Why isn't Python NewType compatible with isinstance and type? - python-typing

Question

This doesn't seem to work:
from typing import NewType
MyStr = NewType("MyStr", str)
x = MyStr("Hello World")
isinstance(x, MyStr)
I don't even get False, but TypeError: isinstance() arg 2 must be a type or tuple of types because MyStr is a function and isinstance wants one or more type.
Even assert type(x) == MyStr or is MyStr fails.
What am I doing wrong?

Answer 1

Cross-reference: inheritance from str or int
Even more detailed in the same question: https://stackoverflow.com/a/2673802/1091677
If you would like to subclass Python's str, you would need to do the following way:
class MyStr(str):
# Class instances construction in Python follows this two-step call:
# First __new__, which allocates the immutable structure,
# Then __init__, to set up the mutable part.
# Since str in python is immutable, it has no __init__ method.
# All data for str must be set at __new__ execution, so instead
# of overriding __init__, we override __new__:
def __new__(cls, *args, **kwargs):
return str.__new__(cls, *args, **kwargs)
Then:
x = MyStr("Hello World")
isinstance(x, MyStr)
returns True as expected

Answer 2

As at python 3.10...
I'd speculate that the answer to
"Why isn't Python NewType compatible with isinstance and type?"
... is "It is a limitation of NewType".
I'd speculate that the answer to
"What am I doing wrong?"
... is "nothing". You are assuming NewType makes a new runtime type, it appears that it doesn't.
And for what it's worth, I wish it did work.

Answer 3

maybe you want a type that is methods like str does but is not a str?
A simple way to get that effect is just:
class MyStr:
value:str
def __init__(self,value:str):
self.value=value
pass
... but that means using all the string methods is "manual", e.g.
x=MyStr('fred')
x.value.startswith('fr')
... you could use #dataclass to add compare etc.
This is not a one-size-fits-all answer, but it might suit your application.

Answer 4

then make that simple
class MyStr:
value:str
def __init__(self,value:str):
self.value=value
...generic like Str in (incomplete) https://github.com/urnest/urnest/blob/master/xju/newtype.py
... and you can write:
class MyStrType:pass; class MyOtherStrType:pass
class MyStr(Str[MyStrType]):
pass
class MyOtherStr(Str[MyOtherStrType]):
pass
x=MyStr('fred')
y=MyOtherStr('fred')
x < y # ! mypy error distinct types MyStr and MyOtherStr
That's what I was after, which might be what you were after? I have to provide Int,Str separately but in my experience distinct int, str, float, bool, bytes types give a lot of readability and error-rejection leverage. I will add Float, Bool, Bytes to xju.newtype soon and give them all the full set of methods.

Answer 5

looks might have been "fixed" in python 3.10:
https://docs.python.org/3/library/typing.html?highlight=typing#newtype
says:
Changed in version 3.10: NewType is now a class rather than a function. There is some additional runtime cost when calling NewType over a regular function. However, this cost will be reduced in 3.11.0.
I don't have 3.10 handy as I write this to try your example.