Edward Ned Harvey (python) wrote:
> I think it's great that for built-in types such as int and str, backward
> compatibility of type hinting annotations is baked into python 3.0 to 3.4.
> In fact, I *thought* python 3.0 to 3.4 would *ignore* annotations, but it
> doesn't...
>
> I'm struggling to create something backward compatible that requires the
> 'typing' module. For example, the following program is good in python 3.5,
> but line 11 is a syntax error in python 3.4:
>
> 1 import sys
> 2
> 3 if sys.version_info[0] < 3:
> 4 raise RuntimeError("Must use at least python version 3")
> 5
> 6 # The 'typing' module, useful for type hints, was introduced in
> python 3.5 7 if sys.version_info[1] >= 5:
> 8 from typing import Optional
> 9
> 10
> 11 def divider(x: int, y: int) -> Optional[float]:
> 12 if y == 0:
> 13 return None
> 14 return x / y
> 15
> 16 print("22 / 7 = " + str(divider(22, 7)))
> 17 print("8 / 0 = " + str(divider(8, 0)))
> 18
>
> When I run this program in python 3.4, I get this:
> Traceback (most recent call last):
> File "./ned.py", line 11, in <module>
> def divider(x: int, y: int) -> Optional[float]:
> NameError: name 'Optional' is not defined
Luckily it's a NameError. To backport you only have to define a symbol. The
easiest way to do that is to get hold of a copy of 3.5's typing.py, so let's
try that first:
$ cp /usr/local/lib/python3.5/typing.py .
$ python3.4
Python 3.4.3 (default, Nov 17 2016, 01:08:31)
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import Optional
>>> def divider(x: int, y: int) -> Optional[float]: pass
...
>>>
Works...
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