从.txt文件读取值后出现KeyError，求原因及解决方法

Let's break down the problem first—your error comes from a mismatch in the data structure you're reading vs. what your code expects.

Looking at your program output:

[(1, 2, 3, 4, 5)]
[(1, 2), (1, 3), (2, 3), (3, 5), (5, 4)]

The first line shows that A (your vertex list) is a list containing a single tuple, not the flat list of integers [1,2,3,4,5] you had in your commented-out code.

Here's why that causes the KeyError:

• When you initialize indegree and outdegree, you loop through elements in A. Right now, A only has one element: the tuple (1,2,3,4,5). So your dictionaries end up with one key: that entire tuple, not the individual integers 1,2,3,4,5.
• Later, when you iterate over edges in B (like (1,2)), you try to increment outdegree[1]—but 1 isn't a key in outdegree (only the tuple is), hence the KeyError.

Two Easy Fixes: #

1. Modify your v.txt file (cleanest solution)

Change the content of v.txt to be a flat list of integers instead of a list with a tuple:

[1,2,3,4,5]

This way, ast.literal_eval will read it directly into the same structure as your original commented-out V variable, and your code will work without changes.

2. Adjust the code to handle the existing file structure

If you can't edit the text file, add a line to unpack the tuple from the list:

import ast

with open('v.txt') as V:
    A = ast.literal_eval(V.read())
    # Unpack the tuple inside the list into a flat integer list
    A = list(A[0])  # Add this line

with open('e.txt') as E:
    B = ast.literal_eval(E.read())

print(A)
print(B)

indegree = {}
outdegree = {}

for x in A:
    indegree[x] = 0
    outdegree[x] = 0

for x,y in B:
    outdegree[x] += 1
    indegree[y] += 1

for x in A:
    print("Outdegree for vertex", x,"=", outdegree[x])
    print("Indegree for vertex", x,"=", indegree[x])

After either fix, A will be the flat integer list your code expects, and the KeyError will disappear.

内容的提问来源于stack exchange，提问作者teddy