"RuntimeError: dictionary changed size during iteration" but it's not changed in the loop

Question

I'm solving this LeetCode problem and here's my code:

class Solution:
    def alienOrder(self, words: List[str]) -> str:
        adjacent = defaultdict(set)
        
        for i in range(1, len(words)):
            w1, w2 = words[i - 1], words[i]
            min_len = min(len(w1), len(w2))
            if len(w1) > len(w2) and w1[:min_len] == w2[:min_len]:
                return ""
            for j in range(min_len):
                if w1[j] != w2[j]:
                    adjacent[w1[j]].add(w2[j])  # modify, not in the loop causing error
                    break
        
        visited = dict()
        res = []
        
        def dfs(c):
            if c in visited:
                return visited[c]
            
            visited[c] = True
            for neighbor in adjacent[c]:  # read only
                if dfs(neighbor):
                    return True
            visited[c] = False
            res.append(c)
            return False
        
        for c in adjacent:  # RuntimeError: dictionary changed size during iteration
            if dfs(c):
                return ''
        return ''.join(reversed(res))

The line for c in adjacent throws "RuntimeError: dictionary changed size during iteration", which I don't understand. I'm not modifying adjacent in dfs(), am I?

Answer 1

The main Problem is when dfs method is called it uses this line

for neighbor in adjacent[c]: 

This just returns the associated value if it exists in defaultdict, if it doesn't exist in it, it creates and adds a key if you try to access key that doesn't exist.

Potential line that triggers accessing adjacent defaultdict without knowing whether it exist or not is

if dfs(neighbor):

neighbor might be or might not be in adjacent defaultdict this causes defaultdict to change. You might check if it exists if not u might want to skip.