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") 'aБb'
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 an encoding 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)
Python 3000 will prohibit decoding of Unicode strings, 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".