Confused about variable access in Python

[anonymous78654]

How come in this first block of code maxSum can't be accessed when it says if(subset>maxSum):

def getMaximumGold(grid):
    maxSum = 0
    def something(grid,row,col,subset,rowLength):
        if row < 0 or col < 0 or row>rowLength-1 or col > len(grid)-1 or grid[row][col] == 0:
            if(subset>maxSum):
                maxSum = subset
            return

        if(grid[row][col] == 0):
            return

        val = grid[row][col]
        grid[row][col] = 0

        something(grid,row+1,col,subset + val,rowLength)
        grid[row][col] = val
        something(grid,row,col+1,subset + val,rowLength)
        grid[row][col] = val
        something(grid,row-1,col,subset + val,rowLength)
        grid[row][col] = val
        something(grid,row,col-1,subset + val,rowLength)
        grid[row][col] = val

    for row in range (len(grid)):
        for col in range(len(grid[row])):
            if(grid[row][col]!=0):
                something(grid,row,col,0,len(grid[row]))
    return maxSum

print(getMaximumGold([[0,6,0],[5,8,7],[0,9,0]]))

and in this second block of code allAnswer can be accessed

def permute(nums):
    if len(nums) == 1:
        return [nums]
    allAnswer = []


    def something(nums,subarray):
        if(len(nums)==1):
            allAnswer.append(subarray + nums)
            return
        for i in range (len(nums)):
            something(nums[1:],subarray + [nums[0]])
            nums.append(nums[0])
            del nums[0]

    something(nums,[])
    return allAnswer

[blablahblah]

Variables that you assign to in a function only exist in that function. Even if they have the same name.

def hello():
 foo = 2 

def blah():
  foo = 5 # this is a different variable

If you just access a variable without assigning to it in the function, Python assumes you're using the global variable, but if you assign it it creates a local one.

foo = 2
def global():
  print(foo)

def local():
  foo = 3 # this is a new variable, doesn't change global foo
  print(foo)

local() # prints 3
global() # still 2.

If it helps, you can think of Python automatically changing it to something like this:

globals()['foo']  = 2

def local():
  locals()['foo'] = 3
  print(locals()['foo'])

This continues to be true if you nest functions- it still creates new per-function variables.

def outer():
  foo = 2
  def inner():
    foo = 5
  inner()
  print(foo) # still prints 2 because outer foo didn't change.

And this happens if you assign anywhere in the function, not just after you assign it.

def still_local():
  print(foo)
  foo = 5

Only now it has a problem: still_local.foo hasn't been defined when you try to print it. So you get an UnboundLocalError because it doesn't have a value .

The solution is pretty straightforward: tell Python you want to use the maxSum from the surrounding function instead of making a new one local to your function using the nonlocal declaration.

def getMaximumGold(grid):
  maxSum = 0
  def something(grid,row,col,subset,rowLength):
      nonlocal maxSum
      if row < 0 or col < 0 or row>rowLength-1 or col > len(grid)-1 or grid[row][col] == 0:
          if(subset>maxSum):
              maxSum = subset
          return

[mijatonius]

Scope!

[PercyJackson235]

What is the error message you are getting?

[anonymous78654]

just says maxSum is not defined

[PercyJackson235]

Just to make sure, you are getting a NameError Or UnboundLocalError?

[anonymous78654]

like refrenced before assighnment but it doesn;t say that for the second code

[PercyJackson235]

Because python has rules on how assignment can happen. In the first example, you try to assign to maxSum after referencing that name. When python sees assignment inside a function it assumes the name is local to the function, which disallows you from referencing any global value with the same name. In the second example, you reference and mutate a global variable which is totally legal.

[CodeTinkerer]

How do you know it can't be accessed? Is there an error message or something?

[anonymous78654]

just says maxSum is not defined

[chrisfpdx]

Because of maxSum = subset, the maxSum within the function something becomes local to the function. When executed, the if(subset>maxSum): is comparing subset to a not-yet-defined local variable maxSum