Using builtin 'str' function in web.py templates: global name not found

I've spent way too much time trying to figure this out:

I'm passing a template a number like so and trying to make it a string:

$def with (num)

$(str(num开发者_如何学运维)) or $str(num)

This generates an error saying global name "str" not found.

Edit & Solution: I'm doing this so I can refer to "img[0-n].png". If num is passed as a number, you can make this string by just saying "img$(num).png". No need to convert num to string explicitly.

Pass str as a global to the render object:

render = web.template.render('templates', globals={ 'str': str })

Or you can use the construct "%d" to convert ints to string, as:

stringified_int="%d" % n

or in the template style using your question:

$ filename="img[0-%d]" % n

I don't know if this works for python2, but for python3 it works.