Re: Where does str class represent its data?

ChrisEdgemon@xxxxxxxxx wrote:
I'd like to implement a subclass of string that works like this:


m = MyString('mail')
m == 'fail'

True

m == 'mail'

False

m in ['fail', hail']

True

My best attempt for something like this is:

class MyString(str):
def __init__(self, seq):
if self == self.clean(seq): pass
else: self = MyString(self.clean(seq))

def clean(self, seq):
seq = seq.replace("m", "f")

but this doesn't work. Nothing gets changed.

I understand that I could just remove the clean function from the
class and call it every time, but I use this class in several
locations, and I think it would be much safer to have it do the
cleaning itself.


The "flat is better than nested" philosophy suggests that clean should be module level and you should initialize a MyString like such:

m = MyString(clean(s))

Where clean is

def clean(astr):
return astr.replace('m', 'f')

Although it appears compulsory to call clean each time you instantiate MyString, note that you do it anyway when you check in your __init__. Here, you are explicit. Such an approach also eliminates the obligation to clean the string under conditions where you know it will already be clean--such as deserialization.

Also, you don't return anything from clean above, so you assign None to self here:

self = MyString(self.clean(seq))

Additionally, it has been suggested that you use __new__. E.g.:

py> class MyString(str):
.... def __new__(cls, astr):
.... astr = astr.replace('m', 'f')
.... return super(MyString, cls).__new__(cls, astr)
....
py> MyString('mail')
'fail'

But this is an abuse of the str class if you intend to populate your subclasses with self-modifying methods such as your clean method. In this case, you might consider composition, wherein you access an instance of str as an attribute of class instances. The python standard library make this easy with the UserString class and the ability to add custom methods to its subclasses:

py> from UserString import UserString as UserString
py> class MyClass(UserString):
.... def __init__(self, astr):
.... self.data = self.clean(astr)
.... def clean(self, astr):
.... return astr.replace('m', 'f')
....
py> MyClass('mail')
'fail'
py> type(_)
<type 'instance'>

This class is much slower than str, but you can always access an instance's data attribute directly if you want fast read-only behavior.

py> astr = MyClass('mail').data
py> astr
'fail'

But now you are back to a built-in type, which is actually the point--not everything needs to be in a class. This isn't java.

James


--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/
.

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