Using typing module in Python 2.7

Question

The typing module is a back port for earlier version of Python to infer input and output datatypes. I am having an issue getting it to work in Python 2.7.

import typing
def greeting(name): # type: (str) -> str
    """documentations"""
    return ('Hello ' + name)
print(greeting.__annotations__) # fails because doesn't exist.

I've also tried this:

import typing
def greeting(name # type: str
             ):
    # type: (...) -> str
    """documentations"""
    return ('Hello ' + name)

And this:

import typing

def greeting(name):
    # type: (str) -> str
    """documentations"""
    return ('Hello ' + name)

This should create an __annotations__ property on the class according to PEP484, but I do not see this happening at all.

What am I doing wrong with the backport code?

Answer 1

typing is a module that was introduced in Python 3.5 . The examples in PEP 484 rely on a Python 3+, and __annotations__ is a Python 3 concept. The backport can only allow to use the types of functions defined in the typing module, but it does not change the Python engine to magically understand all Python 3 concepts.

A discussion in that other SO post suggests that the annotations should be accessible by using inspect.getsourcelines to research the first line right after the function declaration and starting with # type. A typed-ast module exists on pypi and should be able to parse Python 2.7 style annotations. Unfortunately it is only declared at beta level and only compatible with Python 3.