Why does a = [10, 20, 30, 40] not [10, 20] in this case？-CodePudding

a = [10, 20]
b = a  
b  = [30, 40]  
print(a)
print(b)

I understand that b = = [10, 20, 30, 40], but I don't understand why a = [10, 20, 30, 40]?

CodePudding user response：

In Python, a variable is actually a reference to an object.

>>> a = [10, 20]
>>> b = a
>>> id(a)
140351945078976
>>> id(b)
140351945078976

In this case, both a and b were referenced to same array [10, 20]. When you append b, a will have same object and see it.

If you want to have a different object, you should do b = a[:] or b = a.copy()

CodePudding user response：

b and a both refer to the same list object.

CodePudding user response：

Because of b = a, both a and b point out same memory.

You can check detailed information and examples here: https://www.geeksforgeeks.org/is-python-call-by-reference-or-call-by-value/

In python, each variable to which we assign a value/container is treated as an object. When we are assigning a value to a variable, we are actually binding a name to an object.

CodePudding user response：

What you are experiencing is the concept of references. All objects in Python have a reference and when you assign one to two names a and b, this results in both a and b pointing to the same object.

CodePudding user response：

I ran into this problem when working with Python for a bit. When you say b = a in the code above, you are actually creating a pointer to a in memory. Any changes done on b will affect a because b points to a in memory.

If you aren't familiar with pointers I would check them out for the language you are using.

CodePudding user response：

In Python, if two variables are assigned with the same value, then they point to the same object in the memory, like in this case when b=a then a and b are pointing to the same object in the memory when you make change using b to the object like b = [30, 40] then the object pointing by a and b both has changed giving you the final output [10,20,30,40]

if you use the code below then your output will be [10,20] for a and [10,20,30,40] for b as the object pointing was changed after printing a

`a = [10, 20], b = a, print(a),

b = b [30, 40] ,
print(b)`