Can I do math inside Python's string formatting "language"?

Question

I would like to do some simple math while I'm doing string formatting. For example

N = {'number':3}
four = '{number:d + 1}'.format(**N)

This doesn't work (of course). Is there a way to accomplish this that I'm not aware of?

Thanks!

Answer 1

"Is there a way to accomplish this that I'm not aware of?" If by "this" you mean encoding some mathematical logic in the format string using str.format, then no -- not that I'm aware of. However if you use a templating language you can express all kinds of stuff like this.

There are a billion different options for templating languages in Python, so rather than try to say which is best, I'll let you decide. See this article from the Python wiki: http://wiki.python.org/moin/Templating

A favorite of mine is Jinja2, although it's probably overkill for what you're talking about.

Here's an example of how to accomplish what you're after with Jinja2:

N = { 'd' : 3 }
four = Template(u'number:{{ d + 1 }}').render(**N)

The main advantage to a templating system like Jinja2 is that it allows you store templates as files separate from your application control logic such that you can maintain the view/presentation logic for your program in a way that limits or prohibits side effects from presentation execution.

Answer 2

About as close as you can get is to use positional arguments instead of keyword arguments:

four='{0:d}'.format(N['number']+1)

or the shorter old-school:

four='%d'%(N['number']+1)

What's your goal here?