Passing values in Python

Question

When you pass a collection like list, array to another function in python, does it make a copy of it, or is it just a pointer?

Answer 1

The object is passed. Not a copy, but a reference to the underlying object.

Answer 2

Python passes by reference.

Python passes references-to-objects by value (like Java), and everything in Python is an object. This sounds simple, but then you will notice that some data types seem to exhibit pass-by-value characteristics, while others seem to act like pass-by-reference... what's the deal?

It is important to understand mutable and immutable objects. Some objects, like strings, tuples, and numbers, are immutable. Altering them inside a function/method will create a new instance and the original instance outside the function/method is not changed. Other objects, like lists and dictionaries are mutable, which means you can change the object in-place. Therefore, altering an object inside a function/method will also change the original object outside.

Answer 3

It's just a pointer. Yo have to manually copy() it.

Answer 4

A refernce is passed but if the parameter is an immutable object, modifying it within the method will create a new instance.

Here's a thread on the subject.

Answer 5

By reference:

>>> x = [0,1,2,3]
>>> def foo(x_list):
    x_list[0] = 1


>>> foo(x)
>>> x
[1, 1, 2, 3]

Answer 6

I would also recommend looking at the copy module:

Python documentation for copy

It will help you to understand the underlying issues and how to use it to perform your own deep copy.

Answer 7

Thing is, the whole reference/value concept won't fit into python. Python has no "value" of a variable. Python has only objects and names that refer to objects.

So when you call a function and put a "name" inside the parenthesis, like this:

def func(x): # defines a function that takes an argument
    ... # do something here

func(myname) # calling the function

The actual object that myname is pointing is passed, not the name myname itself. Inside the function another name (x) is given to refer to the same object passed.

You can modify the object inside the function if it is mutable, but you can't change what the outside name is pointing to. Just the same that happens when you do

anothername = myname

Therefore I can answer your question with:

it is "pass by value" but all values are just references to objects.