Python Cookbook

Evaluating Code Inside Strings

Credit: Joonas Paalasmaa

Problem

You have a string that contains embedded Python expressions, and you need to copy the string while evaluating those expressions.

Solution

This recipe’s trick is to use the % string-formatting operator’s named-values variant. That variant normally takes a dictionary as the righthand operand, but in fact it can take any mapping, so we just prepare a rather special mapping for the recipe’s purpose:

class Eval:
    """ mapping that does expression evaluation when asked to fetch an item """
    def _ _getitem_ _(self, key):
        return eval(key)

Now we can perform feats such as:

>>> number = 20
>>> text = "python"
>>> print "%(text.capitalize())s %(number/9.0).1f rules!" % Eval(  )
Python 2.2 rules!

Discussion

This recipe can be seen as a templating task, akin to Recipe 3.22 and Recipe 3.23, but it is substantially simpler, because it needs to handle only embedded expressions, not statements. However, because the solution is so much simpler and faster than the general templating ones, it’s better to think of this as a totally separate task.

In Python, the % operator of strings is typically used for normal formatting. The values to be interpolated in the string are the items of the righthand side, which is either a tuple, for unnamed-value formatting, or a mapping, for named-value formatting (where format items have forms such as %(name)s). The mapping is often obtained by functions such as the built-in vars, which returns a dictionary that represents ...