Python字典语法解析咨询：graph["start"]相关代码含义

Hey there! Let's break down this code line by line to clear up your confusion completely.

First, let's set the context: graph must be a dictionary (not a list/array) here. If it were an array, using a string like "start" as an index would throw an error—arrays only accept integer indices.

Let's walk through each line:

1. graph["start"] = {}
   This line adds a new entry to the graph dictionary. The key for this entry is the string "start", and the value we're assigning to it is an empty dictionary ({}). So after this line, graph looks like:

   {"start": {}}
   

   To directly answer part of your question: this is absolutely mapping the key "start" to a dictionary value in the graph dictionary.

2. graph["start"]["a"] = 6
   Here, we're first accessing the value associated with "start" in graph (which we just set to an empty dictionary). Then, we add a new entry to that inner dictionary: the key "a" maps to the integer 6. Now graph becomes:

   {"start": {"a": 6}}
   

Now let's address your specific questions:

• It's not initializing an array—graph is a dictionary, and we're working with nested dictionaries here.
• Yes, it is initializing a mapping where "start" is the key and a dictionary is the value (first line), then extending that inner dictionary.
• Exactly, graph["start"] is a dictionary type—we explicitly set it to {} (an empty dictionary) and then modified it like one.

To make this concrete, here's the full, runnable code with output:

# Initialize the parent dictionary first
graph = {}
graph["start"] = {}
graph["start"]["a"] = 6

print(graph)  # Output: {'start': {'a': 6}}

内容的提问来源于stack exchange，提问作者user3422290