Creating a list of objects in Python

Question

I m trying to create a Python script that opens several databases and compares their contents   In the process of creating that script  I ve run into a problem in creating a list whose contents are objects that I ve created   I ve simplified the program to its bare bones for this posting  First I create a new class  create a new instance of it  assign it an attribute and then write it to a list   Then I assign a new value to the instance and again write it to a list    and again and again     Problem is  it s always the same object so I m really just changing the base object   When I read the list  I get a repeat of the same object over and over     So how do you write objects to a list within a loop   Here s my simplified code  class SimpleClass object       pass  x   SimpleClass   Then create an empty list simpleList       Then loop through from 0 to 3 adding an attribute to the instance  x  of SimpleClass for count in range 0 4                each iteration creates a slightly different attribute value  and then prints it to   prove that step is working   but the problem is  I m always updating a reference to  x  and what I want to add to   simplelist is a new instance of x that contains the updated attribute  x attr1    Bob     count print  Loop Count   s Attribute Value  s     count  x attr1  simpleList append x   print  -  20   And here I print out each instance of the object stored in the list  simpleList    and the problem surfaces   Every element of  simpleList  contains the same      attribute value  y   SimpleClass print  Reading the attributes from the objects in the list  for count in range 0 4       y   simpleList count      print y attr1   So how do I  append  extend  copy or whatever  the elements of simpleList so that each entry contains a different instance of the object instead of all pointing to the same one

User · Answer

I think this simply demonstrates what you are trying to achieve:

# coding: utf-8

class Class():
    count = 0
    names = []

    def __init__(self,name):
        self.number = Class.count
        self.name = name
        Class.count += 1
        Class.names.append(name)

l=[]
l.append(Class("uno"))
l.append(Class("duo"))
print l
print l[0].number, l[0].name
print l[1].number, l[1].name
print Class.count, Class.names

Run the code above and you get:-

[<__main__.Class instance at 0x6311b2c>, 
<__main__.Class instance at 0x63117ec>]
0 uno
1 duo
2 ['uno', 'duo']

User · Answer

If I understand correctly your question  you ask a way to execute a deep copy of an object  What about using copy deepcopy   import copy  x   SimpleClass    for count in range 0 4     y   copy deepcopy x            y attr1    Bob     count   A deepcopy is a recursive copy of the entire object  For more reference  you can have a look at the python documentation  https   docs python org 2 library copy html

User · Answer

It shouldn t be necessary to recreate the SimpleClass object each time  as some are suggesting  if you re simply using it to output data based on its attributes   However  you re not actually creating an instance of the class  you re simply creating a reference to the class object itself   Therefore  you re adding a reference to the same class attribute to the list  instead of instance attribute   over and over   Instead of   x   SimpleClass   you need   x   SimpleClass

User · Answer

I think this simply demonstrates what you are trying to achieve:

# coding: utf-8

class Class():
    count = 0
    names = []

    def __init__(self,name):
        self.number = Class.count
        self.name = name
        Class.count += 1
        Class.names.append(name)

l=[]
l.append(Class("uno"))
l.append(Class("duo"))
print l
print l[0].number, l[0].name
print l[1].number, l[1].name
print Class.count, Class.names

Run the code above and you get:-

[<__main__.Class instance at 0x6311b2c>, 
<__main__.Class instance at 0x63117ec>]
0 uno
1 duo
2 ['uno', 'duo']

User · Answer

To fill a list with seperate instances of a class  you can use a for loop in the declaration of the list  The   multiply will link each copy to the same instance   instancelist     MyClass   for i in range 29     and then access the instances through the index of the list   instancelist 5  attr1    whamma

User · Answer

It shouldn t be necessary to recreate the SimpleClass object each time  as some are suggesting  if you re simply using it to output data based on its attributes   However  you re not actually creating an instance of the class  you re simply creating a reference to the class object itself   Therefore  you re adding a reference to the same class attribute to the list  instead of instance attribute   over and over   Instead of   x   SimpleClass   you need   x   SimpleClass

User · Answer

Create a new instance each time  where each new instance has the correct state  rather than continually modifying the state of the same instance   Alternately  store an explicitly-made copy of the object  using the hint at this page  at each step  rather than the original

User · Answer

To fill a list with seperate instances of a class  you can use a for loop in the declaration of the list  The   multiply will link each copy to the same instance   instancelist     MyClass   for i in range 29     and then access the instances through the index of the list   instancelist 5  attr1    whamma

User · Answer

Create a new instance each time  where each new instance has the correct state  rather than continually modifying the state of the same instance   Alternately  store an explicitly-made copy of the object  using the hint at this page  at each step  rather than the original

User · Answer

To fill a list with seperate instances of a class  you can use a for loop in the declaration of the list  The   multiply will link each copy to the same instance   instancelist     MyClass   for i in range 29     and then access the instances through the index of the list   instancelist 5  attr1    whamma

User · Answer

It shouldn t be necessary to recreate the SimpleClass object each time  as some are suggesting  if you re simply using it to output data based on its attributes   However  you re not actually creating an instance of the class  you re simply creating a reference to the class object itself   Therefore  you re adding a reference to the same class attribute to the list  instead of instance attribute   over and over   Instead of   x   SimpleClass   you need   x   SimpleClass

User · Answer

You demonstrate a fundamental misunderstanding   You never created an instance of SimpleClass at all  because you didn t call it   for count in xrange 4       x   SimpleClass       x attr   count     simplelist append x    Or  if you let the class take parameters  instead  you can use a list comprehension   simplelist    SimpleClass count  for count in xrange 4

User · Answer

If I understand correctly your question  you ask a way to execute a deep copy of an object  What about using copy deepcopy   import copy  x   SimpleClass    for count in range 0 4     y   copy deepcopy x            y attr1    Bob     count   A deepcopy is a recursive copy of the entire object  For more reference  you can have a look at the python documentation  https   docs python org 2 library copy html

User · Answer

You demonstrate a fundamental misunderstanding   You never created an instance of SimpleClass at all  because you didn t call it   for count in xrange 4       x   SimpleClass       x attr   count     simplelist append x    Or  if you let the class take parameters  instead  you can use a list comprehension   simplelist    SimpleClass count  for count in xrange 4

User · Answer

You demonstrate a fundamental misunderstanding   You never created an instance of SimpleClass at all  because you didn t call it   for count in xrange 4       x   SimpleClass       x attr   count     simplelist append x    Or  if you let the class take parameters  instead  you can use a list comprehension   simplelist    SimpleClass count  for count in xrange 4

User · Answer

If I understand correctly your question  you ask a way to execute a deep copy of an object  What about using copy deepcopy   import copy  x   SimpleClass    for count in range 0 4     y   copy deepcopy x            y attr1    Bob     count   A deepcopy is a recursive copy of the entire object  For more reference  you can have a look at the python documentation  https   docs python org 2 library copy html

User · Answer

Create a new instance each time  where each new instance has the correct state  rather than continually modifying the state of the same instance   Alternately  store an explicitly-made copy of the object  using the hint at this page  at each step  rather than the original

User · Answer

It shouldn t be necessary to recreate the SimpleClass object each time  as some are suggesting  if you re simply using it to output data based on its attributes   However  you re not actually creating an instance of the class  you re simply creating a reference to the class object itself   Therefore  you re adding a reference to the same class attribute to the list  instead of instance attribute   over and over   Instead of   x   SimpleClass   you need   x   SimpleClass

User · Answer

If I understand correctly your question  you ask a way to execute a deep copy of an object  What about using copy deepcopy   import copy  x   SimpleClass    for count in range 0 4     y   copy deepcopy x            y attr1    Bob     count   A deepcopy is a recursive copy of the entire object  For more reference  you can have a look at the python documentation  https   docs python org 2 library copy html

User · Answer

You demonstrate a fundamental misunderstanding   You never created an instance of SimpleClass at all  because you didn t call it   for count in xrange 4       x   SimpleClass       x attr   count     simplelist append x    Or  if you let the class take parameters  instead  you can use a list comprehension   simplelist    SimpleClass count  for count in xrange 4

User · Answer

Create a new instance each time  where each new instance has the correct state  rather than continually modifying the state of the same instance   Alternately  store an explicitly-made copy of the object  using the hint at this page  at each step  rather than the original

User · Answer

To fill a list with seperate instances of a class  you can use a for loop in the declaration of the list  The   multiply will link each copy to the same instance   instancelist     MyClass   for i in range 29     and then access the instances through the index of the list   instancelist 5  attr1    whamma

[python] Creating a list of objects in Python

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