Python 3.6 types: Using variable before assignment

Question

New way of type/hinting/assignments is cool, but I don't know how to make such a simple thing work:

class MyContainer:
    def addMyItem(self, item:MyItem):
        pass

class MyItem:
    def __init__(self, container:MyContainer):
        pass

It throw an error: Using variable 'MyItem' before assignment. The best but super ugly workaround I found so far is this:

class MyContainer:
    def addMyItem(self, untypeditem):
        item:MyItem=untypeditem
        pass

class MyItem:
    def __init__(self, container:MyContainer):
        pass

Please tell me that language with #1 principle Beautiful is better than ugly has something better to solve this common typing issue

score 11 · Accepted Answer · answered May 23 '18 at 14:07

11

Forward references are just strings referring to the name (as it is visible in the module).

class MyContainer:
    def addMyItem(self, item: 'MyItem'):
        pass

class MyItem:
    def __init__(self, container: 'MyContainer'):
        pass

If you need to import the name from somewhere else (and you only need the name for type checking, or if it might cause a circular import), you can use

import typing

if typing.TYPE_CHECKING:
    from foo import Thing

TYPE_CHECKING is true only when a type checker is running (i.e. your code is not being evaluated for execution).

answered May 23 '18 at 14:07

AKX

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can't say is really beautiful, but I just tested and it works! Thank you, dear sir! – Philipp Munin May 23 '18 at 14:13
1

I don't know, I think it's as beautiful as you can get for a dynamic language :) You're welcome! – AKX May 23 '18 at 14:14
typescript is also dynamic, but it works that way. I think this issue is typically solved via hoisting – Philipp Munin May 23 '18 at 14:20

Python 3.6 types: Using variable before assignment

1 Answers1