Python equivalent of Typescript interface

Question

Recently I have been working with Typescript a lot, it allows to express things like:

interface Address {     street: string;     housenumber: number;     housenumberPostfix?: string; }  interface Person {     name: string;     adresses: Address[] }  const person: Person = {     name: 'Joe',     adresses: [         { street: 'Sesame', housenumber: 1 },         { street: 'Baker', housenumber: 221, housenumberPostfix: 'b' }     ] } 

Pretty concise and giving all the luxuries as type checking and code completion while coding with Persons.

How is this done in Python?

I have been looking at Mypy and ABC but did not yet succeed in finding the pythonic way to do something similar as the above (my attempts resulted in way too much boilerplate to my taste).

Answer 1

For the code completion and type hinting in IDEs, just add static typing for the Person and Address classes and you are already good to go. Assuming you use the latest python3.6, here's a rough equivalent of the typescript classes from your example:

# spam.py from typing import Optional, Sequence   class Address:     street: str     housenumber: int     housenumber_postfix: Optional[str]      def __init__(self, street: str, housenumber: int,                   housenumber_postfix: Optional[str] = None) -> None:         self.street = street         self.housenumber = housenumber         self.housenumber_postfix = housenumber_postfix   class Person:     name: str     adresses: Sequence[Address]      def __init__(self, name: str, adresses: Sequence[str]) -> None:         self.name = name         self.adresses = adresses   person = Person('Joe', [     Address('Sesame', 1),      Address('Baker', 221, housenumber_postfix='b') ])  # type: Person 

I suppose the boilerplate you mentioned emerges when adding the class constructors. This is indeed inavoidable. I would wish default constructors were generated at runtime when not declared explicitly, like this:

class Address:     street: str     housenumber: int     housenumber_postfix: Optional[str]   class Person:     name: str     adresses: Sequence[Address]   if __name__ == '__main__':     alice = Person('Alice', [Address('spam', 1, housenumber_postfix='eggs')])     bob = Person('Bob', ())  # a tuple is also a sequence 

but unfortunately you have to declare them manually.

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Edit

As Michael0x2a pointed out in the comment, the need for default constructors is made avoidable in python3.7 which introduced a @dataclass decorator, so one can indeed declare:

@dataclass class Address:     street: str     housenumber: int     housenumber_postfix: Optional[str]   @dataclass class Person:     name: str     adresses: Sequence[Address] 

and get the default impl of several methods, reducing the amount of boilerplate code. Check out PEP 557 for more details.

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I guess you could see stub files that can be generated from your code, as some kind of interface files:

$ stubgen spam  # stubgen tool is part of mypy package Created out/spam.pyi 

The generated stub file contains the typed signatures of all non-private classes and functions of the module without implementation:

# Stubs for spam (Python 3.6) # # NOTE: This dynamically typed stub was automatically generated by stubgen.  from typing import Optional, Sequence  class Address:     street: str     housenumber: int     housenumber_postfix: Optional[str]     def __init__(self, street: str, housenumber: int, housenumber_postfix: Optional[str]=...) -> None: ...  class Person:     name: str     adresses: Sequence[Address]     def __init__(self, name: str, adresses: Sequence[str]) -> None: ...  person: Person 

These stub files are also recognized by IDEs and if your original module is not statically typed, they will use the stub file for type hints and code completion.