Why it trows the error "local reference 'a' referenced before assignment"?

Question

#this trows that error
a= [1,2,3]
def add(n):
a+=[n]

add(4)
print(a)

#but if instead of a+=[n], I put a.extend([n]), it works. What is happening here?

Answer 1

I think a (which is a list) is accessible inside and outside the function cause it is declared before using the method extend, so you can apply this method that exists in <class list> without a problem, but when you tried a+= it's like you try to define a new variable called a and you add it to it self ( 'a' as a new variable doesn't have a value so a+= will return an error). if you put a = a +1 a should already have a value or it will return the error you've got.

Answer 2

if you want to modify a inside the function make it global by adding global a inside the function. without it you'll manipulate just local variables inside the function.

def add(n):
global a
a+=[n]

Answer 3

Well, now it makes more sense, but it is still a tricky one...thanks for the help!

Answer 4

But a.extend([n]) inside the function wouldn't also modify a? And it works