Gurobi与Python新手求助：代码运行出现KeyError: 0错误排查

Hey there! Let's break down why you're hitting that KeyError: 0 on the final line of your code, and how to fix it quickly.

The Root Cause #

Looking at your variable definition:

Z = {(a,b): m.addVar() for a in A for b in B}

You’ve created Z as a dictionary where each key is a tuple (a, b) (like (0,0), (0,1), etc.). But in your objective function, you’re trying to access it like a 2D list:

Z[a][b]

This code first tries to fetch Z[a]—meaning it looks for a key that’s just the integer a (like 0 when a=0). Since your dictionary only has tuple keys, there’s no entry for 0, hence the KeyError.

The Fix #

You have two simple options to resolve this:

1. Use the correct dictionary key syntax
   Replace Z[a][b] with Z[a, b] (or Z[(a, b)]—Python lets you omit parentheses for tuple keys):

   m.setObjective(quicksum((PC+X[a]+Y[a][b])*Z[a, b] for a in A for b in B), GRB.MINIMIZE)
   

2. Define Z as a 2D list instead
   If you prefer the Z[a][b] syntax, redefine your variable as a nested list instead of a dictionary:

   # Replace the original Z definition with this
   Z = [[m.addVar() for b in B] for a in A]
   

   Then your original objective function line will work exactly as written.

Either approach will eliminate the KeyError and let your model compile correctly.

内容的提问来源于stack exchange，提问作者Ruby Pa