Function annotations — Python-Future documentation

# Function annotations

Function annotations are a piece of syntax introduced in Python 3.0 that was not backported to Python 2.x. (See PEP 3107: http://www.python.org/dev/peps/pep-3107/). They cause Python 2 to raise a SyntaxError.

To rewrite Python 3 code with function annotations to be compatible with both Python 3 and Python 2, you can replace the annotation syntax with a dictionary called `__annotations__` as an attribute on your functions. For example, code such as this:

    def _parse(self, filename: str, dir='.') -> list:
        pass

can be re-expressed like this:

    def _parse(self, filename, dir='.'):
        pass
    _parse.__annotations__ = {'filename': str, 'return': list}

As described in PEP 3107, the annotation for a function’s return value corresponds to the `'return'` key in the dictionary.

(Note that PEP 3107 describes annotations as belonging to a `func_annotations` attribute. This attribute was renamed in Python 3.2 to `__annotations__`.)

Be aware that some libraries that consume function annotations, such as Reticulated, have their own semantics for supporting earlier Python versions, such as decorators. If you are using such a library, please use its own mechanism for providing compatibility with earlier Python versions, rather than the generic equivalent above.