Refactoring A Python Beginner’s Code

ああ、PyConJPの講演資料っていうんですか、まだ0%ですね。

Today’s Presentation & source

• http://shimizukawa.bitbucket.org/
• https://bitbucket.org/shimizukawa/refactoring-python-beginners-code

Who am I / おまえ誰よ

• http://清水川.jp/ @shimizukawa
• BeProud
• PyConJP 2012 vice-chairman, Sphinx-users.jp vice-chairman

2 Python books

9/16、Expert Python Reading PyConJP2012 Edition

• at 9/16 OpenSpace
• Gift something...

My Confusions when I started Python programing

• I didn’t know what is good coding style in Python
• I tried to port code from C/C++ to Python
• That code looks like Java or C++

My code in 2004

class PlayerMaker:
    def __init__(self):
        self.name = ""
        self.hp = 0
        self.at = 0
        ...

    def setName( self, item ):
        self.name = item.text

    def setHp( self, item ):
        self.hp = int(item.text)

    def setAt( self, item ):
        self.at = int(item.text)

    def setDf( self, item ):
        self.df = int(item.text)

    def setAg( self, item ):
        self.ag = int(item.text)

   ...

A code I just glanced a few months ago

public class ISBNConverter {
   public static void main(String[] args) {
      ...
      char [] cisbn = isbn.toCharArray();
      int [] iisbn;
      iisbn = new int[10];
      ...
      for (int idx=0; idx<9; idx++) {
          iisbn[idx] = Character.digit(cisbn[idx], 10);
          if ((iisbn[idx]<0)||(iisbn[idx]>9)){
              System.out.println("Error: Not a number is included in the 9 numbers.");
              System.exit(0);
      ...

I tried to rewrite by python straightly

def main(args):
    ...
    cisbn = isbn
    iisbn = [None] * 10;
    ...
    for idx in range(9):
        if not cisbn[idx].isdigit():
            print("Error: Not a number is included in the 9 numbers.")
            sys.exit(0)
        iisbn[idx] = int(cisbn[idx])

    if cisbn[9] == 'X':
        iisbn[9] = 10
    else:

Why the code does not seems like Python?

CamelCase getter/setter

class PlayerMaker:
    def __init__(self):
        self.name = ""
        self.hp = 0
        ...

    def setName( self, item ):
        self.name = item.text

    def setHp( self, item ):
        self.hp = int(item.text)

    ...

range for-loop

Like for(i=0; i<9; i++) {...}

for idx in range(9):

For example...

for i in range(9):
    if not cisbn[i].isdigit():
    ...

Prepare fixed-length array

Initializing array...

iisbn = [None] * 10;

For example...

for idx in range(9):
    ...
    iisbn[idx] = int(cisbn[idx])

そうだ、リファクタリングしよう

OK, Let’s refactoring!

First, we need Test-Case

• We NEED test case before any changes
• Changes SHOULD NOT effect the results
• For now, I test only printed text.

Practice

The points of refactoring

• Remove range loop
• Decrease variables
• Separate some processing in a loop
• Add new function to split code
• Replace complex loop by simple one
• Use Exceptions instead of print/exit
• Write Test-Case for new functions

Questions?

Thank You!