Python:无法在函数中调用备忘录

Question

无法启动备忘录。如果我将其设置为memo全局，那么后续的打印函数将在第一次打印期间获取存储的备忘录。请告知

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）
• 你不需要帮助

Reference Efficient memorization in Python使用已发布的解决方案#2

修订代码

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

解释

函数签名为：howsum（targetsum，number，combo=None，memo=None）：

参数函数定义中的值称为默认参数。 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

默认值仅在传递实际值时使用

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