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Apologies if this is te wrong place to post - I realise the question
is pretty basic...

I have a simple python script that parses a text file and extracts data
from it. Currently
this data is stored in a list by a function, each element of which is
an instance of a class with
member variables to take the data. So for example I have:

camera_list.append(camera(alpha,beta,gamma....))

where

class camera:
def __init__(self,name,site,address,text):
self.name=name
self.site=site
self.address=address
self.text=text

Every time I append an item to this list I pass in the constructor
parameters so end up with all my data in the list which can then be
accessed by doing myList[x].name (for example)

This seemed like a nice solution until I tried to put the entire
parsing program into its own class. I did this so that I could parse
different types of file:

thistype.getdata()
thattype.getdata()
....
thisfile and thatfile would have the same function definitions, but
different implementations as needed.

But now my list generating funtion needs to create inner "camera"
classes when it is itself a member funcition of a class. This seems to
be causing problems - I coudl possibly use nested dictionaries, but
this sounds messy. Ideally I would use structs defined inside the
outer class, but pythn doesn't seem to support these.

I hope I haven't rambled too much here - I'm new to python so have
probably done some silly things

Cheers,

Ben

Ah I see. So istead of creating the classes diectly I use a facroty
class as a buffer - I tell it what I want and it makes the appropriate
instances. I am not enitely sure that applies here though (I may be
wrong)

Currently I have:

camera_list[]

class camera:
def __init__(self,alpha,beta,gamma...):
self.alpha=alpha
self.beta=beta
self.gamma=gamma
...

for (some conditions)
camera_list.append(camera(alpha,beta,gamma...)

the append command creates an instance of the camera class and shoves
it at the end of a list for each itteration of the loop.

However, I want to recover various types of information from the text
file I am parsing - not just (as in the example above) camera data.
However I am trying to keep the interface the same.

so I can have collectSomeData.getData() as well as
collectSomeOtherData.getData()

In each case the getData impementation will be different to suit the
required task.

So something like:

class camera:
def __init__(self,alpha,beta,gamma...):
self.alpha=alpha
self.beta=beta
self.gamma=gamma
...

class list_type1
def createList() :
for (some conditions)
camera_list.append(camera(alpha,beta,gamma...)

class list_type2
def createList() :
for (some other conditions)
camera_list.append(camera(alpha,beta,gamma...)



data1=list_type1()
data2=list_type2()


data1.createList()
data2.createList()

The only change above is that I have taken the list appending loop and
put it into a class of its own.

However, wheras when the method list_type_2 was not in a class tings
worked fine, when put into a class as above and the method attempts to
create an instance of the camera class to append to its list it
complains.

I'm pretty sure there is a solution, and I think I will kick myself
when I work it out (or have it pointed out)!

Cheers,

Ben

Ah - I think I've sorted it out.

I can now have data1.function()

or

data2.function()


etc


However, I can still call function() directly:

function()

which seems very odd Indeed. The only instances of function are within
classes data1 and data2, and these definitions are different, so I
don't see why I can get away with calling it without reference to a
base class - for a start how does it know whivh one to call... will
investigate :-p

Ben wrote:
> Apologies if this is te wrong place to post - I realise the question
> is pretty basic...
>
> I have a simple python script that parses a text file and extracts data
> from it. Currently
> this data is stored in a list by a function, each element of which is
> an instance of a class with
> member variables to take the data. So for example I have:
>
> camera_list.append(camera(alpha,beta,gamma....))
>
> where
>
> class camera:
> def __init__(self,name,site,address,text):
> self.name=name
> self.site=site
> self.address=address
> self.text=text
>
> Every time I append an item to this list I pass in the constructor
> parameters so end up with all my data in the list which can then be
> accessed by doing myList[x].name (for example)
>
> This seemed like a nice solution until I tried to put the entire
> parsing program into its own class. I did this so that I could parse
> different types of file:
>
> thistype.getdata()
> thattype.getdata()
> ....
> thisfile and thatfile would have the same function definitions, but
> different implementations as needed.
>
> But now my list generating funtion needs to create inner "camera"
> classes when it is itself a member funcition of a class. This seems to
> be causing problems - I coudl possibly use nested dictionaries, but
> this sounds messy. Ideally I would use structs defined inside the
> outer class, but pythn doesn't seem to support these.
>
> I hope I haven't rambled too much here - I'm new to python so have
> probably done some silly things
>
Sounds like a fairly simple problem, but just the kind to tax a beginner ...

I think you are mistaken in your belief that the camera classes have to
be declared inside the file-handler classes: it's quite possible to
declare them independently and use them anyway. Here's a class whose
method creates an instance of another class and returns it (though it
could of course just as easily return a list of such objects):

In [9]: class Camera1:
...: def __init__(self, p1, p2):
...: self.p1 = p1
...: self.p2 = p2
...:

In [10]: class Camera2:
....: def __init__(self, p1, p2):
....: self.p1 = p1
....: self.p2 = p2
....:

In [11]: class factory:
....: def CreateCamera(self, x):
....: if x == 1:
....: return Camera1("a", "b")
....: else:
....: return Camera2("x", "y")
....:

In [12]: f = factory()

In [13]: c1 = f.CreateCamera(1)

In [14]: c2 = f.CreateCamera("something else")

In [15]: c1
Out[15]: <gs.model.Camera1 instance at 0x017E99E0>

In [16]: c2
Out[16]: <gs.model.Camera2 instance at 0x0180D940>

Does this help?

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://holdenweb.blogspot.com
Recent Ramblings http://del.icio.us/steve.holden

Finally got it all sorted

I got slightly confused because it seems that if you refer to class
variables from methods within that class you need to explicitly state
that they are self, otherwise, since class variables are all public in
python, things could get quite confusing.

Ben


Ben wrote:
> Ah - I think I've sorted it out.
>
> I can now have data1.function()
>
> or
>
> data2.function()
>
>
> etc
>
>
> However, I can still call function() directly:
>
> function()
>
> which seems very odd Indeed. The only instances of function are within
> classes data1 and data2, and these definitions are different, so I
> don't see why I can get away with calling it without reference to a
> base class - for a start how does it know whivh one to call... will
> investigate :-p