Python: Unexpected behaviour with dict and a colon. ( my_dict['key']: None ) what does the colon ( : ) do?

Question

I have encountered some unexpected behavior with dicts in python.
It might be because of annotations, but i'm not sure.
Please see the snippet below:

>>> d = {}  # lets create a dictionary and add something to it.
>>> d['a'] = 'a'
>>> d
{'a': 'a'}
>>> d['a']
'a'
>>> # ok all well and good, we know how dicts work, right ?
...
>>> d['z']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'z'
>>>
>>> # yea no key 'z' was inserted. Lets add a colon (:) in the mix 
...
>>> d['z']: d
>>>
>>> # nothing happend! weird...
...
>>> d['z']: d['z']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'z'

>>> # again 'z' was not added to the dict, but what did the colon ???
...
>>> d['z']: d['z']: None
  File "<stdin>", line 1
    d['z']: d['z']: None
                  ^
SyntaxError: invalid syntax
>>>
>>> # NOW the colon gives an SyntaxError !?!
>>> # Lets try to directly use a colon after the dict
...
>>> {'a': 'a'}: ()
  File "<stdin>", line 1
SyntaxError: illegal target for annotation
>>>
>>> # so annotation huh ?
>>> # is it possible to annotate on the fly ?
...
>>> def func(x): return x
... 
>>> d = {'f': func}  # overwrite previous dict 
>>> d['f']: callable
>>> d['f'].__annotations__
{}
>>> # doesn't look like it was taken over
... # lets check if d['f'] returns the callable function: func
...
>>>
>>> def func(x: str) -> callable: return x
... 
>>> d = {'f': func}
>>> d['f'].__annotations__  # lets check for annotations !!
{'x': <class 'str'>, 'return': <built-in function callable>}
>>>
>>> # I'm confused, what does the colon do?
...
>>>

I'm curious why the dict syntax doesn't always react on the given colon (:).
Also the annotations doesn't seem to be the case (or my conclusion is wrong)

So dear readers, what is the purpose of the colon used in the context described above?

Answer 1

Colon is used to separate key and value but it works only if it is in a bracket.

As the document say

Placing a comma-separated list of key:value pairs within the braces adds initial key:value pairs to the dictionary; this is also the way dictionaries are written on output

https://docs.python.org/3/tutorial/datastructures.html

Answer 2

After long and hard thinking i came to conclusion that objects in dictionaries can be iterables too.
That means the colon is allowed after the square brackets.

>>> d = {'a': 'abcd'}
>>> for letter in d['a']: None # <- see here, look at the placement of the colon
...