Python undefined variable issue

Question

During python programming, i met an undefined error.

The error is about 'flag' variable and i can use it in the above print sentence without any error.

Why the error occur only for index of array?

If you have any ideas, please share it for me.

Thank you for your interest.

class Solution:
    def rob(self, nums: List[int]) -> int:
        global memo
        
        memo = [[0 for j in range(len(nums))] for i in range(2)]
        flag = 0
        #print("===========================")
        def dp(i : int) -> int:
            global flag
            
            if i >= len(nums):
                #print(i,"return 0")
                return 0

            if i == 1:
                flag = 1
                #print(str(i) + " : " + str(globals), memo)

            if memo[flag][i] is not 0: # <= undefined error occur because of flag
                #print(i,"*",memo)
                return memo[i]
            memo[i] = max(dp(i+1), (dp(i+2) + nums[i]))
            if i is len(nums) - 1 and i is not 0:
                result = nums[i] * flag
                flag = 0
                #print(str(i) + " ! " , result)
                return result
            #print(i,"-",memo)
            return memo[i]
        return max(dp(0), dp(1))

NameError: name 'flag' is not defined. Did you mean: 'flags'?
    if memo[flag][i] is not 0:
Line 21 in dp (Solution.py)
    return max(dp(0), dp(1))
Line 37 in rob (Solution.py)
    ret = Solution().rob(param_1)
Line 57 in _driver (Solution.py)
    _driver()
Line 68 in <module> (Solution.py)
  if i is len(nums) - 1 and i is not 0:
  if memo[flag][i] is not 0:

They're the same for small integers, but I don't think you should depend on that. — Barmar, Jun 04 '22 at 02:22
Well, @JoeyM `is not` accomplish what is required but how it is using here is not 'best way' because is recommended when you try to compare with None not with 0. — Brad Figueroa, Jun 04 '22 at 02:24
@JoeyM https://stackoverflow.com/questions/1504717/why-does-comparing-strings-using-either-or-is-sometimes-produce-a-differe — kmoser, Jun 04 '22 at 02:25
Oh wow @kmoser I didn't know this. 4th year Python programmer and still learning every day — Joey M, Jun 04 '22 at 02:28
@JoeyM Python is not English, or in Python: `"python != "english"`. `is not` compares object identity, while `!=` compares object contents. — SethMMorton, Jun 04 '22 at 02:30
@Barmar "They're the same for small integers, but I don't think you should depend on that." - and that optimization is only true for CPython, making your recommendation to not depend on it so much more valuable. — SethMMorton, Jun 04 '22 at 02:32
@JoeyM **absolutely not**. `is not` and `!=` are *entirely different thigns — juanpa.arrivillaga, Jun 04 '22 at 02:47
@Barmar no, it is not the same for small integers. It is never the same — juanpa.arrivillaga, Jun 04 '22 at 02:48
Noted, @SethMMorton thank you so much. This will definitely help me in the future :) — Joey M, Jun 04 '22 at 02:50

score 0 · Accepted Answer · answered Jun 04 '22 at 02:22

Well, the problem happens because you have a function inside another function so global key word won't work. There are four scopes in Python: Local, Enclose, Global and Built-in and rob function is in Enclose scope, so what you have to do is change for nonlocal key word as following:

def dp(i : int) -> int:
    nonlocal flag

    if i >= len(nums):
       #print(i,"return 0")
       return 0

BTW using local or nonlocal aren't recommended so try to use in other way.

Oh i see. now i've solved my problems. thank you for providing keywords. — setgo 1, Jun 04 '22 at 03:17

Python undefined variable issue

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