Python中的__all__

__all__ = ['MNIST', 'FashionMNIST', 'CIFAR10', 'CIFAR100',
           'ImageRecordDataset', 'ImageFolderDataset']

__all__ = ['x', 'y', 'test']

x = 2
y = 3
z = 4

def test():
    print('test')

from foo import *
print('x: ', x)

print('y: ', y)
print('z: ', z)

test()

x:  2
y:  3
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    print('z: ', z)
NameError: name 'z' is not defined

from foo import *
from foo import z
print('x: ', x)

print('y: ', y)
print('z: ', z)

test()

x:  2
y:  3
z:  4
test