Unicode strings cannot be used as keys in dictionaries passed as keyword argument to a function. For example:

Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC v.1310 32 bit (Intel)] on
win32
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>>> def fn(**kw):
...     print repr(kw)
...
>>> fn(**{u'x':1})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: fn() keywords must be strings
>>>

Unicode strings should be converted to str automatically using the site specific default encoding or something similar solution.

This bug caused problems when decoding dictionaries from data formats such as XML or JSON. Usually unicode strings are returned from such modules, such as simplejson. Manual encoding from unicode to str causes performance loss if this cannot be done by Python automatically.