Error accessing class objects in python

Question

I am having some problem accessing class instances. I am calling the class from a procedure, name of instance is defined in some variable. I want the instance name to be created of that value and then want to access it, but when i access it is giving error. Can some one please help to resolve this issue.

class myclass:
  def __init__(self,object):
    self.name = object

def mydef():
   global a1
   b = "a1"
   b = myclass(b)

mydef()
print a1.name

Second Problem: In my actual script, I have to create a large number of such instances from this function (around 100). So defining their name as global would be painful, is there a way i could access those instances outside function without having to declare them as global.

Modification:

class myclass:
    def __init__(self,object,typename):
        self.name = object
        self.typeid = typename

def mydef():
    file_han = open(file,"r")
    while True:
      line = file_han.readline()
      if not line:
        break
      start = line.find('"')
      end = line.find('"',start+1)
      string_f = line[start+1:end]
      myclass(string_f,'a11')        

mydef(file)

print def.name
print def.typeid

File Contents are :
a11 "def"
a11 "ghi"
a11 "eff"

You need to fix your indentation; it is not clear at what level mydef() is supposed to be declared, for example. For help with formatting, see How do I format my code blocks? — Martijn Pieters
– Martijn Pieters, Commented Aug 2, 2012 at 10:33
I've rolled this question back. There was some missing context with the accepted answer (didn't see where you were using globals save for the revision history). — Makoto
– Makoto, Commented Aug 2, 2012 at 13:02

Lauritz V. Thaulow · Accepted Answer · 2012-08-02 12:10:07Z

3

Here's how I'd do it. I don't know why you're messing around with globals, if you'd care to explain, I'll update my answer.

class Myclass(object):
    def __init__(self, name):
        self.name = name

def mydef():
   return Myclass("a1")

a1 = mydef()
print a1.name

Gather your instances in a list:

instances = []
for x in range(1000):
    instances.append(Myclass("Instance {0}".format(x)))
print instance[42].name

Note the changes:

Class names should be capitalized
Use object as the base class of your classes (since python 2.2, but no longer necessary in 3.x)
Don't shadow the built-in object with your parameter name
Just use the string "a1" directly as a parameter instead of assigning it to a variable
Return something from the function instead of passing the result by global variable

RE: Comment

You haven't said anything about the format of these files, so I'll just give an example where the file to be read contains one class name per line, and nothing else:

def mydef(filename):
    ret = []
    with open(filename) as f:
        for line in f:
            # Call `strip` on line to remove newline and surrounding whitespace
            ret.append(Myclass(line.strip()))
    return ret

So if you have several files and wish to add all your instances from all your files to a large list, do it like this:

instances = []
for filename in ["myfile1", "myfile2", "myfile3"]:
    instances.extend(mydef(filename))

RE: OP Edit

def mydef(filename):
    ret = []
    with open(filename, "r") as file_han:
        for line in file_han:
            string_f = line.split('"')[1]
            ret.append(Myclass(string_f))
    return ret

i = mydef("name_of_file")

RE: Comment

Oh, you want to access them by name. Then return a dict instead:

def mydef(filename):
    ret = {}
    with open(filename, "r") as file_han:
        for line in file_han:
            string_f = line.split('"')[1]
            ret[string_f] = Myclass(string_f)
    return ret

i = mydef("name_of_file")
print i["ghi"].name  # should print "ghi"

RE: Comment

If I understand you correctly, you want to have it both ways -- index by both line number and name. Well then why don't you return both a list and a dictionary?

def mydef(filename):
    d = {}
    L = []
    with open(filename, "r") as file_han:
        for line in file_han:
            string_f = line.split('"')[1]
            instance = Myclass(string_f)
            d[string_f] = instance
            L.append(instance)
    return L, d

L, d = mydef("name_of_file")
print d["ghi"].name
print L[3]
print L.index(d["ghi"])

edited Aug 2, 2012 at 12:10

answered Aug 2, 2012 at 10:39

Lauritz V. Thaulow

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8 Comments

sarbjit Over a year ago

Actually in my original script, mydef corresponds to a function which when called will open a file and as per contents of file, will create instance name based on value read from file. Also i will be having multiple classes which needs to be called inside that function (depending upon some keyword in file text). So i can't use return statement as you used because i need to extract the instance name from text which is defined in file.

sarbjit Over a year ago

thx for your help, please see my modified code, as you can see i have to store the value fetched from file in another variable before calling class definition.

Lauritz V. Thaulow Over a year ago

@sarbjit It's no problem to do so, it's just that it was unnecessary in the code you first showed. Just modify your code to return a list of instances, and you're good to go.

sarbjit Over a year ago

I tried returning the list, but it doesn't seems to work, looks like i am missing something.

Lauritz V. Thaulow Over a year ago

@sarbjit I see your updated code. You need to actually return the list at the bottom of the function, and you need to assign it to a variable, like instances = mydef(file). I strongly recommend you look through the python tutorial as you seem to lack the grasp of some basic python concepts.

|

Odomontois · Accepted Answer · 2012-08-02 10:55:44Z

0

You could use class as repository for your instances, for example

class Named(object):
    def __init__(self,name):
        self.name = name

    def __new__(cls,name):
        instance = super(type,cls).__new__(cls,name)
        setattr(cls,name,instance)
        return instance

    def __repr__(self):
        return 'Named[%s]'%self.name

Named('hello')
Named('x123')
Named('this is not valid attribute name, but also working')

print(Named.hello,Named.x123,getattr(Named,'this is not valid attribute name, but also working'))

answered Aug 2, 2012 at 10:55

Odomontois

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