Python typing not working when declaring a variable

Question

I'm using python 3.6, and wanted to use typing, because it's nice, you get better linting and you can catch errors before runtime, also autocomplete will be available.

I have this method :

def add_to_dynamic_dict(filenames_by_satustag: Dict[str, List[str]], status_tag: str, filename: str) -> Dict[str, List[str]]:

So before calling it I created a dictionary like this :

filenames_by_satustag = Dict[str, List[str]]

But when running the script I get

TypeError: typing.Dict[str, typing.List[str]] is not a generic class

on that line.

But it is correct according to the documentation :

https://docs.python.org/3.6/library/typing.html (26.1.1 type aliases, second bloc)

What did I do wrong?

Thanks.

Can you post the full error and a minimal reproducible example of the code raising it? I am not getting any errors from that line... — Tomerikoo
– Tomerikoo, Commented Nov 25, 2020 at 14:38
filenames_by_satustag = Dict[str, List[str]] does not create a dictionary... — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Nov 25, 2020 at 14:39

Tristan Nemoz · Accepted Answer · 2020-11-25 14:41:52Z

3

I think you are confused with type aliases. The method you declare expects a first argument, which you called filenames_by_satustag with type Dict[str, List[str]]. However, if I'm not mistaken, you call your method with a variable filenames_by_satustag which has value Dict[str, List[str]]. Hence, its type is the type of Dict[str, List[str]], that is typing._GenericAlias. If you want to create a new type called filenames_by_satustag, you should do the following:

filenames_by_satustag = Dict[str, List[str]]

# The function return can also be replaced by filenames_bu_satustag
def add_to_dynamic_dict(some_variable: filenames_by_satustag, status_tag: str, filename: str) -> Dict[str, List[str]]:

If what you wanted, however, was not to create a type alias, but just to create a variable filenames_by_satustag, you could do it like this:

filenames_by_satustag: Dict[str, List[str]] = {}

answered Nov 25, 2020 at 14:41

Tristan Nemoz

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1 Comment

Heetola Over a year ago

oooh, I get it now, what I did was say : now you can use filenames_by_satustag as a Dict[str, List[str]] type Thanks

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