generator expression works in shell, NameError in script

[ssc]

Hello,

I am trying to generate this list of tuples:
[(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')]

My code works fine in the Python shell:

>>> titles = ['Dr', 'Miss', 'Mr', 'Mrs', 'Ms',]
>>> title_choices = [(0, '')] + list((titles.index(t)+1, t) for t in titles)
>>> title_choices
[(0, ''), (1, 'Dr'), (2, 'Miss'), (3, 'Mr'), (4, 'Mrs'), (5, 'Ms')]

The same code run in a script fails with
NameError: global name 'titles' is not defined

Does anybody know why ? How can I fix the error ?

Thank you very much :-)

Steve