Why Does List Argument in Python Behave Like ByRef?

Question

This may be for most languages in general, but I'm not sure. I'm a beginner at Python and have always worked on copies of lists in C# and VB. But in Python whenever I pass a list as an argument and enumerate through using a "for i in range," and then change the value of the list argument, the input values actually changes the original list. I thought Python was supposed to pass arguments by value by default so that once the function is finished I still have the original values from before I called the function. What am I missing? Thanks!

Answer 1

Python does pass arguments by value but the value you are receiving is a copy of the reference (incidentally this is the exact same way that C#, VB.NET, and Java behave as well).

This is the important thing to remember:

Objects are not passed by reference - object references are passed by value.

Since you have a copy of the reference, any operation on what that reference points to will be just as if you were holding the original reference itself.