CODEWITHSUNDEEP
pythonscope
Hans
Hans

Reputation: 1379

In Python, what is the operating principle to change from within a function an object outside of a function?

In Python, I find the following behavior of function related to the variable scope somewhat puzzling. It seems that a function can alter an element of an array outside of the function but not replace the array completely. What is this operating principle?

a = np.array([2,3,5])
b = np.array([2,3,5])

def fun1():
    a[1] = 100
    return

def fun2():
    b = -1.1
    return

fun1()
print(a)

fun2()
print(b)

Upvotes: 0

Views: 62

Answers (1)

zero
zero

Reputation: 172

Python scope go in the LEGB order, First checking Local, Enclosing, Global Scope, and Bult-in scope When you search inside a array element, python searchs for a existing variable and references it, since there is no variable inside the function scope it goes to up in the scope

def fun3():
    c[0] = 0
    return

fun3 would print a error C is not defined wouldn't make c = [0], it would try to find the variable C first in the local scope (inside the function in the case) after it will go up in scope for each search

but when you do b = 1.1, it is going straight to assigning the variable it prioritizes the local scope and simple define its value inside the function

def fun5():
     b = -1.1
     print(b) # prints local value, -1.1
     return
print(b) #prints original array)

in the mean while

def fun6():
    b = [2,3,4]
    b[1] = 500
    print(b) #would print [2,500,4]
    return
print(b) #would print original array

now in fun6 since the new b array is in inside the scope, the b[1] operation references the local scope first, and them only changes the local array without changing the original b array this happens because its the closer reference to b is in the local scope. if you comment the first line, the next closer reference would be the original b declared in the beginning of the file therefore the change would affect the variable b of that scope

user juanpa.arrivillaga also mentioned it on a reply first

Upvotes: 1

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