Re: How to get/set class attributes in Python

[Simon Brunning]

On 6/12/05, Steve Jorgensen <[EMAIL PROTECTED]> wrote:
> Oops - I thought I cancelled that post when I relized I was saying nothing,

Would that everyone cancelled their posts when they realised that they
weren't saying anything. ;-)

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Re: How to get/set class attributes in Python

[Kalle Anke]

On Tue, 14 Jun 2005 06:40:51 +0200, Terry Hancock wrote
(in article <[EMAIL PROTECTED]>):

> I find the biggest problem coming to Python from a language
> like C, C++, or Java is that you overthink things and try to
> do them the hard way.  A lot of times, you find out that the
> "Python way" to do the thing is so blindingly obvious that
> it just didn't occur to you that it could be that simple.

It's very possible that this is the case, I'm not experienced enough in 
Python yet to have a opinion on this yet. I tend to think in terms of larger 
systems with several programmers ... instead of what I'm actually doing 
writing code for my own pleasure.

But I'm learning new stuff every day ... so in 6 months I'll probably have an 
opinion.


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Re: How to get/set class attributes in Python

[Terry Hancock]

On Monday 13 June 2005 03:50 pm, Kalle Anke wrote:
> On Mon, 13 Jun 2005 20:41:48 +0200, Terry Hancock wrote
> (in article <[EMAIL PROTECTED]>):
> 
> > 1) Assume the variables are of a sensible type (not 
> > necessarily the one you expected, though), and provide
> > exception handling to catch the case where their interface
> > does not match what you expect.
> 
> The problem I have with this is that I might discover an error at runtime 
> instead of compile time, this means that I need to really exercise all 
> possible test cases to make sure that I've covered everything (which of 
> course is a good thing) but it would be easier to discover this at "compile 
> time".

The Python solution to this problem is called "unittest". It is
at least as easy to do unit testing in Python as it is to properly
handle type-checking in C or Java.

> > Let's face it -- should it matter if you "are a programmer" 
> > or only if you "can program"?  This is the difference in
> > philosophy behind a dynamically-typed language like
> > Python and a statically typed one like Java.
> 
> I don't really understand what you're meaning (English isn't my native 
> language as you probably already have discovered)

(Actually no, you're quite fluent, AFAICT).  I was making
an analogy:

Requiring a variable to be, say, a "float" is like insisting that
only people with the job title "Programmer" be allowed to
see the language documentation.

In both situations, real life will confront you with situations
where you will wish that you had been more tolerant.  Lots
of us are not "Programmers", but we do quite a bit of programming.

Likewise, you'll be happier to discover that ints or complex
numbers work with your code too, without having to jump
through hoops to make it happen.

I find the biggest problem coming to Python from a language
like C, C++, or Java is that you overthink things and try to
do them the hard way.  A lot of times, you find out that the
"Python way" to do the thing is so blindingly obvious that
it just didn't occur to you that it could be that simple.

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Re: How to get/set class attributes in Python

[Mike Meyer]

Kalle Anke <[EMAIL PROTECTED]> writes:

> On Mon, 13 Jun 2005 20:41:48 +0200, Terry Hancock wrote
> (in article <[EMAIL PROTECTED]>):
>
>> 1) Assume the variables are of a sensible type (not 
>> necessarily the one you expected, though), and provide
>> exception handling to catch the case where their interface
>> does not match what you expect.
>
> The problem I have with this is that I might discover an error at runtime 
> instead of compile time, this means that I need to really exercise all 
> possible test cases to make sure that I've covered everything (which of 
> course is a good thing) but it would be easier to discover this at "compile 
> time".

But static typing only catches *some* of the errors that might
occur. Other errors will occur at run time even with static typing -
so you need to really exercise all possible test cases to make sure
you've covered everything in any case.

> (note: I'm not trying to change Python, only that to me statically typing has 
> it advantages also)

Static typing saves you some time by catching a small subset of
programming errors at compile time. On the other hand, it costs you
time by forcing you to declare a type for - well, everything that
you're going to catch errors for.

It's not clear which cost is larger.

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Re: How to get/set class attributes in Python

[Kalle Anke]

On Mon, 13 Jun 2005 20:41:48 +0200, Terry Hancock wrote
(in article <[EMAIL PROTECTED]>):

> 1) Assume the variables are of a sensible type (not 
> necessarily the one you expected, though), and provide
> exception handling to catch the case where their interface
> does not match what you expect.

The problem I have with this is that I might discover an error at runtime 
instead of compile time, this means that I need to really exercise all 
possible test cases to make sure that I've covered everything (which of 
course is a good thing) but it would be easier to discover this at "compile 
time".

(note: I'm not trying to change Python, only that to me statically typing has 
it advantages also)

> Let's face it -- should it matter if you "are a programmer" 
> or only if you "can program"?  This is the difference in
> philosophy behind a dynamically-typed language like
> Python and a statically typed one like Java.

I don't really understand what you're meaning (English isn't my native 
language as you probably already have discovered)

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Re: How to get/set class attributes in Python

[Terry Hancock]

I really think the problem is that you are trying to use 
techniques whose only reason for existing is that they make 
up for deficiencies in other languages:

For example, the *reason* for the Java or C++ use of get/set
methods is to allow for the future possibility of needing to
make those operations dynamic.  Python solves this by 
allowing you to create get/set methods *only when needed*
by using "properties", using exactly the same interface as
provided by ordinary attributes. Thus there is no need to
artificially add get/set methods in the general case.

This results in much cleaner and more concise code.

The *reason* for taking so much care that a variable is of 
the right type in classes in C++ or Java is because the 
program will crash if they are not.  In Python, you would 
be better served to:

1) Assume the variables are of a sensible type (not 
necessarily the one you expected, though), and provide
exception handling to catch the case where their interface
does not match what you expect.

2) Check for the specific interface methods that you need,
only  (if all you plan to do is read from a "file", you 
shouldn't worry about whether the "file" object you've been 
handed actually has a "write" method or not).
This is often called "duck typing".

3) Use "interfaces" (either from Zope or PyProtocols) to 
specify the required *behavior* (not type).

Let's face it -- should it matter if you "are a programmer" 
or only if you "can program"?  This is the difference in
philosophy behind a dynamically-typed language like
Python and a statically typed one like Java.


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Re: How to get/set class attributes in Python

[Axel Straschil]

Hello!

> You can 'hide' you getsetters using a property attribute[1]:

For me, the property attribute is a beast:

class A(object):
def getP(self): return 'A'
p = property(getP)
class A2(A):
def getP(self): return 'A2'
a = A()
a2 = A2()
print a.getP(), a2.getP()
print a.p, a2.p

So, property is instance-level super() tool ;-)

Lg,
AXEL.
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Re: How to get/set class attributes in Python

[Chris Spencer]

Peter Dembinski wrote:
> Chris Spencer <[EMAIL PROTECTED]> writes:
> 
> 
>>Peter Dembinski wrote:
>>
>>Of course, in that Python is dynamically typed as opposed to the
>>static typing of Java or C++. Please excuse my previous mis-wording :)
> 
> 
> Mis-wording?

I previously said Python was "untyped", when it's more accurate to say 
"dynamically typed".
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Re: How to get/set class attributes in Python

[Peter Dembinski]

Chris Spencer <[EMAIL PROTECTED]> writes:

> Peter Dembinski wrote:
>> Bruno Desthuilliers <[EMAIL PROTECTED]> writes:
>>
>>> Nope. Python *is* typed. But it doesnt confuse implementation with
>>> semantic.
>> Python is typed.  And its type system may look strange for anyone
>> who did only Java or C++ programming before :>
>
> Of course, in that Python is dynamically typed as opposed to the
> static typing of Java or C++. Please excuse my previous mis-wording :)

Mis-wording?
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Re: How to get/set class attributes in Python

[Chris Spencer]

Peter Dembinski wrote:
> Bruno Desthuilliers <[EMAIL PROTECTED]> writes:
> 
>>Nope. Python *is* typed. But it doesnt confuse implementation 
>>with semantic.
> 
> 
> Python is typed.  And its type system may look strange for anyone who
> did only Java or C++ programming before :>

Of course, in that Python is dynamically typed as opposed to the static 
typing of Java or C++. Please excuse my previous mis-wording :)

Chris
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Re: How to get/set class attributes in Python

[Chris Spencer]

Bruno Desthuilliers wrote:
> Chris Spencer a écrit :
> 
>> I was providing the original poster with a simple way to ensure 
>> appropriate type.
> 
> 
> s/appropriate type/specific implementation/
> 
> Hint : the appropriate type for print >> XXX is "whatever has a 
> 'write(anything_that_can_be_coerced_to_a_string)' method".

Certainly, although now I think you're splitting hairs over my 
terminology. Let's synchronize our wording with a point made in the 
Python docs. Section 2.1 of the Library Reference recommends that the 
builtin function isinstance() be used for testing the object type. Does 
this then mean that "type" refers to the class of an object? In which 
case, wouldn't "interface" refer to a collection of methods, that when 
present in a class create an implementation?

Chris
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Re: How to get/set class attributes in Python

[Bruno Desthuilliers]

Chris Spencer a écrit :
> Bruno Desthuilliers wrote:
> 
>> And *this* is highly unpythonic. And un-OO too, since it makes foo() 
>> dependant on *class* Bar, when it should most probably be enough that 
>> it only depends on (probably part of) the *interface* of class Bar. 
> 
> I was providing the original poster with a simple way to ensure 
> appropriate type.

s/appropriate type/specific implementation/

Hint : the appropriate type for print >> XXX is "whatever has a 
'write(anything_that_can_be_coerced_to_a_string)' method".
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Re: How to get/set class attributes in Python

[Peter Dembinski]

Bruno Desthuilliers <[EMAIL PROTECTED]> writes:

[snap]

>> Being an untyped language, Python does not require you to enforce
>> types.
>
> Nope. Python *is* typed. But it doesnt confuse implementation 
> with semantic.

Python is typed.  And its type system may look strange for anyone who
did only Java or C++ programming before :>
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Re: How to get/set class attributes in Python

[Chris Spencer]

Bruno Desthuilliers wrote:

> And *this* is highly unpythonic. And un-OO too, since it makes foo() 
> dependant on *class* Bar, when it should most probably be enough that it 
> only depends on (probably part of) the *interface* of class Bar.

I was providing the original poster with a simple way to ensure 
appropriate type. You can assert whatever aspect you wish. If you want 
to ensure compatibility with some other property simply use "assert 
blah" subsituting blah for whatever test you wish to perform (e.g. 
assert MyInterface.implementedBy(bar) to test for interface implementation).
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Re: How to get/set class attributes in Python

[Bruno Desthuilliers]

Chris Spencer a écrit :
> Kalle Anke wrote:
> 
>> On Sun, 12 Jun 2005 13:59:27 +0200, deelan wrote
>> (in article <[EMAIL PROTECTED]>):
>>
>> void doSomething( data : SomeClass ){ ... }
>>
>> and I would be sure at compile time that I would only get SomeClass 
>> objects as parameters into the method.
> 
> 
> Being an untyped language,  Python does not require you to enforce types.

Nope. Python *is* typed. But it doesnt confuse implementation with semantic.

> However, for those that require such functionality, you can get away 
> with using the "assert" statement. For example, if I wanted to make sure 
> my function foo was only given instances of class Bar, I'd write 
> something like:
> 
>  >>> class Bar: pass
>  >>> def foo(bar):
> ... assert isinstance(bar, Bar), "argument is not of type Bar"
> ... print "argument must be of type Bar"
  ...
>  >>> bar = Bar()
>  >>> foo(bar)
> argument must be of type Bar
>  >>> foo(123)
> Traceback (most recent call last):
>   File "", line 1, in ?
>   File "", line 2, in foo
> AssertionError: argument is not of type Bar
>  >>>

And *this* is highly unpythonic. And un-OO too, since it makes foo() 
dependant on *class* Bar, when it should most probably be enough that it 
only depends on (probably part of) the *interface* of class Bar.

Suppose I want to adapt my own class Baaz so it's compatible with Bar. 
Now I *must* make the adapter a subclass of Bar (and then depends on 
Bar's *implementation*) when it only needs to expose the same (probably 
subset of) interface.

Except for exceptions (please pardon the pun), one most usually don't 
have to worry about the real class of an object. In 5 years programming 
Python, I only had to check for the type of an object a couple of times, 
and each time only for more genericity (I mostly needed to dispatch on 
some 'generic types' like string-like, sequence-like, map-like, 
numeric-like, callable, and others), and this was never used as a way to 
restrict the possible types (exceptions are good enough for this).

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Re: How to get/set class attributes in Python

[Peter Dembinski]

"vincent wehren" <[EMAIL PROTECTED]> writes:

> "Peter Dembinski" <[EMAIL PROTECTED]> schrieb im Newsbeitrag 
> news:[EMAIL PROTECTED]
> | Kalle Anke <[EMAIL PROTECTED]> writes:
> |
> | [snap]
> |
> | >> sys.maxint = -12345
> | >
> | > I don't really understand what you're meaning.
> |
> | He meant None = 1 :>
>
> I'm sure you know that has become a no-no in Python 2.4+ ;)

Yep.  Still waiting for delegalisation of others :>
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Re: How to get/set class attributes in Python

[Steven D'Aprano]

On Sun, 12 Jun 2005 15:35:46 +0200, Kalle Anke wrote:

> Anyway, I got another "problem" (read: being used to do it like this in other 
> languages). I'm used to use statically typed languages and for me one of the 
> advantages is that I can be sure that a parameter is of a certain type. So in 
> Java I could write
> 
> void doSomething( data : SomeClass ){ ... }
> 
> and I would be sure at compile time that I would only get SomeClass objects 
> as parameters into the method.
> 
> In learning Python I've understood that I should write code in such a way 
> that it can handle different data and this is fine with me. But what if I 
> have a class where different attributes should only have values of a certain 
> type and everything else is an error.
> 
> For example, if I have an class that defines three attributes: first and last 
> name plus email address. The only valid data types for the first two are 
> strings and for the last an EmailAddress class.
> 
> How should I handle this in Python? 
> 
> Should I use just don't care (but I'm going to send the data to a database so 
> I need to ensure that the data is OK)? Should I use 'isinstance' and check 
> manually? Or should I do something else?

As you have worked out, Python doesn't do automatic type-checking for you.
But if you really do need type-checking, you can do it yourself with
isinstance() and/or type(). It isn't forbidden :-)

Or, you can try just coercing the data you have to the type you want. eg
instead of testing to see if obj is an integer, you might do:

try:
obj = int(obj)
except:
raise TypeError("can't convert to integer")
insert_in_database(obj)

This may be appropriate for your application.

Another method that is sometimes useful: you are expecting an instance of
Waterfowl class, but actually you are happy to use duck-typing: if it
looks like a duck, swims like a duck, and quacks like a duck, it is
close-enough to a duck:

def process_waterfowl(duck):
"""do things to an instance of Waterfowl, or equivalent"""
try:
look, swim, quack = duck.look, duck.swim, duck.quack
except AttributeError:
raise TypeError("object is not a waterfowl")
# process any object that has look, swim and quack attributes
# as if it were a waterfowl
duck.look()
duck.swim()
duck.quack()


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Re: How to get/set class attributes in Python

[vincent wehren]

"Peter Dembinski" <[EMAIL PROTECTED]> schrieb im Newsbeitrag 
news:[EMAIL PROTECTED]
| Kalle Anke <[EMAIL PROTECTED]> writes:
|
| [snap]
|
| >> sys.maxint = -12345
| >
| > I don't really understand what you're meaning.
|
| He meant None = 1 :>

I'm sure you know that has become a no-no in Python 2.4+ ;)

>>> None = 1
SyntaxError: assignment to None
>>>


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Re: How to get/set class attributes in Python

[Peter Dembinski]

Kalle Anke <[EMAIL PROTECTED]> writes:

[snap]

>> sys.maxint = -12345
>
> I don't really understand what you're meaning.

He meant None = 1 :>
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Re: How to get/set class attributes in Python

[Steven D'Aprano]

On Sun, 12 Jun 2005 14:40:26 +, Chris Spencer wrote:

> Being an untyped language, Python does not require you to enforce types. 
> However, for those that require such functionality, you can get away 
> with using the "assert" statement.

Assuming that Python isn't executed with the optimize switch, which
disables assert.

$ python -O
Python 2.3.3 (#1, May  7 2004, 10:31:40)
[GCC 3.3.3 20040412 (Red Hat Linux 3.3.3-7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
py>
py>
py> def tester(x):
... assert type(x) == type(0)
... print "%s is an integer." % x
...
py> tester(3)
'3 is an integer'
py> tester("hello")
'hello is an integer'


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Re: How to get/set class attributes in Python

[Chris Spencer]

Kalle Anke wrote:
> On Sun, 12 Jun 2005 13:59:27 +0200, deelan wrote
> (in article <[EMAIL PROTECTED]>):
> 
> void doSomething( data : SomeClass ){ ... }
> 
> and I would be sure at compile time that I would only get SomeClass objects 
> as parameters into the method.

Being an untyped language, Python does not require you to enforce types. 
However, for those that require such functionality, you can get away 
with using the "assert" statement. For example, if I wanted to make sure 
my function foo was only given instances of class Bar, I'd write 
something like:

 >>> class Bar: pass
 >>> def foo(bar):
... assert isinstance(bar, Bar), "argument is not of type Bar"
... print "argument must be of type Bar"
...
 >>> bar = Bar()
 >>> foo(bar)
argument must be of type Bar
 >>> foo(123)
Traceback (most recent call last):
   File "", line 1, in ?
   File "", line 2, in foo
AssertionError: argument is not of type Bar
 >>>

Chris
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Re: How to get/set class attributes in Python

[George Sakkis]

"alex23" wrote:

> Kalle Anke wrote:
> > I'm coming to Python from other programming languages. I like to
> > hide all attributes of a class and to only provide access to them
> > via methods.
>
> I'm pretty fond of this format for setting up class properties:
>
> class Klass(object):
> def propname():
> def fget: pass
> def fset: pass
> def fdel: pass
> def doc: """pass"""
> return locals()
> propname = property(**propname())
>
> (Replacing 'pass' in the getters & setters et.al with the actual
> functionality you want, of course...)
>
> This method minimises the leftover bindings, effectively leaving the
> getter/setter methods bound only to the property, ensuring they're only
> called when the property is acted upon.
>
> Incidentally, kudos & thanks to whomever I originally stole it from :)
>
> -alex23

And a slight improvement in readability IMHO (for python 2.4+) is the
following recipe:
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/410698.
Using the Property decorator, the property declaration above becomes:

class Klass(object):
@Property # <--- the capitalized 'P' is not a typo
def propname():
'''Documentation'''
def fget: pass
def fset: pass
def fdel: pass

The Property decorator peeks automagically the fget, fset, fdel and
__doc__ from the property's locals(), instead of having the property
return locals() explicitly. Also, it doesn't break when the property
defines local variables other than [fget, fset, fdel, doc]. 

George

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Re: How to get/set class attributes in Python

[Dan Sommers]

On Sun, 12 Jun 2005 15:35:46 +0200,
Kalle Anke <[EMAIL PROTECTED]> wrote:

> In learning Python I've understood that I should write code in such a
> way that it can handle different data and this is fine with me. But
> what if I have a class where different attributes should only have
> values of a certain type and everything else is an error.

> For example, if I have an class that defines three attributes: first
> and last name plus email address. The only valid data types for the
> first two are strings and for the last an EmailAddress class.

> How should I handle this in Python?

> Should I use just don't care (but I'm going to send the data to a
> database so I need to ensure that the data is OK)? Should I use
> 'isinstance' and check manually? Or should I do something else?

One key phrase here is "duck typing":  if it walks like a duck, and
swims like a duck, and quacks like a duck, then it's a duck, or at least
you can assume it's a duck.  For example:

def put_something( file_open_for_writing, something ):
file_open_for_writing.write( str( something ) )

I don't care if file_open_for_writing is "really a file," as long it has
a "write" method that writes a string somewhere.

The other key phrase is "we're all adults here":  if I import sin from
the math module and pass it a unicode string, I get what I deserve.

In lower-level methods/functions, I usually just assume that parameters
are correct, and let the higher-level code catch any exceptions that
occur becuase it (the higher-level code) messed up and passed the wrong
kind of parameter.  For example, UI code *has* to check things that
users type, but once that happens, there's no need for my program to
recheck every parameter on every function call all the way down.  Either
everything works, or that same UI code catches and logs a TypeError or
ValueError or KeyError exception and asks the user what to do next.

Regards,
Dan

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Re: How to get/set class attributes in Python

[Kalle Anke]

On Sun, 12 Jun 2005 15:35:15 +0200, John Machin wrote
(in article <[EMAIL PROTECTED]>):

> OTOH, I beseech you to consider an attitude transplant :-)

;-)

> I.e. put your effort into writing code that allows people to do useful 
> things, rather than opaque guff full of __blahblah__ that stops them 
> from doing dopey or evil things they're usually smart enough or 
> righteous enough not to do anyway.

I'm just trying to protect myself from myself :-) No, I'm playing around with 
different ways of doing things, trying to learn Python and how do things in a 
proper "pythonic" way. 

In this case I'm going to have a class with some properties that are going to 
be stored in a database, I don't want to read all the properties everytime I 
recreate the object from the database but I still want to give the impression 
that the attributes exists and is available. So my idea was to "hide" the 
actual database stuff ...

> BTW, what do you think of this:
> 
> sys.maxint = -12345

I don't really understand what you're meaning.

   jem


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Re: How to get/set class attributes in Python

[John Machin]

Kalle Anke wrote:
> On Sun, 12 Jun 2005 12:20:29 +0200, tiissa wrote
> (in article <[EMAIL PROTECTED]>):
> 
> 
> 
>>You can 'hide' you getsetters using a property attribute[1]:
>>
>>[1]http://docs.python.org/lib/built-in-funcs.html
> 
> 
> Thanks, this is exactly what I was looking for
> 
> 

OTOH, I beseech you to consider an attitude transplant :-)

I.e. put your effort into writing code that allows people to do useful 
things, rather than opaque guff full of __blahblah__ that stops them 
from doing dopey or evil things they're usually smart enough or 
righteous enough not to do anyway.

BTW, what do you think of this:

sys.maxint = -12345

Cheers & HTH,
John

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Re: How to get/set class attributes in Python

[Kalle Anke]

On Sun, 12 Jun 2005 13:59:27 +0200, deelan wrote
(in article <[EMAIL PROTECTED]>):

> the pythonic way is to use "property" (as others have already explained)
> only when is *stricly necessary*. this may clarify things up:

Thanks for the link (although Java was only one of the languages I was 
thinking of). 

Anyway, I got another "problem" (read: being used to do it like this in other 
languages). I'm used to use statically typed languages and for me one of the 
advantages is that I can be sure that a parameter is of a certain type. So in 
Java I could write

void doSomething( data : SomeClass ){ ... }

and I would be sure at compile time that I would only get SomeClass objects 
as parameters into the method.

In learning Python I've understood that I should write code in such a way 
that it can handle different data and this is fine with me. But what if I 
have a class where different attributes should only have values of a certain 
type and everything else is an error.

For example, if I have an class that defines three attributes: first and last 
name plus email address. The only valid data types for the first two are 
strings and for the last an EmailAddress class.

How should I handle this in Python? 

Should I use just don't care (but I'm going to send the data to a database so 
I need to ensure that the data is OK)? Should I use 'isinstance' and check 
manually? Or should I do something else?

(I've been trying to figure out this and other things but I haven't found 
much in books or web sites)

   jem


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Re: How to get/set class attributes in Python

[Kalle Anke]

On Sun, 12 Jun 2005 12:20:29 +0200, tiissa wrote
(in article <[EMAIL PROTECTED]>):


> You can 'hide' you getsetters using a property attribute[1]:
> 
> [1]http://docs.python.org/lib/built-in-funcs.html

Thanks, this is exactly what I was looking for


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Re: How to get/set class attributes in Python

[deelan]

Kalle Anke wrote:
> I'm coming to Python from other programming languages. I like to
> hide all attributes of a class and to only provide access to them
> via methods. 
(...)
> Is this the "Pythonic" way of doing it or should I do it in a different
> way or do I have to use setX/getX (shudder)

the pythonic way is to use "property" (as others have already explained)
only when is *stricly necessary*. this may clarify things up:

"Python Is Not Java"


HTH.

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Re: How to get/set class attributes in Python

[alex23]

Kalle Anke wrote:
> I'm coming to Python from other programming languages. I like to
> hide all attributes of a class and to only provide access to them
> via methods.

I'm pretty fond of this format for setting up class properties:

class Klass(object):
def propname():
def fget: pass
def fset: pass
def fdel: pass
def doc: """pass"""
return locals()
propname = property(**propname())

(Replacing 'pass' in the getters & setters et.al with the actual
functionality you want, of course...)

This method minimises the leftover bindings, effectively leaving the
getter/setter methods bound only to the property, ensuring they're only
called when the property is acted upon.

Incidentally, kudos & thanks to whomever I originally stole it from :)

-alex23

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Re: How to get/set class attributes in Python

[Steve Jorgensen]

On Sun, 12 Jun 2005 03:15:27 -0700, Steve Jorgensen <[EMAIL PROTECTED]>
wrote:

...
>>Is this the "Pythonic" way of doing it or should I do it in a different
>>way or do I have to use setX/getX (shudder)
>
>I'm totally new to Python myself, but my understanding is that
...

Oops - I thought I cancelled that post when I relized I was saying nothing,
but somehow, it got posted.
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Re: How to get/set class attributes in Python

[tiissa]

Kalle Anke wrote:
> I'm coming to Python from other programming languages. I like to
> hide all attributes of a class and to only provide access to them
> via methods. Some of these languages allows me to write something
> similar to this
> 
> int age( )
> {
>   return theAge
> }
> 
> void age( x : int )
> {
>   theAge = x
> }
> 
> (I usually do more than this in the methods).

You can 'hide' you getsetters using a property attribute[1]:

  >>> class person(object):
  ... def __init__(self):
  ... self.age = 0
  ... def set_age(self, age):
  ... print 'set %d' % age
  ... self.__age = age
  ... def get_age(self):
  ... print 'get'
  ... return self.__age
  ... age = property(get_age, set_age)
  ...
  >>> joe = person()
  set 0
  >>> joe.age = 20
  set 20
  >>> print joe.age
  get
  20
  >>>

[1]http://docs.python.org/lib/built-in-funcs.html
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Re: How to get/set class attributes in Python

[Steve Jorgensen]

On Sun, 12 Jun 2005 11:54:52 +0200, Kalle Anke <[EMAIL PROTECTED]> wrote:

>I'm coming to Python from other programming languages. I like to
>hide all attributes of a class and to only provide access to them
>via methods. Some of these languages allows me to write something
>similar to this
>
>int age( )
>{
>  return theAge
>}
>
>void age( x : int )
>{
>  theAge = x
>}
>
>(I usually do more than this in the methods). I would like to do
>something similar in Python, and I've come up with two ways to do
>it: The first one uses the ability to use a variable number of
>arguments ... not very nice. The other is better and uses 
>__setattr__ and __getattr__ in this way:
>
>class SuperClass:
>   def __setattr__( self, attrname, value ):
>   if attrname == 'somevalue':
>   self.__dict__['something'] = value
>   else:
>   raise AttributeError, attrname
>
>   def __str__( self ):
>   return str(self.something)
>
>class Child( SuperClass ):
>   def __setattr__( self, attrname, value ):
>   if attrname == 'funky':
>   self.__dict__['fun'] = value
>   else:
>   SuperClass.__setattr__( self, attrname, value )
>
>   def __str__( self ):
>   return SuperClass.__str__( self ) + ', ' + str(self.fun)
>
>Is this the "Pythonic" way of doing it or should I do it in a different
>way or do I have to use setX/getX (shudder)

I'm totally new to Python myself, but my understanding is that
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How to get/set class attributes in Python

[Kalle Anke]

I'm coming to Python from other programming languages. I like to
hide all attributes of a class and to only provide access to them
via methods. Some of these languages allows me to write something
similar to this

int age( )
{
  return theAge
}

void age( x : int )
{
  theAge = x
}

(I usually do more than this in the methods). I would like to do
something similar in Python, and I've come up with two ways to do
it: The first one uses the ability to use a variable number of
arguments ... not very nice. The other is better and uses 
__setattr__ and __getattr__ in this way:

class SuperClass:
def __setattr__( self, attrname, value ):
if attrname == 'somevalue':
self.__dict__['something'] = value
else:
raise AttributeError, attrname

def __str__( self ):
return str(self.something)

class Child( SuperClass ):
def __setattr__( self, attrname, value ):
if attrname == 'funky':
self.__dict__['fun'] = value
else:
SuperClass.__setattr__( self, attrname, value )

def __str__( self ):
return SuperClass.__str__( self ) + ', ' + str(self.fun)

Is this the "Pythonic" way of doing it or should I do it in a different
way or do I have to use setX/getX (shudder)




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