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[英]Pytest raises exception but assert doesn't catch it, although they are the same type and same value
[英]Creating value type that raises exception on reading
如何創建讀取時引發異常的值類型?
例如:
from dataclasses import dataclass, field
Missing = ...
@dataclass
class A:
a: int = field(default=None) # <- value can be None
b: int = field(default=Missing) # <- can be Missing until you try to access it
def print(self):
for i in [self.a, self.b]:
print(i) # <- raises ValueError if i is Missing
在 Python 中,似乎總有辦法解決任何問題 :-)
您可以通過巧妙地將描述符值用作數據類字段來解決此問題,如下圖所示。 我還將閱讀有關驗證器部分的更多信息,以了解更多關於描述符如何工作的信息。
from dataclasses import dataclass
# create `_MissingType` class
_MissingType = type('_MissingType', (), {'__bool__': lambda self: False})
# create a singleton for that class
Missing = _MissingType()
class MissingValidator:
__slots__ = ('default', 'private_name')
# You may or may not want a default value
def __init__(self, default=Missing):
self.default = default
def __set_name__(self, owner, name):
self.private_name = '_' + name
# override __get__() to return a default value if one is not passed in to __init__()
def __get__(self, obj, obj_type=None):
try:
value = getattr(obj, self.private_name)
if value is Missing:
cls_name = obj_type.__qualname__
public_name = self.private_name.lstrip('_')
raise ValueError(f'Missing value for field `{public_name}` in class `{cls_name}`')
return value
except AttributeError:
return self.default
def __set__(self, obj, value):
setattr(obj, self.private_name, value)
@dataclass
class A:
a: int = None # <- value can be None
b: int = MissingValidator() # <- can be Missing until you try to access it
def print(self):
for i in [self.a, self.b]:
print(i) # <- raises ValueError if i is Missing
A(b=3).print()
# None
# 3
A(a=42).print()
# raises:
# ValueError: Missing value for field `b` in class `A`
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