Re: My fight with classes :)

On Wed, 11 Jun 2008 22:16:56 +0800, TheSaint <fc14301589@xxxxxxxxxxx> wrote:
Hi,
I'm very new with classes. I still reading something around ;)

I got started to try a concatenation of 2 type of string, which have a
particular property to start with A or D.

My class here:
""" Small class to join some strings according to the leading first
letter"""

def __init__(self):
self.valueA= ''
self.valueD= ''

def __add__(self, value):
if not isinstance(value, str): return
if value.lower().startswith('a'):
self.valueA += value
if value.lower().startswith('d'):
self.valueD += value
return self.valueA ,self.valueD

__call__= __add__
__iadd__= __add__

my test on the shell:

[snip]
k +'aks'
('aks', '')
k +'daks'
('aks', 'daks')
[snip]
k += 'liu'
k += 'aliu'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can only concatenate tuple (not "str") to tuple

You have designed your class in a confusing way, and then you
were confused by it.

It was confusing to design your class in such a way that "k+'aks'"
modifies k, because "k+'aks'" appears to be just an expression.
Changing k as a side-effect of evaluating that expression is
unnecessarily confusing. See if you can figure out how to design
your class so that you modify k either by writing "k.append( 'aks' )"
or "k += 'aks'".

Speaking of which, you discovered that "k += 'liu';
k += 'aliu'" fails. It fails because "k += 'liu'" replaces k
with a tuple (because you return a tuple from your __add__
function), so k is no longer an instance of your class.


Do I miss something?

I'd rather like to avoid class, but a function won't allow me to store so
easily data between several call.

Classes are wonderfully useful, and it would be sad for you to
use Python while shunning classes. I strongly recommend looking
at some good examples of simple class programming; I apologize
for having no specific recommendations.

--
To email me, substitute nowhere->spamcop, invalid->net.
.

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