KraftDiner wrote:
> c = [a, b]
> for c in [a,b]:
> c.getName()
>
> but when does baseClass ever get used?
> Why did i even have to define it?
>
One reason for using base classes are for logical reasons. Oranges and
Apples are different, but they are both fruits. Python has both
unicode strings and 8-bit ASCII strings. Both are strings and share a
common base class: the 'basestring' class.
A second reason is for code sharing. Let's say Python's string class
does everything you want already... except for one little thing. You
want the split method to return a list with some extra information.
Why re-invent the wheel trying to implement the other string methods
when you can reuse everything that's already been done?
>>> class MyString(str):
... def split(self):
... "Perform some extra work this version of split"
... wordList = str.split(self) # Call the original method
... return ['Extra'] + wordList + ['Information'] # Do additional
work!
...
>>>
In Python, we often rely on duck typing. "If it looks like a duck,
quacks like a duck, it's a duck." If we can treat it like a string, we
can consider it a string. If we can't treat it like a string, Python
will let us know by raising an exception. We can catch this exception
and try something different, or let the exception stop the program and
let the user know something went wrong.
Duck typing allows us to re-use code very efficiently. I'll
demonstrate it with a function and the class defined above.
>>> def GetWords(stringValue):
... "Print the list of words in a string"
... print 'Words are: %s' % stringValue.split()
...
>>> stringValue = str('Hello, world!')
>>> unicodeValue = unicode('These are different strings')
>>> myStringValue = MyString('good, hopefully useful')
>>>
>>> GetWords(stringValue)
Words are: ['Hello,', 'world!']
>>> GetWords(unicodeValue)
Words are: [u'These', u'are', u'different', u'strings']
>>> GetWords(myStringValue)
Words are: ['Extra', 'good,', 'hopefully', 'useful', 'Information']
>>>
As shown above, the GetWords() function works fine with my new string
class. Any methods that I didn't redefine keep their old behavior.
For example, I didn't define the upper() method in MyString, but I can
still use it:
>>> stringValue.upper()
'HELLO, WORLD!'
>>> myStringValue.upper()
'GOOD, HOPEFULLY USEFUL'
>>>
While we rely on duck typing in Python, we occassionally want special
behavior for certain types of data. Currently, you can pass anything
into the GetWords() function that has a method named 'split'. It does
not have to be a string:
>>> class NotAString(object):
... def split(self):
... return 'I am not a string!'
...
>>> otherDataValue = NotAString()
>>> GetWords(otherDataValue)
Words are: I am not a string!
>>>
Sometimes, we want some specialized behavior. Lists, tuples, and
strings all act like sequences (meaning, you can get their length and
use them in for-loops). Often, though, you'll want to treat strings
differently. You can check the type directly, or you can check by
using the isinstance() built-in function. isinstance() checks to see
if a variable is an instance of a class or any of its subclasses.
Remember the first reason given, of using a base class to logically
organize other classes? This is it in practice. I'll demonstrate
below:
>>> def CheckOnlyBuiltinStrings(stringValue):
... "Tells us whether or not stringValue is a str or unicode string."
... if type(stringValue) is str or type(stringValue) is unicode:
... print 'A built-in string type: %s' % stringValue
... else:
... print 'Not a built-in string type: %s' % stringValue
...
>>> def CheckAllStrings(stringValue):
... "Tells us whether or not stringValue is a string."
... # The basestring class is a superclass for all string classes.
... if isinstance(stringValue, basestring):
... print 'Is a string: %s' % stringValue
... else:
... print 'Not a string: %s' % stringValue
...
>>> CheckOnlyBuiltinStrings(stringValue)
A built-in string type: Hello, world!
>>> CheckOnlyBuiltinStrings(unicodeValue)
A built-in string type: These are different strings
>>> CheckOnlyBuiltinStrings(myStringValue)
Not a built-in string type: good, hopefully useful
>>>
>>> CheckAllStrings(stringValue)
Is a string: Hello, world!
>>> CheckAllStrings(unicodeValue)
Is a string: These are different strings
>>> CheckAllStrings(myStringValue)
Is a string: good, hopefully useful
>>> CheckAllStrings(42)
Not a string: 42
>>>
How do you know when you should use type() checks, when you should use
isinstance(), and when you should just try to use the data? That
depends, and there have been many lively debates on this subject in the
newsgroup. I recommend that you should only use as much type checking
as needed, and the less is better.
A bit long, but I hope this helps you out.
--Jason
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