Django+PuLP运输问题WebApp开发：NameError报错求助

先帮你搞定眼前的NameError——这确实是个容易疏忽的小错误，咱们看这段仓库约束的代码：

for x in Warehouses:
    prob += lpSum([route_vars[x][y] for x in Distributors]) <= supply[x], "Sum of Products out of Warehouse %s"%x

这里有两个明显的问题：

• 内层列表推导式里你用了x作为循环变量，直接覆盖了外层循环的x（也就是当前遍历的仓库），逻辑完全乱了；
• 变量y根本没在推导式里定义过，Python自然会报它未赋值的错误。

正确的写法应该是，对每个仓库x，遍历所有分销商y，计算从x到每个y的运输量之和：

for x in Warehouses:
    prob += lpSum([route_vars[x][y] for y in Distributors]) <= supply[x], "Sum of Products out of Warehouse %s"%x

同样的，你的分销商约束代码也犯了一模一样的错误：

for y in Distributors:
    prob += lpSum([route_vars[x][y] for y in Warehouses]) >= demand[y], "Sum of Products into Distributors %s"%y

这里内层循环用y覆盖了外层的y，而且x没定义，正确的写法应该是遍历仓库x：

for y in Distributors:
    prob += lpSum([route_vars[x][y] for x in Warehouses]) >= demand[y], "Sum of Products into Distributors %s"%y

再说说你之前遇到的TypeError——当用字符串命名Warehouses和Distributors时（比如Warehouses = ["A", "B", "C"]），出现list indices must be integers or slices, not str的错误，原因是你的costs是二维列表，只能用整数索引，但你却用字符串（比如"A"）去索引它了。

解决这个问题的办法是把costs改成字典结构，和supply、demand的键类型保持一致，比如：

Warehouses = ["A", "B", "C"]
supply = {"A": deliver1, "B": deliver2, "C": deliver3}
Distributors = ["D", "E", "F"]
demand = {"D": receiver1, "E": receiver2, "F": receiver3}
costs = {
    "A": {"D":3, "E":5, "F":7},
    "B": {"D":12, "E":10, "F":9},
    "C": {"D":13, "E":3, "F":9},
}

这样后续用costs[x][y]的时候，不管x和y是字符串还是数字，都能正常索引了。

最后把修正后的完整核心代码贴出来，方便你参考：

Warehouses = [0,1,2]
supply = { 0: deliver1, 1: deliver2, 2: deliver3 }
Distributors = [0, 1, 2]
demand = { 0: receiver1, 1: receiver2, 2: receiver3 }
costs = [ 
    [3, 5, 7],#A Warehouse
    [12, 10, 9],#B Warehouse
    [13, 3, 9],#C Warehouse
]
prob = LpProblem("Transportation Problem",LpMinimize)
Routes = [(x,y) for x in Warehouses for y in Distributors]
route_vars = LpVariable.dicts("Route",(Warehouses,Distributors),0,None,LpInteger)

# 目标函数
prob += lpSum([route_vars[x][y]*costs[x][y] for (x,y) in Routes]), "Sum of Transporting Costs"

# 仓库供应约束（已修正）
for x in Warehouses:
    prob += lpSum([route_vars[x][y] for y in Distributors]) <= supply[x], "Sum of Products out of Warehouse %s"%x

# 分销商需求约束（已修正）
for y in Distributors:
    prob += lpSum([route_vars[x][y] for x in Warehouses]) >= demand[y], "Sum of Products into Distributors %s"%y

prob.writeLP("TransportationProblem.lp")
prob.solve()
print("Status:", LpStatus[prob.status])
for v in prob.variables():
    print(v.name, "=", v.varValue)
print("Total Cost of transportation = ", value(prob.objective))

内容的提问来源于stack exchange，提问作者Marcin Mirzyński