我正在尝试创建一个类来表示命名元组的列表,但在按名称访问元素时遇到问题。这是一个例子:
from typing import NamedTuple
class Record(NamedTuple):
id: int
name: str
age: int
class NamedTupleList:
def __init__(self, data):
self._data = data
def attempt_access(self, row, column):
print(f'{self._data[row].age=}')
r = self._data[row]
print(f'{type(column)=}')
print(f'{column=}')
print(f'{r[column]=}')
data = [Record(1, 'Bob', 30),
Record(2, 'Carol', 25),
Record(3, 'Ted', 29),
Record(4, 'Alice', 28),
]
class_data = NamedTupleList(data)
print('show data')
print(f'{data[0]=}')
print(f'{type(data[0])=}')
print(f'{data[0].age=}')
print('\nshow class_data')
print(f'{type(class_data)=}')
print('\nattempt_access by index')
class_data.attempt_access(0, 2)
print('\nattempt_access by name')
class_data.attempt_access(0, Record.age) # why can't I do this?
产品:
data[0]=Record(id=1, name='Bob', age=30)
type(data[0])=<class '__main__.Record'>
data[0].age=30
show class_data
type(class_data)=<class '__main__.NamedTupleList'>
attempt_access by index
self._data[row].age=30
type(column)=<class 'int'>
column=2
r[column]=30
attempt_access by name
self._data[row].age=30
type(column)=<class '_collections._tuplegetter'>
column=_tuplegetter(2, 'Alias for field number 2')
print(f'{r[column]=}')
~^^^^^^^^
TypeError: tuple indices must be integers or slices, not _collections._tuplegetter
因此我可以通过索引成功访问“行”和“列”,但是如果我想通过方法调用访问列(即namedtuple属性),则会收到错误。有趣的是,列值是
_tuplegetter(2, 'Alias for field number 2')
,因此索引在我的方法中是已知的,但我无法访问它。有谁知道如何访问该值以便将名称传递给该方法?我试图避免将名称作为字符串传递 - 我真的很想利用命名空间,因为这毕竟是命名元组的优点之一。
有趣的问题。该字段是一个描述符,因此您可以调用它:
class NamedTupleList:
def __init__(self, data):
self._data = data
def attempt_access(self, row, column):
r = self._data[row]
try:
val = r[column]
except TypeError:
val = column.__get__(r)
return val
演示:
>>> class_data.attempt_access(0, 2)
30
>>> class_data.attempt_access(0, Record.age)
30
您需要将
attempt_access
修改为:
def attempt_access(self, row, column_name):
r = self._data[row]
if isinstance(column_name, int):
print(f'{r[column_name]=}')
else:
column_index = r._fields.index(column_name)
print(f'{r[column_index]=}')
然后调用:
class_data.attempt_access(0, 'age')
问题是在命名元组中使用整数。
结果输出:
show data
data[0]=Record(id=1, name='Bob', age=30)
type(data[0])=<class '__main__.Record'>
data[0].age=30
show class_data
type(class_data)=<class '__main__.NamedTupleList'>
attempt_access by index
r[column_name]=30
attempt_access by name
r[column_index]=30