An “empty” feature is one with a point set that coincides with the empty set; not None, but like set([]). Empty features can be created by calling the various constructors with no arguments. Almost no operations are supported by empty features.

空要素是指其点集为空集，不是None，而是像set([])。空要素可以通过调用各种没有参数的构造函数来创建。空要素几乎不支持任何操作。

>>> line = LineString()
>>> line.is_empty
True
>>> line.length
0.0
>>> line.bounds
(nan, nan, nan, nan)
>>> list(line.coords)
[]