[DougBTX]

Type systems and static checks in general are great. What do you think of the example type in the post above? It seems like the type system might not be expressive enough to handle existing APIs, that’s something which was hard for TypeScript too, but has slowly been getting better. Perhaps that will be the case in Python too.

[chc]

Personally, I use type aliases in cases where I need to describe very dynamic types that make intuitive sense but are too wordy to pattern-match visually. So you might decompose that example like:

    FileSourceType = Union[basestring, file]
    FileSpecType = Union[
        # (filename, file_source)
        Tuple[basestring, Optional[FileSourceType]],
        # (filename, file_source, content_type)
        Tuple[basestring, Optional[FileSourceType], Optional[basestring]],
        # (filename, file_source, content_type, custom_headers)
        Tuple[basestring, Optional[FileSourceType], Optional[basestring], Optional[Headers]]
    ]

    ...

    files: Optional[
        Union[
            Mapping[basestring, FileSpecType], 
            Iterable[Tuple[basestring, FileSpecType]
        ]
    ]

You theoretically lose some "glance value" because now you have to look in two places for the type...but in practice, I think you can figure it out a lot easier than the original example. Obviously you don't want to do this in simple cases, but it can make pathological cases like the above a lot easier to chew on.

[strbean]

Some parts of that example seem pretty silly. URLs for example: he describes how just about anything can be passed in as a URL, and requests attempts to call __str__ or __unicode__ on it and then parse it. Considering (essentially?) all types in Python have __str__, this is a perfectly reasonable place to use the Any type. The issue isn't with the expressiveness of the typing system, but with the absurd flexibility of input handling by requests.

[anderskaseorg]

The right type is ‘object’, not ‘Any’.

‘object’ is the base class of all types: you can put anything into an ‘object’, and you can only do very generic operations like str() on what you get out of an ‘object’ without further checks like isinstance().

‘Any’ is an unsound escape hatch that disables type checking: you can put anything into an ‘Any’, and you can do anything with what you get out of an ‘Any’, and the type checker will make no effort to stop you from doing something wrong.

https://docs.python.org/3/library/typing.html#the-any-type

https://mypy.readthedocs.io/en/latest/dynamic_typing.html#an...

[takeda]

Actually the proper way to handle is to use protocols. They provide already SupportsBytes type, and you can similarly create custom SupportsStr, but IMO it is silly to do that. This approach increases chance of bugs, and it's just easy to use str type and for developer to wrap such value in str(). I believe that's why SupportsStr was not introduced.

[weberc2]

You should check out the Literal type in Python. It provides for some of the stupider things in libraries, like return types that vary based on the value of an input parameter, as in Python’s open() method, which might return a text stream or a bytestream based on the value of the second parameter.