Refactoring
SIS
July 2024
What Are Your Goals When Coding?
- Compiled code communicates to the computer
- Source code communicates to people
What is Refactoring?
The process of changing a software system in such a way that it does not alter the external behavior of the code, yet improves its internal structure.
Martin Fowler, Refactoring (1999)
What is Refactoring?
- Refactoring does not change the behavior of a program.
- If the program is called twice (before and after a refactoring) with the same set of inputs, the resulting set of output values should be the same.
- Refactoring supports software design and evolution by restructuring a program in the way that allows other changes to be made more easily.
William Opdyke‘s PhD Thesis (1992)
Extension-Refactoring Cycle
A little non-coding writing example:
- Extend
- The dog, which was long and thin and also very fast, ate.
- Refactor
- The long, thin, fast dog ate.
- Extend
- The long, thin, fast dog ate a biscuit.
- Refactor
- The greyhound dog ate a biscuit.
Why Refactoring?
- 📖 Makes your code easier to read
- ✏️ Makes your code easier to modify
- 🐞 Makes it easier to find and fix bugs
- 🚀 Makes it possible to develop faster
When Should You Refactor?
When should you edit your writing?
All the time! 🧐
Bad Smells: Signs That a Refactoring is Necessary
| 🤪 | Cryptic names for variables, etc. | ⇒ | Rename to have clearer names |
| 😵 | Long function / script body | ⇒ | Break into smaller units |
| 🤯 | Duplicated code | ⇒ | Extract units and share |
| 🤔 | Learn a better algorithm | ⇒ | Replace algorithm |
Some Useful Refactorings
- Rename Unit (Class, Method, Function, Variable)
- Introduce Explaining Variable
- Extract Method/Function
- Extract Class
- Make Temporary Variable an Instance Variable
- Substitute Algorithm/Data Structure
How to Refactor
- Use version control (e.g. git)
- Run unit tests
- Use tools (if available)
- Make one change at a time
- Iterate
A Comprehensive Example
Rename Variable
Extract Function
logisticMap <- function(x) { 3.6 * x * (1 - x) }
x <- 0.3
symbols <- c("symbol 1", "symbol 2", "stuff", "more stuff", "lorem ipsum")
numberSequence <- c()
symbolSequence <- c()
for (i in 1:100) {
x <- logisticMap(x)
numberSequence <- c(numberSequence, x)
x <- logisticMap(x)
symbolSequence <- c(symbolSequence, symbols[floor(x * 5) + 1])
}Extract Function
logisticMap <- function(x) { 3.6 * x * (1 - x) }
pickSymbol <- function(symbols, x) {
symbols[floor(x * length(symbols)) + 1]
}
x <- 0.3
symbols <- c("symbol 1", "symbol 2", "stuff", "more stuff", "lorem ipsum")
numberSequence <- c()
symbolSequence <- c()
for (i in 1:100) {
x <- logisticMap(x)
numberSequence <- c(numberSequence, x)
x <- logisticMap(x)
symbolSequence <- c(symbolSequence, pickSymbol(symbols, x))
}Substitute Algorithm
logisticMap <- function(x) { 3.6 * x * (1 - x) }
pickSymbol <- function(symbols, x) {
symbols[floor(x * length(symbols)) + 1]
}
x <- 0.3
symbols <- c("symbol 1", "symbol 2", "stuff", "more stuff", "lorem ipsum")
numberSequence <- numeric(100)
symbolSequence <- character(100)
for (i in 1:100) {
x <- logisticMap(x)
numberSequence[i] <- x
x <- logisticMap(x)
symbolSequence[i] <- pickSymbol(symbols, x)
}Introduce Explaining Variable
logisticMap <- function(x) { 3.6 * x * (1 - x) }
pickSymbol <- function(symbols, x) {
symbols[floor(x * length(symbols)) + 1]
}
x <- 0.3
symbols <- c("symbol 1", "symbol 2", "stuff", "more stuff", "lorem ipsum")
N <- 100
numberSequence <- numeric(N)
symbolSequence <- character(N)
for (i in 1:N) {
x <- logisticMap(x)
numberSequence[i] <- x
x <- logisticMap(x)
symbolSequence[i] <- pickSymbol(symbols, x)
}Basic Refactorings
- Rename Unit (Class, Method, Function, Variable)
- Introduce Explaining Variable
- Extract Method/Function
- Extract Class
- Make Temporary Variable an Instance Variable
- Substitute Algorithm/Data Structure
Rename Variable
def increment_depth(request):
temp = request["depth"]
temp = temp + 1
request["depth"] = temp
return temp- Decide on a new name.
- Check that name isn‘t used in scope.
- Find all references to the name and replace it with the new name.
Rename Variable
Introducing Explaining Variable
class ShoppingCartEntry:
def __init__(self, item_price, quantity):
self.item_price = item_price
self.quantity = quantity
def price(self):
"""price is base price - quantity discount + shipping"""
return self.quantity * self.item_price - \
max(0, self.quantity - 500) * self.item_price * 0.05 + \
min(self.quantity * self.item_price * 0.1, 100.0)- Introduce a variable.
- Set the value to the result of part of a complex expression.
- Replace that part of the expression with the variable.
class ShoppingCartEntry:
def __init__(self, item_price, quantity):
self.item_price = item_price
self.quantity = quantity
def price(self):
"""price is base price - quantity discount + shipping"""
+ base_price = self.quantity * self.item_price
- return self.quantity * self.item_price \
+ return base_price \
- max(0, self.quantity - 500) * self.item_price * 0.05 \
- + min(self.quantity * self.item_price * 0.1, 100.0)
+ + min(base_price * 0.1, 100.0)Extract Function
class ShoppingCartEntry:
def __init__(self, item_price, quantity):
self.item_price = item_price
self.quantity = quantity
def price(self):
"""price is base price - quantity discount + shipping"""
return self.quantity * self.item_price - \
max(0, self.quantity - 500) * self.item_price * 0.05
+ min(self.quantity * self.item_price * 0.1, 100.0)Extract Function
- Pick a good name.
- Create a new function by copy existing code.
- Bring over local variables or provide them as parameters.
- Replace extracted code with a call to the function.
Extract Function
class ShoppingCartEntry:
def __init__(self, item_price, quantity):
self.item_price = item_price
self.quantity = quantity
def price(self):
"""price = base price - discount + shipping"""
- return self.quantity * self.item_price - \
+ return self.base_price() - \
max(0, self.quantity - 500) \
* self.item_price * 0.05 \
- + min(self.quantity * self.item_price * 0.1,
+ + min(self.base_price() * 0.1,
100.0)
+
+ def base_price(self):
+ return self.quantity * self.item_price