Use string method format() to extract values from string

Question

In python I can do the following:

who = "tim"
what = "cake"
print "{0} likes {1}".format(who, what)

to yield "tim likes cake".

However the inverse operation is not as straightforward since I need to use regular expressions. I mean, to parse a string of known structure and extract the portions I know it contains, and store them into my variables. This extraction I perform by:

import re

expression = "([a-z]*) likes ([a-z]*)"
input_line = "tim likes cake"

who, what = re.search(expression, inputline).groups()

which is neat enough for small amount of parameters, but it has two main drawbacks for me compared to my idea of "ideal inverse" to format():

• Parameters extracted are always strings, they need to be converted to float with extra lines. Format handles internally the conversion needed, from any value to string.
• I need to define different templates for input and output, because the input template in regular expression form "([a-z]*) likes ([a-z]*)" cannot be reused for the "exporting" of the data, in the format function.

So, my question is, does a function like this exists, which would parse automatically the string and get the values the same way as we print them to the string, following almost the same syntax like
"{0} likes {1}".extract(who,what,input_line="tim likes cake")

I am aware I can create my custom "extract" function which behaves as desired, but I don't want to create it if there is already one available.

Answer 1

There doesn't seem to be a built-in solution beyond splitting the string and casting the components or using re.

Which is a little weird, because format can be used to specify types on input: "{0:03d}_{1:f}".format(12, 1) gives '012_3.000000', so I'm not sure why there's no "012_3.000000".extract("{0:03d}_{1:f}", [a, b]), but .. maybe only people coming from C want such a thing.

In any case, you may find the parse module useful, as suggested in this answer.

Answer 2

Here's an idea.

import re 

template ="{0} likes {1}"
str_re = r"\w+"
re.search(template.format(str_re, str_re), ...) 

Though, seems messy

Answer 3

who = "tim"
what = "cake"
print "{0} likes {1}".format(who, what)

This works because you know exactly where who and what are in the string. If that's the case, you don't need regex. Strings are lists of characters :)

def extract_who_what_from_string(string):
    words = string.split(" ")
    who = words[0]
    what = words[-1] 
    return who, what

Anything more complicated than this is, in fact, natural language processing and would be very much out of my scope.