii' codec can't encode character u'\u0411' in position 0: ordinal not in range(128)

CategoryUnicode