Question

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What is the difference between LIST.append(1) and LIST = LIST + [1] (Python)

I have a doubt on how parameters are passed to functions and their mutability, especially in the case of lists.

Consider the following…

def add_list(p):
    p = p + [1]

def append_list(p):
   p.append(1)

p = [1, 2, 3]

add_list(p)
print p

append_list(p)
print p

The output I get is…

[1, 2, 3]
[1, 2, 3, 1]

Why does the original list change when I append to it in a function, but is unchanged if I use the operator +?

Answer 1

Assignment operator within a function creates a new local variable.
In the *add_list* function your p is local variable.