Question

So, I’ve searched for a while and couldn’t find anything, so I’ve decided to turn to the experts on SO to help me clarify what is going on.

I am learning Python and whilst learning about regular expressions, I’ve come across an interesting bit of syntax that I can’t quite figure out. In this example, a function is defined which runs a regular expression on the input parameter, returning the match of the integer as a float, or throwing an exception if the input doesn’t match something that looks like a number:

import re
def getNumber(token):
  r'-?[1-9][0-9]*.?[0-9]*'
  return float(token)

this function can be called like so:

print getNumber('123.123')
print getNumber('123.123')+40

which would output:

123.123
163.123

I am trying to understand the mechanics of how this is happening. I understand that we are declaring a regular expression object with the call to r’STRING’, but somehow just declaring that regular expression causes the token parameter to get passed into the expression as well. Is this a trait with functions explicitly containing a parameter called “token”? Is there a behavior associated with multiple parameters? It definitely seems that some work has been done here to provide a pythonic syntax, I would just like to know the details of how it works and how to use it in the future. Pointing me to documentation would be great, as I couldn’t find anything on the subject.

Answer 1

This function isn’t running a regular expression on the input … It’s calling float on the input which is converting the input to a floating point number. The “regular expression” that is there is nothing more than a docstring on the function.

Note that r"this is a string" simply creates a “raw string” which has nothing to do with regex ("\t" is tab whereas r"\t" is the literal characters '\' and 't') .