How to create a list of objects?

Question

How do I go about creating a list of objects (class instances) in Python?

Or is this a result of bad design? I need this cause I have different objects and I need to handle them at a later stage, so I would just keep on adding them to a list and call them later.

Do you mean a list of type objects (`[type(x), ... ]`) or instances of a class (`[1, 2, 3]`)? — Skurmedel, Jul 05 '10 at 21:12
@Skurmedel: class instances. I mentioned it specifically in the question :) — user225312, Jul 06 '10 at 05:07
@Alfred: hehe, I just find class instances a bit ambiguous, but I see what you mean now through the selected answer :P — Skurmedel, Jul 06 '10 at 11:29

score 86 · Accepted Answer · edited Sep 16 '16 at 14:43

86

Storing a list of object instances is very simple

class MyClass(object):
    def __init__(self, number):
        self.number = number

my_objects = []

for i in range(100):
    my_objects.append(MyClass(i))

# later

for obj in my_objects:
    print obj.number

edited Sep 16 '16 at 14:43

Umair Ahmad

11
4

answered Jul 05 '10 at 21:24

yanjost

4,933
2
24
27

Where you pass "object" in the first line, is that an actual object or just a number signifying that object instance's number? If I were to write my_objects(1), I would call to the first object, but I don't think the 1 being passed in is an object. This might be entirely orthogonal to the educational purposes of the original response, but it would help me on a different, related topic I'm struggling with. Thanks. – Brad English Aug 04 '17 at 16:31
2

T'was hoping there was a shiny special syntax for this :( – SIGSTACKFAULT Mar 20 '18 at 12:53
@Brad English, that indicates that `MyClass` inherits the properties of an `object` – ChootsMagoots May 26 '18 at 23:10
1

The `i` in `MyClass (i)` refers to the `number` in the class's `__init__` function. – ChootsMagoots May 26 '18 at 23:17

score 21 · Answer 2 · answered Feb 07 '19 at 07:38

21

You can create a list of objects in one line using a list comprehension.

class MyClass(object): pass

objs = [MyClass() for i in range(10)]

print(objs)

answered Feb 07 '19 at 07:38

cryptoplex

1,165
8
14

1

This is really clean syntax. – Galaxy Apr 22 '20 at 03:42
1

Slight detail: objs = [MyClass(i) for i in range(10)] – Brainless Sep 04 '20 at 21:33

score 5 · Answer 3 · answered Jul 05 '10 at 21:13

5

The Python Tutorial discusses how to use lists.

Storing a list of classes is no different than storing any other objects.

def MyClass(object):
    pass

my_types = [str, int, float, MyClass]

answered Jul 05 '10 at 21:13

Chris B.

78,620
24
94
135

score 4 · Answer 4 · answered Jul 06 '10 at 01:26

4

In Python, the name of the class refers to the class instance. Consider:

class A: pass
class B: pass
class C: pass

lst = [A, B, C]

# instantiate second class
b_instance = lst[1]()
print b_instance

answered Jul 06 '10 at 01:26

A. Jesse Jiryu Davis

71
3

score 2 · Answer 5 · answered Jan 15 '18 at 16:59

I have some hacky answers that are likely to be terrible... but I have very little experience at this point.

a way:

class myClass():
    myInstances = []
    def __init__(self, myStr01, myStr02):
        self.myStr01 = myStr01
        self.myStr02 = myStr02
        self.__class__.myInstances.append(self)

myObj01 = myClass("Foo", "Bar")
myObj02 = myClass("FooBar",  "Baz")

for thisObj in myClass.myInstances:
    print(thisObj.myStr01)
    print(thisObj.myStr02)

A hack way to get this done:

import sys
class myClass():
    def __init__(self, myStr01, myStr02):
        self.myStr01 = myStr01
        self.myStr02 = myStr02

myObj01 = myClass("Foo", "Bar")
myObj02 = myClass("FooBar",  "Baz")

myInstances = []
myLocals = str(locals()).split("'")
thisStep = 0
for thisLocalsLine in myLocals:
    thisStep += 1
    if "myClass object at" in thisLocalsLine:
        print(thisLocalsLine)
        print(myLocals[(thisStep - 2)])
        #myInstances.append(myLocals[(thisStep - 2)])
        print(myInstances)
        myInstances.append(getattr(sys.modules[__name__], myLocals[(thisStep - 2)]))

for thisObj in myInstances:
    print(thisObj.myStr01)
    print(thisObj.myStr02)

Another more 'clever' hack:

import sys
class myClass():
    def __init__(self, myStr01, myStr02):
        self.myStr01 = myStr01
        self.myStr02 = myStr02

myInstances = []
myClasses = {
    "myObj01": ["Foo", "Bar"],
    "myObj02": ["FooBar",  "Baz"]
    }

for thisClass in myClasses.keys():
    exec("%s = myClass('%s', '%s')" % (thisClass, myClasses[thisClass][0], myClasses[thisClass][1]))
    myInstances.append(getattr(sys.modules[__name__], thisClass))

for thisObj in myInstances:
    print(thisObj.myStr01)
    print(thisObj.myStr02)

score 0 · Answer 6 · answered Mar 24 '16 at 08:59

0

if my_list is the list that you want to store your objects in it and my_object is your object wanted to be stored, use this structure:

my_list.append(my_object)

answered Mar 24 '16 at 08:59

Kooranifar

1
1

score -2 · Answer 7 · answered Jul 06 '10 at 00:45

I think what you're of doing here is using a structure containing your class instances. I don't know the syntax for naming structures in python, but in perl I could create a structure obj.id[x] where x is an incremented integer. Then, I could just refer back to the specific class instance I needed by referencing the struct numerically. Is this anything in the direction of what you're trying to do?

How to create a list of objects?

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