python - 为什么我不能比较 python 2.7 中的集合和不可变集合

Question

为什么不能使用子集运算符 <= 比较集合和 ImmutableSet？例如。运行以下代码。这里有什么问题？任何帮助表示赞赏。我正在使用 Python 2.7。

>>> from sets import ImmutableSet
>>> X = ImmutableSet([1,2,3])
>>> X <= set([1,2,3,4])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sets.py", line 291, in issubset
    self._binary_sanity_check(other)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sets.py", line 328, in _binary_sanity_check
    raise TypeError, "Binary operation only permitted between sets"
TypeError: Binary operation only permitted between sets
>>> 

Answer 1

使用 frozenset object反而; sets module已被弃用，无法与内置类型相媲美:

>>> X = frozenset([1,2,3])
>>> X <= set([1,2,3,4])
True

来自 sets 模块的文档:

Deprecated since version 2.6: The built-in set/frozenset types replace this module.

如果您对使用 sets 模块的代码感到困惑，请在比较时只使用它的类型:

>>> from sets import Set, ImmutableSet
>>> Set([1, 2, 3]) <= set([1, 2, 3, 4])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mj/Development/Libraries/buildout.python/parts/opt/lib/python2.7/sets.py", line 291, in issubset
    self._binary_sanity_check(other)
  File "/Users/mj/Development/Libraries/buildout.python/parts/opt/lib/python2.7/sets.py", line 328, in _binary_sanity_check
    raise TypeError, "Binary operation only permitted between sets"
TypeError: Binary operation only permitted between sets
>>> ImmutableSet([1, 2, 3]) <= Set([1, 2, 3, 4])
True

Python set 和 frozenset 确实接受许多运算符和函数的任何序列，因此反转您的测试也有效:

>>> X
frozenset([1, 2, 3])
>>> set([1,2,3,4]) >= X
True

这同样适用于 sets.ImmutableSet 和 sets.Set 类上的 .issubset() 函数:

>>> X.issubset(set([1,2,3,4]))
True

但不混合已弃用的类型和新的内置类型完全是最佳选择。