Author Dusty Phillips Reviewers

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 148 
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This code is not very safe. You can very easily move past the ending 
position, and if you try to go home on an empty file, it will crash. 
These examples are kept short to make them readable, but that doesn't 
mean they are defensive! You can improve the error checking of this 
code as an exercise; it might be a great opportunity to expand your 
exception handling skills.
The 
Document
class itself is hardly changed, except for removing the two methods 
that were moved to the 
Cursor
class:
class Document:
def __init__(self):
self.characters = []
self.cursor = Cursor(self)
self.filename = ''
def insert(self, character):
self.characters.insert(self.cursor.position,
character)
self.cursor.forward()
def delete(self):
del self.characters[self.cursor.position]
def save(self):
f = open(self.filename, 'w')
f.write(''.join(self.characters))
f.close()
We simply updated anything that accessed the old cursor integer to use the new object 
instead. We can test that the 
home
method is really moving to the newline character:
>>> d = Document()
>>> d.insert('h')
>>> d.insert('e')
>>> d.insert('l')
>>> d.insert('l')
>>> d.insert('o')
>>> d.insert('\n')
>>> d.insert('w')
www.it-ebooks.info

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Chapter 5
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 149 
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>>> d.insert('o')
>>> d.insert('r')
>>> d.insert('l')
>>> d.insert('d')
>>> d.cursor.home()
>>> d.insert("*")
>>> print("".join(d.characters))
hello
*world
Now, since we've been using that string 
join
function a lot (to concatenate the 
characters so we can see the actual document contents), we can add a property
to the 
Document
class to give us the complete string:
@property
def string(self):
return "".join(self.characters)
This makes our testing a little simpler: