Python: 无法在函数中调用 Memoization

Question

无法启动记忆。如果我将其设置为 memoglobal，那么随后的打印功能将在第一次打印时获取存储的备忘录。请指教，

def howsumhelper(targetsum,numbers):
      memo = dict() #this memoization will not initiate. Why?
      return howsum(targetsum,numbers,[])


def howsum(targetsum,numbers,combo):
      print("Debug==",memo)
      if targetsum in memo: return memo[targetsum]
      if targetsum == 0:return combo
      if targetsum < 0: return None

      for number in numbers:
            remainder = targetsum - number
            if howsum(remainder,numbers,combo) != None:
                  combo.append(number)
                  memo[targetsum] = combo
                  return combo
      memo[targetsum] = None
      return None

print(howsumhelper(7,[3,4])) #output should be [3,4]
print(howsumhelper(8,[2,3])) #output should be [2,2,2,2]
print(howsumhelper(7,[2,4])) #output should be None

Answer 1

建议进行以下更改

使 memo 成为 howsum 的可变默认参数（使其在函数中可调用）
你不需要 howsumhelper

Reference--Efficient memorization in Python--使用发布的解决方案 #2

修改后的代码

def howsum(targetsum,numbers,combo=None,memo=None):
      # Initiaze default arguments
      if combo is None:
        combo = []
      if memo is None:
        memo = {}
        
      #print("Debug==",memo)
      if targetsum in memo: return memo[targetsum]
      if targetsum == 0:return combo
      if targetsum < 0: return None

      for number in numbers:
            remainder = targetsum - number
            if howsum(remainder,numbers,combo,memo) != None:
                  combo.append(number)
                  memo[targetsum] = combo
                  return combo
      memo[targetsum] = None
      return None

print(howsum(7,[3,4])) #output should be [3,4] #actually or [4, 3]
print(howsum(8,[2,3])) #output should be [2,2,2,2]
print(howsum(7,[2,4])) #output should be None

输出

[4, 3]
[2, 2, 2, 2]
None

说明

函数签名是：howsum(targetsum,numbers,combo=None,memo=None):

函数定义中的参数值称为默认参数。所以 combo 和 memo 都有默认参数。

Python 的默认参数在定义函数时计算一次，而不是每次调用函数时计算一次。

只有在传递实际值时才使用默认值。

因此：

howsum(7,[3,4]) # uses default values, so called with combo = None and memo = None
howsum(8,[2,3]) # uses default values, so called with combo = None and memo = None
howsum(7,[2,4]) # uses default values, so called with combo = None and memo = None

但是：

howsum(remainder,numbers,combo,memo) # does not use defaults for combo and memo
                                     # since we are passing values for these arguments

Python: 无法在函数中调用 Memoization

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