Creating an Instance of a Class with a variable in Python

Question

I'm trying to create a game for my little sister. It is a Virtual Pet sort of thing and the Pet has toys to play with.

I created a class Toy and want to create a function, getNewToy(name, data1, data2, data3, data4, data5).

I want this function to create a new instance of the class Toy, and I want the function to be able to be called multiple times each time creating a new instance.

In my experience you create an instance with:

class Toy:
    def __init__(self, name, data1, data2, data3, data4, data5):
        pass

myToy = Toy(myToy, 1, 2, 3, 4, 5)

then to use methods from the class with:

myToy.method1()

Seeing as I want to have the ability to have multiple toys, each with a playWith() method I want the instance to reflect the name of the Toy each time one is called.

I want the instance to be different each time I call the method getNewToy(,...) and the instance to reflect the name.

Answer 1

Given your edit i assume you have the class name as a string and want to instantiate the class? Just use a dictionary as a dispatcher.

class Foo(object):
    pass

class Bar(object):
    pass

dispatch_dict = {"Foo": Foo, "Bar": Bar}
dispatch_dict["Foo"]() # returns an instance of Foo

Answer 2

If you haven't found it yet, here is Dive into Python's chapter on object-oriented programming.

Here are some more examples, scroll to BankAccount.

---

You can call a class directly to create an instance. Parameters are passed to the __init__ method.

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

imouto = Tamago('imouto')
oba = Tamago('oba')
oba.name # 'oba'
imouto.name # 'imouto'

Answer 3

You can create variable like this:

x = 10 
print(x)

Or this:

globals()['y'] = 100
print(y)

Lets create a new class:

class Foo(object):
    def __init__(self):
        self.name = 'John'

You can create class instance this way:

instance_name_1 = Foo()

Or this way:

globals()['instance_name_2'] = Foo()

Lets create a function:

def create_new_instance(class_name,instance_name):
    globals()[instance_name] = class_name()
    print('Class instance '{}' created!'.format(instance_name))

Call a function:

create_new_instance(Foo,'new_instance') #Class instance 'new_instance' created!
print(new_instance.name) #John

Also we can write generator function:

def create_instance(class_name,instance_name):
    count = 0
    while True:
        name = instance_name + str(count)
        globals()[name] = class_name()
        count += 1
        print('Class instance: {}'.format(name))
        yield True

generator_instance = create_instance(Foo,'instance_') 

for i in range(5):
    next(generator_instance)

#out
#Class instance: instance_0
#Class instance: instance_1
#Class instance: instance_2
#Class instance: instance_3
#Class instance: instance_4

print(instance_0.name) #john
print(instance_1.name) #john
print(instance_2.name) #john
print(instance_3.name) #john
print(instance_4.name) #john

#print(instance_5.name) #error.. we only created 5 instances.. 

next(generator_instance) #Class instance: instance_5
print(instance_5.name) #John  Now it works.. 

Answer 4

Let's say you have three classes: Enemy1, Enemy2, Enemy3. This is how you instantiate them directly:

Enemy1()
Enemy2()
Enemy3()

but this will also work:

x = Enemy1
x()
x = Enemy2
x()
x = Enemy3
x()

Is this what you meant?

Answer 5

If you just want to pass a class to a function, so that this function can create new instances of that class, just treat the class like any other value you would give as a parameter:

def printinstance(someclass):
  print someclass()

Result:

>>> printinstance(list)
[]
>>> printinstance(dict)
{}

Answer 6

Rather than use multiple classes or class inheritance, perhaps a single Toy class that knows what "kind" it is:

class Toy:
    num = 0
    def __init__(self, name, kind, *args):
        self.name = name
        self.kind = kind
        self.data = args
        self.num = Toy.num
        Toy.num += 1

    def __repr__(self):
        return ' '.join([self.name,self.kind,str(self.num)])

    def playWith(self):
        print self

def getNewToy(name, kind):
    return Toy(name, kind)

t1 = Toy('Suzie', 'doll')
t2 = getNewToy('Jack', 'robot')
print t1
t2.playWith()

Running it:

$ python toy.py 
Suzie doll 0
Jack robot 1

As you can see, getNewToy is really unnecessary. Now you can modify playWith to check the value of self.kind and change behavior, you can redefine playWith to designate a playmate:

def playWith(self, who=None):
    if who:  pass
    print self

t1.playWith(t2)

Answer 7

This is a very strange question to ask, specifically of python, so being more specific will definitely help me answer it. As is, I'll try to take a stab at it.

I'm going to assume what you want to do is create a new instance of a datastructure and give it a variable. For this example I'll use the dictionary data structure and the variable mydictionary.

mydictionary = dict()

This will create a new instance of the dictionary data structure and place it in the variable named mydictionary. Alternatively the dictionary constructor can also take arguments:

mydictionary = dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])

Finally, python will attempt to figure out what data structure I mean from the data I give it. IE

mydictionary = {'jack': 4098, 'sape': 4139}

These examples were taken from Here

Answer 8

I think you can use eval. Something like this

def toclass(strcls):
    return eval(strcls)()