What does this UnicodeEncodeError mean?

It means that a given unicode object (i.e. the Python internal representation for a sequence of internationalized characters conforming to the Unicode standard) failed to be converted to a str object (i.e. a sequence of bytes). The failure means that there was a character which couldn't be represented by an appropriate sequence of bytes in the chosen output encoding.

In practice, the default conversion being a "strict" one using the default encoding, which is 'ascii' most of the time, the error will happen as soon as the unicode object contains characters outside of the ASCII range of characters (codepoint 0 to 127).

This error was frequently seen during the transition to internal use of unicode that happened in Trac 0.10, and can still be seen now and then with Trac plugins that are not using the Trac API the way they should.

Examples:

>>> str(u'chaîne de caractères')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
UnicodeEncodeError: 'ascii' codec can't encode character u'\xee'
                    in position 3: ordinal not in range(128)
>>> 

The above is actually equivalent to the following, as sys.getdefaultencoding() is frequently 'ascii':

>>> u'chaîne de caractères'.encode('ascii')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
UnicodeEncodeError: 'ascii' codec can't encode character u'\xee' 
                    in position 3: ordinal not in range(128)

One possibility to force the conversion to happen would be to use a non-strict conversion:

>>> u'chaîne de caractères'.encode('ascii', 'ignore')
'chane de caractres'
>>> u'chaîne de caractères'.encode('ascii', 'replace')
'cha?ne de caract?res'

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See also: TracDev/UnicodeGuidelines, UnicodeDecodeError