Re: Where does str class represent its data?

On Jul 11, 9:49 pm, James Stroud <jstr...@xxxxxxxxxxxx> wrote:
ChrisEdge...@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.

Initially, I tried simply calling a clean function on a regular
string, without any of this messy subclassing. However, I would end
up accidentally cleaning it more than once, and transforming the
string was just very messy. I thought that it would be much easier to
just clean the string once, and then add methods that would give me
the various transformations that I wanted from the cleaned string.
Using __new__ seems to be the solution I was looking for.


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:

What constitutes an abuse of the str class? Is there some performance
decrement that results from subclassing str like this? (Unfortunately
my implementation seems to have a pretty large memory footprint, 400mb
for about 400,000 files.) Or do you just mean from a philsophical
standpoint? I guess I don't understand what benefits come from using
UserString instead of just str.

Thanks for the help,
Chris


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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