How to create a new typing.Dict[str,str] and add items to it

Question

There is a great Q/A here already for creating an untyped dictionary in Python. I'm struggling to figure out how to create a typed dictionary and then add things to it.

An example of what I am trying to do would be...

return_value = Dict[str,str]
for item in some_other_list:
  if item.property1 > 9:
    return_value.update(item.name, "d'oh")
return return_value

... but this gets me an error of descriptor 'update' requires a 'dict' object but received a 'str'

I've tried a few other permutations of the above declaration

return_value:Dict[str,str] = None

errors with 'NoneType' object has no attribute 'update'. And trying

return_value:Dict[str,str] = dict() 

or

return_value:Dict[str,str] = {}

both error with update expected at most 1 arguments, got 2. I do not know what is needed here to create an empty typed dictionary like I would in C# (var d = new Dictionary<string, string>();). I would rather not eschew type safety if possible. What am I missing or doing incorrectly?

Coelacanth

return_value isn't a dict; it's an object that represents the type of a dict. The things you import from typing are only for type hints, not actual values. – Grizelda

Note, type annotations do not give you type safety. There is no "typed dict" in Python, not built-in anyway. Annotations are for documentation or by use from third-party static time type-checkers, like mypy. – Acknowledge

Also, assign a single key-value pair in a dict, you do my_dict[key] = value. .update takes another mapping, e.g. another dict, and updates the key-value pairs in the mapping you pass as an argument. So you could do my_dict.update({key:value}) but that is rather wasteful for a single item(creating a completely unnecessary intermediate dict object). Alternatively, you can pass keyword arguments to .update, like so: my_dict.update(foo='bar', baz='bing') which sometimes is nice from a readability perspective – Acknowledge

Answer 1

The last 2 ones are the right usage of Dict, but you updated the dictionary with incorrect syntax inside the for loop.

return_value: Dict[str, str] = dict()
for item in some_other_list:
    if item.property1 > 9:
        return_value[item.name] = "d'oh"
return return_value

Obtrude

Thanks, I figured this out about 15 minutes after posting. It only took me a few hours to get to that point =/. The examples for Dict[] that I could find had .update(key,value). – Coelacanth

Answer 2

Things like Dict aren't intended to be used at runtime; they are objects that represent types used for static type analysis. If you want a dict, you have to use

return_value = dict()

You can't use Dict to create an object with a restricted runtime type.

Grizelda

or more idiomatically, {} ;) – Acknowledge

Just wanted to emphasize that dict and Dict are different :) – Grizelda

in the same vein, would lists be more commonly specified as () instead of list()? – Coelacanth

() is the empty tuple; [] is the empty list. Using the literals is more efficient, as no name lookup is required. – Grizelda