python supports type hints:
https://docs.python.org/3/library/typing.html
I was wondering if these hints can also be used to dynamically enforce types during runtime.
For example:
class C:
def __init__(self):
self.a : int = 0
def __str__(self):
return str(self.a)
@classmethod
def get(cls,**kwargs):
c = cls()
for k,v in kwargs.items():
setattr(c,k,v)
# ValueError exception thrown here ?
return c
attrs = {"a":"a"} # developer wanted an int !
c = C.get(**attrs)
print(c)
In short, I'd like to avoid to re-enter the type of the attribute "a" in the get function:
@classmethod
def get(cls,**kwargs):
c = cls()
for k,v in kwargs.items():
if k=="a" and not isinstance(v,int):
raise ValueError()
setattr(c,k,v)
return c
Would there be a way to "reuse" the information given in the constructor that "a" is expected to be an int ?
Note: answer to this question shows that at least for functions introspection on the arguments type hints can be accessed:
I was wondering if these hints can also be used to dynamically enforce types during runtime.
In some cases and with external lib - the answer is yes. Read below.
If the actual use-case you have is simple like your C class I would go and use dataclass and a library like dacite. You will not be able to create c2 since you are not passing an int.
So dacite managed to "see" that a should be int and raised an exception
from dataclasses import dataclass
from dacite import from_dict
@dataclass
class C:
a:int = 0
d1 = {'a':3}
c1: C = from_dict(C,d1)
print(c1)
print(C.__annotations__)
d2 = {'a':'3'}
c2: C = from_dict(C,d2)
output
C(a=3)
{'a': <class 'int'>}
Traceback (most recent call last):
File "main.py", line 14, in <module>
c2: C = from_dict(C,d2)
File "/opt/virtualenvs/python3/lib/python3.8/site-packages/dacite/core.py", line 68, in from_dict
raise WrongTypeError(field_path=field.name, field_type=field.type, value=value)
dacite.exceptions.WrongTypeError: wrong value type for field "a" - should be "int" instead of value "3" of type "str"