[Tutor] Where is this tr function?

[Kelie]

Hello,

I see a lots tr('some string') in codes, especially the ones related to
wxPython. Where is this tr function defined and what does it do? To make the
argument a raw string?

When I type print tr('some string') in PyShell that comes with wxPython, no
error. But if I type that line in a .py file and run it, I get an error:
NameError: name 'tr' is not defined

Thanks!