How to return a value from __init__ in Python?

I have a class with an __init__ function.

How can I return an integer value from this function when an object is created?

I wrote a program, where __init__ does command line parsing and I need to have some value set. Is it OK set it in global variable and use it in other member functions? If so how to do that? So far, I declared a variable outside class. and setting it one function doesn’t reflect in other function ??

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

Why would you want to do that?

If you want to return some other object when a class is called, then use the __new__() method:

class MyClass(object):
    def __init__(self):
        print "never called in this case"
    def __new__(cls):
        return 42

obj = MyClass()
print obj

Method 2

__init__ is required to return None. You cannot (or at least shouldn’t) return something else.

Try making whatever you want to return an instance variable (or function).

>>> class Foo:
...     def __init__(self):
...         return 42
... 
>>> foo = Foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() should return None

Method 3

From the documentation of __init__:

As a special constraint on constructors, no value may be returned; doing so will cause a TypeError to be raised at runtime.

As a proof, this code:

class Foo(object):
    def __init__(self):
        return 2

f = Foo()

Gives this error:

Traceback (most recent call last):
  File "test_init.py", line 5, in <module>
    f = Foo()
TypeError: __init__() should return None, not 'int'

Method 4

Sample Usage of the matter in question can be like:

class SampleObject(object):

    def __new__(cls, item):
        if cls.IsValid(item):
            return super(SampleObject, cls).__new__(cls)
        else:
            return None

    def __init__(self, item):
        self.InitData(item) #large amount of data and very complex calculations

...

ValidObjects = []
for i in data:
    item = SampleObject(i)
    if item:             # in case the i data is valid for the sample object
        ValidObjects.append(item)

Method 5

The __init__ method, like other methods and functions returns None by default in the absence of a return statement, so you can write it like either of these:

class Foo:
    def __init__(self):
        self.value=42

class Bar:
    def __init__(self):
        self.value=42
        return None

But, of course, adding the return None doesn’t buy you anything.

I’m not sure what you are after, but you might be interested in one of these:

class Foo:
    def __init__(self):
        self.value=42
    def __str__(self):
        return str(self.value)

f=Foo()
print f.value
print f

prints:

42
42

Method 6

__init__ doesn’t return anything and should always return None.

Method 7

You can just set it to a class variable and read it from the main program:

class Foo:
    def __init__(self):
        #Do your stuff here
        self.returncode = 42
bar = Foo()
baz = bar.returncode

Method 8

We can not return value from init. But we can return value using new.

class Car:
    def __new__(cls, speed, unit):
        return (f"{speed} with unit {unit}")


car = Car(42, "km")
print(car)

Method 9

init() return none value solved perfectly

class Solve:
def __init__(self,w,d):
    self.value=w
    self.unit=d
def __str__(self):
    return str("my speed is "+str(self.value)+" "+str(self.unit))
ob=Solve(21,'kmh')
print (ob)

output:
my speed is 21 kmh

Method 10

Just wanted to add, you can return classes in __init__

@property
def failureException(self):
    class MyCustomException(AssertionError):
        def __init__(self_, *args, **kwargs):
            *** Your code here ***
            return super().__init__(*args, **kwargs)

    MyCustomException.__name__ = AssertionError.__name__
    return MyCustomException

The above method helps you implement a specific action upon an Exception in your test

Method 11

solution here
Yes,
trying to return from the init method in python returns errors as it is a constructor of the class you can only assign values for the scope of the class but not return a specific value.
if you want to return a value but do not wish to create a method, you can use
str method

def __init__(self,a):
    self.value=a

   def __str__(self):
    return str("all my return values are possible here")`

Method 12

Well, if you don’t care about the object instance anymore … you can just replace it!

class MuaHaHa():
def __init__(self, ret):
    self=ret

print MuaHaHa('foo')=='foo'

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0