How is the typing.NewType syntax working?

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The Python documentation shows use of NewType like this:

import typing

foo = typing.NewType('foo', int)
some_id = foo(524313)

I don't understand exactly what the syntax is doing. How does that differ from this:

import typing

bar = typing.NewType('foo', int)
some_id = bar(524313)

What are the consequences of 'foo' != 'bar' in the source above?

Or, maybe worse:

import typing

bar = typing.NewType('foo', int)
some_id = foo(524313)
type-hinting typing python
1 Answers

Two different purposes. The parameter to NewType is used to describe the type for diagnostic purposes, like error messages. It can't know what name you actually stored the type in. The return value is the actual class object. Mixing them, while possible, will generally result in tears.

Your third example will fail because there is no foo.

The idea is that, later, you can enforce type checking:

some_id : foo = bar(524313)

That triggers a diagnostic.

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