Categories

See More
Popular Forum

MBA (4887) B.Tech (1769) Engineering (1486) Class 12 (1030) Study Abroad (1004) Computer Science and Engineering (988) Business Management Studies (865) BBA (846) Diploma (746) CAT (651) B.Com (648) B.Sc (643) JEE Mains (618) Mechanical Engineering (574) Exam (525) India (462) Career (452) All Time Q&A (439) Mass Communication (427) BCA (417) Science (384) Computers & IT (Non-Engg) (383) Medicine & Health Sciences (381) Hotel Management (373) Civil Engineering (353) MCA (349) Tuteehub Top Questions (348) Distance (340) Colleges in India (334)
See More

How do I pass a variable by reference?

General Tech Bugs & Fixes
Max. 2000 characters
Replies

usr_profile.png
Shreya Bansal

User

( 7 months ago )

The Python documentation seems unclear about whether parameters are passed by reference or value, and the following code produces the unchanged value 'Original'

 

class PassByReference:

    def __init__(self):

        self.variable = 'Original'

        self.change(self.variable)

        print(self.variable)

 

    def change(self, var):

        var = 'Changed'

Is there something I can do to pass the variable by actual reference?

usr_profile.png
Jadav Payeng

User

( 7 months ago )

The problem comes from a misunderstanding of what variables are in Python. If you're used to most traditional languages, you have a mental model of what happens in the following sequence:

a = 1
a = 2

You believe that a is a memory location that stores the value 1, then is updated to store the value 2. That's not how things work in Python. Rather, a starts as a reference to an object with the value 1, then gets reassigned as a reference to an object with the value 2. Those two objects may continue to coexist even though a doesn't refer to the first one anymore; in fact they may be shared by any number of other references within the program.

When you call a function with a parameter, a new reference is created that refers to the object passed in. This is separate from the reference that was used in the function call, so there's no way to update that reference and make it refer to a new object. In your example:

def __init__(self):
    self.variable = 'Original'
    self.Change(self.variable)

def Change(self, var):
    var = 'Changed'

self.variable is a reference to the string object 'Original'. When you call Change you create a second reference var to the object. Inside the function you reassign the reference var to a different string object 'Changed', but the reference self.variable is separate and does not change.

The only way around this is to pass a mutable object. Because both references refer to the same object, any changes to the object are reflected in both places.

def __init__(self):         
    self.variable = ['Original']
    self.Change(self.variable)

def Change(self, var):
    var[0] = 'Changed'

what's your interest


forum_ban8_5d8c5fd7cf6f7.gif