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

I use #3. Same for long lists, tuples, etc. It doesn't require adding any extra spaces beyond the indentations. As always, be consistent.

mydict = {    "key1": 1,    "key2": 2,    "key3": 3,}mylist = [    (1, 'hello'),    (2, 'world'),]nested = {    a: [        (1, 'a'),        (2, 'b'),    ],    b: [        (3, 'c'),        (4, 'd'),    ],}

Similarly, here's my preferred way of including large strings without introducing any whitespace (like you'd get if you used triple-quoted multi-line strings):

data = (    "iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAABG"    "l0RVh0U29mdHdhcmUAQWRvYmUgSW1hZ2VSZWFkeXHJZTwAAAEN"    "xBRpFYmctaKCfwrBSCrRLuL3iEW6+EEUG8XvIVjYWNgJdhFjIX"    "rz6pKtPB5e5rmq7tmxk+hqO34e1or0yXTGrj9sXGs1Ib73efh1"    "AAAABJRU5ErkJggg==")

First of all, like Steven Rumbalski said, "PEP8 doesn't address this question", so it is a matter of personal preference.

I would use a similar but not identical format as your format 3. Here is mine, and why.

my_dictionary = { # Don't think dict(...) notation has more readability    "key1": 1, # Indent by one press of TAB (i.e. 4 spaces)    "key2": 2, # Same indentation scale as above    "key3": 3, # Keep this final comma, so that future addition won't show up as 2-lines change in code diff    } # My favorite: SAME indentation AS ABOVE, to emphasize this bracket is still part of the above code block!the_next_line_of_code() # Otherwise the previous line would look like the begin of this part of codebad_example = {               "foo": "bar", # Don't do this. Unnecessary indentation wastes screen space               "hello": "world" # Don't do this. Omitting the comma is not good.} # You see? This line visually "joins" the next line when in a glancethe_next_line_of_code()btw_this_is_a_function_with_long_name_or_with_lots_of_parameters(    foo='hello world',  # So I put one parameter per line    bar=123,  # And yeah, this extra comma here is harmless too;              # I bet not many people knew/tried this.              # Oh did I just show you how to write              # multiple-line inline comment here?              # Basically, same indentation forms a natural paragraph.    ) # Indentation here. Same idea as the long dict case.the_next_line_of_code()# By the way, now you see how I prefer inline comment to document the very line.# I think this inline style is more compact.# Otherwise you will need extra blank line to split the comment and its code from others.some_normal_code()# hi this function is blah blahsome_code_need_extra_explanation()some_normal_code()

Since your keys are strings and since we are talking about readability, I prefer :

mydict = dict(    key1 = 1,    key2 = 2,    key3 = 3)