[Tutor] Refactoring

[Steven D'Aprano]

Ejaj Hassan wrote:
> Hi,
>    I have been hearing this refactoring of code. Well does it  have
> any thing like this in Python and if it is then what is it all about.

"Refactoring" is a general technique that applies to any language, not just 
Python. Refactoring means to take a program which is written in a complicated 
way, not easy to understand, and gradually simplify it so that it becomes 
easier to understand and maintain without starting from scratch or changing 
the behaviour.

The trick with refactoring is to find parts of the program that have common, 
related code. Here is a toy example -- suppose I had code that looks like this:


# Ask the user for a number between one and ten.
print "Please enter a number between 1 and 10"
while True:
     a = int(raw_input("My number is: "))
     if 1 <= a <= 10:
         break
     print "I'm sorry, your number was not between 1 and 10"
     print "please try again"

# Ask the user for a number between ten and twenty.
print "Please enter a number between 10 and 20"
while True:
     b = int(raw_input("My number is: "))
     if 10 <= a <= 20:
         break
     print "I'm sorry, your number was not between 10 and 20"
     print "please try again"

# And one more between twenty and 100.
print "Please enter a number between 20 and 100"
while True:
     c = int(raw_input("My number is: "))
     if 20 <= c <= 100:
         break
     print "I'm sorry, your number was not between 1 and 10"
     print "please try again"

print a+b+c


Look at all that duplicated code! And if you look carefully, you'll see a 
silly bug in it as well.


You might re-factor that code like so:

def get_number(low, high):
     # Ask the user for a number between low and high.
     print "Please enter a number between", low, "and", high
     while True:
         n = int(raw_input("My number is: "))
         if low <= n <= high:
             break
         print "I'm sorry, your number was not between", low, "and", high
         print "please try again"
     return n


a = get_number(1, 10)
b = get_number(10, 20)
c = get_number(20, 100)
print a+b+c


Notice that in refactoring, the aim should be to end up with code that is 
easier to maintain in the future. In this example, all the repeated code is 
now in one place, so instead of having to make changes to it in three 
different places, we can make it in one place.



-- 
Steven