Hi
I'm starting a new proyect and i'm in doubt about diferent interfaces
for my clases. My clases will have a lot of attributes and i'm want to
know what aproach could be the best
1) Define one SetAttribute/GetAttribute pair of method for each
attribute.
2) Define one SetAttribute/GetAttribute which argument is a key=value
format.
Any advaice?
Thanks in advance
Zunbeltz Izaola
--
Remove XXX from email: zu******@wm.lc. ehu.esXXX 12 1993
On Wed, 10 Dec 2003 09:38:55 +0100, Zunbeltz Izaola wrote: I'm starting a new proyect and i'm in doubt about diferent interfaces for my clases. My clases will have a lot of attributes and i'm want to know what aproach could be the best
1) Define one SetAttribute/GetAttribute pair of method for each attribute. 2) Define one SetAttribute/GetAttribute which argument is a key=value format.
I asked a similar question myself when I started out with Python, and I
got some very good answers that served me well. http://groups.google.com/groups?hl=e...net%26rnum%3D2
My preferred technique is to just set attributes directly on the object,
like this:
shoe = Shoe()
shoe.laced_up = True
shoe.is_on_foot = True
shoe.resoled = False
You may think that it's a bit nasty because it breaks encapsulation, etc.,
but Python is designed to let you do stuff quickly and cleanly. You're
not always in a situation where you need to enforce access so
strictly. Using attributes directly is more of a Python idiom (see the
last two posts in the thread I posted earlier).
Since that earlier thread Python has been given a nice way of
customising attribute access; properties. Basically, they let you
define get/set methods that get run when you access/assign to an
attribute. You benefit by getting a cleaner interface to your class, and
you only have to define get/set methods when you actually need them (i.e.
when they have side effects other than getting/setting the attribute).
For example:
class Shoe:
def __init__(self):
self._laced_up = False
self.is_on_foot = False
self.resoled = False
def _set_laced_up(s elf, boolean):
self._laced_up = boolean
if boolean:
self.is_on_foot = True
def _get_laced_up(s elf):
return self._laced_up
laced_up = property(_get_l aced_up, _set_laced_up)
I've not run that so it may have syntax errors. It should illustrate the
principle though; you can use attributes directly until you want to take
actions when you set them. Then you can make the attribute "private" and
replace it with a property that access the real attribute, and does
whatever else you want to do when it's accessed.
Hope that makes sense.
-- Graham
"Zunbeltz Izaola" <zu******@wm.lc .ehu.es.XXX> wrote in message
news:m1******** ****@lcpxdf.wm. lc.ehu.es... Hi
I'm starting a new proyect and i'm in doubt about diferent interfaces for my clases. My clases will have a lot of attributes and i'm want to know what aproach could be the best
1) Define one SetAttribute/GetAttribute pair of method for each attribute. 2) Define one SetAttribute/GetAttribute which argument is a key=value format.
Any advaice?
Thanks in advance
Zunbeltz Izaola
I presume you're talking about constructing the object,
not about state changes after it's been constructed and
released into the wild?
In that case, the "best" pattern is to never let the object
appear half constructed. There are a wide variety of ways
of doing this, from passing all the needed parameters into
the constructor, to numerous variations on the factory
pattern. And I wouldn't be adverse to the other responder's
practice of simply plugging values into the instance while
it's still in the factory: most real world objects don't contain
the logic for their construction, so why should our programming
objects?
John Roth
On Wed, 2003-12-10 at 03:24, Graham Ashton wrote: [snip]
I've not run that so it may have syntax errors. It should illustrate the principle though; you can use attributes directly until you want to take actions when you set them. Then you can make the attribute "private" and replace it with a property that access the real attribute, and does whatever else you want to do when it's accessed.
A couple of points on Graham's example:
1. You have to subclass from object in order to use property.
2. When Graham nominalizes private, he means that you can still access
the attribute variable directly--and that's by design.
See example below.
Cheers,
// m
#!/usr/bin/env python
class Shoe:
def __init__(self):
self._size = None
def getSize(self):
print "In getSize..."
return self._size
def setSize(self, size):
print "In setSize..."
self._size = size
size = property(getSiz e, setSize)
class Shoe2(object):
def __init__(self):
self._size = None
def getSize(self):
print "In getSize..."
return self._size
def setSize(self, size):
print "In setSize..."
self._size = size
size = property(getSiz e, setSize)
s = Shoe()
# Since we haven't subclassed from object, the property descriptor
# doesn't work; notice that setSize() isn't getting called...
s.size = 1
print s.size
# We can still access the "private" member variable. In Python,
# private is merely a convention.
print s._size
# Now that we've subclassed from object, our get/set methods are
# called.
s2 = Shoe2()
s2.size = 1
print s2.size
# And we can still access the "private" member variable.
print s2._size
On Wed, 2003-12-10 at 03:24, Graham Ashton wrote: [snip]
I've not run that so it may have syntax errors. It should illustrate the principle though; you can use attributes directly until you want to take actions when you set them. Then you can make the attribute "private" and replace it with a property that access the real attribute, and does whatever else you want to do when it's accessed.
A couple of points on Graham's example:
1. You have to subclass from object in order to use property.
2. When Graham nominalizes private, he means that you can still access
the attribute variable directly--and that's by design.
See example below.
Cheers,
// m
#!/usr/bin/env python
class Shoe:
def __init__(self):
self._size = None
def getSize(self):
print "In getSize..."
return self._size
def setSize(self, size):
print "In setSize..."
self._size = size
size = property(getSiz e, setSize)
class Shoe2(object):
def __init__(self):
self._size = None
def getSize(self):
print "In getSize..."
return self._size
def setSize(self, size):
print "In setSize..."
self._size = size
size = property(getSiz e, setSize)
s = Shoe()
# Since we haven't subclassed from object, the property descriptor
# doesn't work; notice that setSize() isn't getting called...
s.size = 1
print s.size
# We can still access the "private" member variable. In Python,
# private is merely a convention.
print s._size
# Now that we've subclassed from object, our get/set methods are
# called.
s2 = Shoe2()
s2.size = 1
print s2.size
# And we can still access the "private" member variable.
print s2._size
Mark McEahern <ma*******@mcea hern.com> wrote in
news:ma******** *************** *************@p ython.org: # We can still access the "private" member variable. In Python, # private is merely a convention. print s._size
Except that in Python the convention for private variables is to begin them
with two underscore characters, and then it is more than just a convention: class Shoe2(object):
def __init__(self):
self.__size = None
def getSize(self):
print "In getSize..."
return self.__size
def setSize(self, size):
print "In setSize..."
self.__size = size
size = property(getSiz e, setSize)
s2 = Shoe2() s2.size = 1
In setSize... print s2.size
In getSize...
1 print s2.__size
Traceback (most recent call last):
File "<pyshell#1 1>", line 1, in -toplevel-
print s2.__size
AttributeError: 'Shoe2' object has no attribute '__size'
Of course, it doesn't absolutely prevent you accessing the private variable
if you know what you are doing, but then neither do C++ or Java:
print s2._Shoe2__size
1
--
Duncan Booth du****@rcp.co.u k
int month(char *p){return(1248 64/((p[0]+p[1]-p[2]&0x1f)+1)%12 )["\5\x8\3"
"\6\7\xb\1\x9\x a\2\0\4"];} // Who said my code was obscure?
Zunbeltz Izaola <zu******@wm.lc .ehu.es.XXX> writes: Hi
I'm starting a new proyect and i'm in doubt about diferent interfaces for my clases. My clases will have a lot of attributes and i'm want to know what aproach could be the best
1) Define one SetAttribute/GetAttribute pair of method for each attribute. 2) Define one SetAttribute/GetAttribute which argument is a key=value format.
Any advaice?
3) Not have any setters or getters at all.
Ask yourself: "what is the point of getters and setters?".
The most common answers are:
a) Data hiding, privacy, etc.
b) To protect myself against breaking the interface when I change the
implementation of my object.
c) Give you and your users typing exercise.
In Python there is no enforced data hiding or privacy: you use a
leading underscore in the identifier name to indicate to your users
that something is _not_ part of the interface ... so (a) is irrelevant.
As for (b), Python provides properties, which allow you to replace an
attribute with a setter and/or a getter method, while maintaining the
illusion that there is only an attribute. Properties also allow you to
implement read-only (or write-only! (or delete-only!!)) attributes.
(I won't comment on (c) :-)
If you are a fan of Bertrand Meyer, you might also answer
d) Attribute access and method calls should look the same.
I'm not aware of a Python mechanism to make that irrelevant.
Duncan Booth <du****@NOSPAMr cp.co.uk> writes: Mark McEahern <ma*******@mcea hern.com> wrote in news:ma******** *************** *************@p ython.org:
# We can still access the "private" member variable. In Python, # private is merely a convention. print s._size
Except that in Python the convention for private variables is to begin them with two underscore characters, and then it is more than just a convention:
Now, I always believed that "private" in Python is spelt "_", while
"mangle this name for me because I want to avoid name clashes in
multiple inheritance scenarios" is spelt "__".
However, I've heard _so_ many people make the (in my opinion
incorrect) claim that "private" is spelt "__", that I'm beginning to
question my sanity.
Where is the Python spelling of "private" set in stone ?
I find the suggestion that a mangled name is somehow "more private"
than one with a single leading underscore, mildly insulting to the
clients of your code. The implcation is that they can neither work out
the (very complicated and cryptic[*]) mangling scheme, not use dir().
If a Python programmer wants to acces a private attribute, he can do
so, regardless of the number of leading underscores.
[*] Sarcasm alert. (This alert was brought to you by the International
Campaign for Promotion of Understanding between Cultures.)
Jacek Generowicz <ja************ **@cern.ch> wrote in
news:ty******** *****@pcepsft00 1.cern.ch: Except that in Python the convention for private variables is to begin them with two underscore characters, and then it is more than just a convention: Now, I always believed that "private" in Python is spelt "_", while "mangle this name for me because I want to avoid name clashes in multiple inheritance scenarios" is spelt "__".
However, I've heard _so_ many people make the (in my opinion incorrect) claim that "private" is spelt "__", that I'm beginning to question my sanity.
Where is the Python spelling of "private" set in stone ?
Section 5.2.1 of the Python reference manual in the paragraph headed
'Private name mangling'. I find the suggestion that a mangled name is somehow "more private" than one with a single leading underscore, mildly insulting to the clients of your code. The implcation is that they can neither work out the (very complicated and cryptic[*]) mangling scheme, not use dir().
Not at all, the mangling is clearly documented. Its purpose is most
emphatically NOT to prevent access to your clients. If a Python programmer wants to acces a private attribute, he can do so, regardless of the number of leading underscores.
Good points. I suppose it all depends on your definition of 'private'. To
me, there are two reasons why someone might want to use 'private'
variables. One is to prevent accidental nameclashes when someone subclasses
one of your classes, the other is because you don't trust those poxy
programmers who are going to use your class, so you want to burn their
fingers if they try.
The first of these, Python provides. Ok, it doesn't get it quite right (the
name mangling should maybe include the module and package as well as the
class), but it does a reasonable job of minimising accidental nameclashes.
Python does not attempt to address the second of these issues, preferring
to believe that all Python programmers are mature & responsible.
C++ fails dismally on the first count, it doesn't hide the names so adding
a 'private member' to a base class can break the derived classes. It also
fails on the second count, since you can easily bypass private if you wish.
To my mind, data hiding is a good reason for using private variables, but
preventing knowledgeable users from bypassing the privacy is dubious at
best. You are of course free to disagree.
--
Duncan Booth du****@rcp.co.u k
int month(char *p){return(1248 64/((p[0]+p[1]-p[2]&0x1f)+1)%12 )["\5\x8\3"
"\6\7\xb\1\x9\x a\2\0\4"];} // Who said my code was obscure?
Jacek Generowicz wrote: Duncan Booth <du****@NOSPAMr cp.co.uk> writes:
Mark McEahern <ma*******@mcea hern.com> wrote in news:ma******** *************** *************@p ython.org:
# We can still access the "private" member variable. In Python, # private is merely a convention. print s._size
Except that in Python the convention for private variables is to begin them with two underscore characters, and then it is more than just a convention:
Now, I always believed that "private" in Python is spelt "_", while "mangle this name for me because I want to avoid name clashes in multiple inheritance scenarios" is spelt "__".
However, I've heard _so_ many people make the (in my opinion incorrect) claim that "private" is spelt "__", that I'm beginning to question my sanity.
Distinguish "private" and "protected" , as in C++ and perhaps wider usage.
"private" is spelled __ in Python and is supported in a limited fashion
by the language, while *protected* is spelled _ and is by convention.
-Peter This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
by: Herman |
last post by:
Hi everyone, I recently installed the Sun J2SE SDK on my machine, and
I am having trouble running the java.exe interpreter for my Java
compiled code. I remember that I had to set my environment CLASSPATH
variable to whatever the current directory is, but I forgot the exact
characters to denote this. Or that there was an extra call to the...
|
by: Brandon J. Van Every |
last post by:
What's better about Ruby than Python? I'm sure there's something. What is
it?
This is not a troll. I'm language shopping and I want people's answers. I
don't know beans about Ruby or have any preconceived ideas about it. I have
noticed, however, that every programmer I talk to who's aware of Python is
also talking about Ruby. So it...
|
by: Edward K. Ream |
last post by:
Hello all,
First of all, my present state of mind re pep 318 is one of sheepish
confusion. I suspect pep 318 will not affect Leo significantly, but I am
most surprised that an apparently controversial feature is being added
without a notice, say, in comp.lang.python.announce. I say sheepish,
because everyone knows that one should make...
|
by: lechequier |
last post by:
Let's say I define a list of pairs as follows:
>>l =
Can anyone explain why this does not work?
>>h = {}.update(l)
and instead I have to go:
>>h = {}
>>h.update(l)
to initialize a dictionary with the given list of pairs?
|
by: googleboy |
last post by:
Hi there.
I have defined a class called Item with several (about 30 I think)
different attributes (is that the right word in this context?). An
abbreviated example of the code for this is:
class Item(object):
def __init__(self, height, length, function):
params = locals()
| |
by: happyvalley |
last post by:
I just wonder how to pass arguments to this function with a char**
void oldmain(int argv, char**argc)
{
........
}
void main(void)
{
int argv;
|
by: fdu.xiaojf |
last post by:
Hi all, I'm not skilled at programming, so sorry for my ignorance.
My questions:
(1)
which is the better way to calculate the value of attributes of a class ?
for example:
(A)
def cal_attr(self, args):
#do some calculations
|
by: Steven W. Orr |
last post by:
I have a structure I need to pack. I call struct.pack about a dozen times
and each call takes about 53 arguments.
I create a sequence of the arguments:
a1 = 1
a2 = 2
a3 = 3
etc...
a54 = 88
myseq = (a1, a2, a3, a4 etc... a53)
|
by: Hilton |
last post by:
Hi,
for (int i = 0; i < list.Count; i++)
has a hidden performance hit; i.e. list.Count gets evaluated each time, so
we write something like:
int listCount = list.Count;
for (int i = 0; i < listCount; i++)
|
by: lorlarz |
last post by:
I still have a question regarding the following code,
in a commonly used routine. First, Here's the code in
question:
Function.prototype.bind = function(){
var fn = this, args = Array.prototype.slice.call(arguments), object =
args.shift();
return function(){
return fn.apply(object,
args.concat(Array.prototype.slice.call(arguments)));
|
by: marktang |
last post by:
ONU (Optical Network Unit) is one of the key components for providing high-speed Internet services. Its primary function is to act as an endpoint device located at the user's premises. However, people are often confused as to whether an ONU can Work As a Router. In this blog post, we’ll explore What is ONU, What Is Router, ONU & Router’s main...
| |
by: Hystou |
last post by:
Overview:
Windows 11 and 10 have less user interface control over operating system update behaviour than previous versions of Windows. In Windows 11 and 10, there is no way to turn off the Windows Update option using the Control Panel or Settings app; it automatically checks for updates and installs any it finds, whether you like it or not. For...
|
by: agi2029 |
last post by:
Let's talk about the concept of autonomous AI software engineers and no-code agents. These AIs are designed to manage the entire lifecycle of a software development project—planning, coding, testing, and deployment—without human intervention. Imagine an AI that can take a project description, break it down, write the code, debug it, and then...
|
by: isladogs |
last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM).
In this session, we are pleased to welcome a new presenter, Adolph Dupré who will be discussing some powerful techniques for using class modules.
He will explain when you may want to use classes...
|
by: conductexam |
last post by:
I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and then checking html paragraph one by one.
At the time of converting from word file to html my equations which are in the word document file was convert...
|
by: TSSRALBI |
last post by:
Hello
I'm a network technician in training and I need your help.
I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs.
The last exercise I practiced was to create a LAN-to-LAN VPN between two Pfsense firewalls, by using IPSEC protocols.
I succeeded, with both firewalls in...
|
by: adsilva |
last post by:
A Windows Forms form does not have the event Unload, like VB6. What one acts like?
| |
by: muto222 |
last post by:
How can i add a mobile payment intergratation into php mysql website.
|
by: bsmnconsultancy |
last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating...
| |