Hi,
I have a basic programming question regarding classes
in python. I want to have a list of "primaryCla ss"
instances, and in each instance of primaryClass I would
like a list of "subClass" instances. When each primaryClass
instance is created I want to put one instance of subClass
into the "sClasses" list (see code). The attached
code is what I came up with, but it doesn't do what I
want. For some reason the "sClasses" list in each primaryClass
instance is the same! That is, when a new primaryClass is instantiated,
and the .append(subClas s()) is called in the constructor, the
subClass is appended to ALL instances of primaryClass, instead
of just the new instance of primaryClass. The
print statements in the code confirm this.
I think I must be missing something fundamental. Thanks
in advance.
jgw
=============== =============== =============== =============== ==========
############### ############### ############### ############### ############### ####
class primaryClass:
name=""
sClasses = []
def __init__(self,n ame):
self.name = name
self.sClasses.a ppend(subClass( "default"))
############### ############### ############### ############### ############### ####
class subClass:
name=""
def __init__(self,n ame):
self.name = name
############### ############### ############### ############### ############### ####
port = [] # make a list
port.append(pri maryClass("firs tclass"))
print 'port contents'
for i in range(len(port) ):
print i, port[i].name
for j in range(len(port[i].sClasses)):
print i, j, port[i].sClasses[j].name
port.append(pri maryClass("seco ndclass"))
print 'port contents'
for i in range(len(port) ):
print i, port[i].name
for j in range(len(port[i].sClasses)):
print i, j, port[i].sClasses[j].name
port.append(pri maryClass("thir dclass"))
print 'port contents'
for i in range(len(port) ):
print i, port[i].name
for j in range(len(port[i].sClasses)):
print i, j, port[i].sClasses[j].name 2 1971
John Wohlbier wrote: class primaryClass:
Remove the following two lines. Variables you put here are shared between
all instances of the class.
name="" sClasses = []
def __init__(self,n ame):
self.sClasses = [] # now you'll get a new list for each instance self.name = name self.sClasses.a ppend(subClass( "default"))
############### ############### ############### ############### ############### #### class subClass:
The following line is superfluous.
name=""
def __init__(self,n ame): self.name = name
############### ############### ############### ############### ############### ####
port = [] # make a list port.append(pri maryClass("firs tclass"))
print 'port contents'
More idiomatic is:
for p in port:
print p.name
for s in s.sClasses:
#...
Or, if you really need the indices:
for i, p in enumerate(port) :
print i, p.name
#...
for i in range(len(port) ): print i, port[i].name for j in range(len(port[i].sClasses)): print i, j, port[i].sClasses[j].name
port.append(pri maryClass("seco ndclass"))
Hey, I've seen the following section before. Never duplicate code - that's
what functions are for.
print 'port contents' for i in range(len(port) ): print i, port[i].name for j in range(len(port[i].sClasses)): print i, j, port[i].sClasses[j].name
port.append(pri maryClass("thir dclass")) print 'port contents' for i in range(len(port) ): print i, port[i].name for j in range(len(port[i].sClasses)): print i, j, port[i].sClasses[j].name
Peter
> class primaryClass: name="" sClasses = []
def __init__(self,n ame): self.name = name self.sClasses.a ppend(subClass( "default"))
Try the below instead.
class primaryClass:
def __init__(self,n ame):
self.name = name
self.sClasses = [subClass("defau lt")]
In general, everything defined in the class-level scope is shared among
all instances of that class, even before creation. When you re-assign
'name' in __init__ in your original version, or really anywhere else,
'name' is no longer referring to the shared 'primaryClass.n ame' object,
it is referring to the object assigned. When you used
self.sClasses.a ppend(), you were modifying the shared list in-place,
which is why all instances of primaryClass shared the list.
Also, you don't need to define all class variables (or really any)
outside in the class-level scope. It is convenient for some shared
immutables (strings, integers, tuples, floats, functions) on occasion,
but unless you know what you are doing with the mutables (lists,
dictionaries, arrays (from the array module), etc.), you should be careful.
- Josiah This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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