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?
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
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.
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.
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.
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.