Python equivalent for typedef

Question

What is the python way to define a (non-class) type like:

typedef Dict[Union[int, str], Set[str]] RecordType

Hibben

In what context - do you want to define a new type that you can then use for type hints? – Report

GenericAlias – Xenocrates

Answer 1

This would simply do it?

from typing import Dict, Union, Set

RecordType = Dict[Union[int, str], Set[str]]


def my_func(rec: RecordType):
    pass


my_func({1: {'2'}})
my_func({1: {2}})

This code will generate a warning from your IDE on the second call to my_func, but not on the first. As @sahasrara62 indicated, more here https://docs.python.org/3/library/stdtypes.html#types-genericalias

Since Python 3.9, the preferred syntax would be:

from typing import Union

RecordType = dict[Union[int, str], set[str]]

The built-in types can be used directly for type hints and the added imports are no longer required.

Since Python 3.10, the preferred syntax is

RecordType = dict[int | str, set[str]]

The | operator is a simpler way to create a union of types, and the import of Union is no longer required.

Since Python 3.12, the preferred syntax is

type RecordType = dict[int | str, set[str]]

The type keyword explicitly indicates that this is a type alias.

Report

I love how elegant and easy to understand Python's syntax is – Canvass

Answer 2

In case users look for a distinct nominal typedef:

from typing import Dict, Union, Set, NewType

RecordType = Dict[Union[int, str], Set[str]]
DistinctRecordType = NewType("DistinctRecordType", Dict[Union[int, str], Set[str]])

def foo(rec: RecordType):
    pass

def bar(rec: DistinctRecordType):
    pass

foo({1: {"2"}})
bar(DistinctRecordType({1: {"2"}}))
bar({1: {"2"}}) # <--- this will cause a type error

This snippet demonstrates that only explicit casting will do.

$ mypy main.py
main.py:14: error: Argument 1 to "bar" has incompatible type "Dict[int, Set[str]]"; expected "DistinctRecordType"
Found 1 error in 1 file (checked 1 source file)

Hibben