How do I get str.translate to work with Unicode strings?

Question

I have the following code:

import string def translate_non_alphanumerics(to_translate, translate_to='_'):     not_letters_or_digits = u'!"#%\'()*+,-./:;<=>?@[\]^_`{|}~'     translate_table = string.maketrans(not_letters_or_digits,                                        translate_to                                          *len(not_letters_or_digits))     return to_translate.translate(translate_table) 

Which works great for non-unicode strings:

>>> translate_non_alphanumerics('<foo>!') '_foo__' 

But fails for unicode strings:

>>> translate_non_alphanumerics(u'<foo>!') Traceback (most recent call last):   File "<stdin>", line 1, in <module>   File "<stdin>", line 5, in translate_non_alphanumerics TypeError: character mapping must return integer, None or unicode 

I can't make any sense of the paragraph on "Unicode objects" in the Python 2.6.2 docs for the str.translate() method.

How do I make this work for Unicode strings?

Answer 1

The Unicode version of translate requires a mapping from Unicode ordinals (which you can retrieve for a single character with ord) to Unicode ordinals. If you want to delete characters, you map to None.

I changed your function to build a dict mapping the ordinal of every character to the ordinal of what you want to translate to:

def translate_non_alphanumerics(to_translate, translate_to=u'_'):     not_letters_or_digits = u'!"#%\'()*+,-./:;<=>?@[\]^_`{|}~'     translate_table = dict((ord(char), translate_to) for char in not_letters_or_digits)     return to_translate.translate(translate_table)  >>> translate_non_alphanumerics(u'<foo>!') u'_foo__' 

edit: It turns out that the translation mapping must map from the Unicode ordinal (via ord) to either another Unicode ordinal, a Unicode string, or None (to delete). I have thus changed the default value for translate_to to be a Unicode literal. For example:

>>> translate_non_alphanumerics(u'<foo>!', u'bad') u'badfoobadbad'