What is the proper way to format a multi-line dict in Python?

In Python, I want to write a multi-line dict in my code. There are a couple of ways one could format it. Here are a few that I could think of:

1. mydict = { "key1": 1,"key2": 2,"key3": 3, }

2. mydict = { "key1": 1,"key2": 2,"key3": 3,         }

3. mydict = {"key1": 1,"key2": 2,"key3": 3,}

I know that any of the above is syntactically correct, but I assume that there is one preferred indentation and line-break style for Python dicts. What is it?

Note: This is not an issue of syntax. All of the above are (as far as I know) valid Python statements and are equivalent to each other.