当所有元素都被迭代时，pymongo 游标会发生什么？

Question

我想使用 pymongo 从我的数据库中获取一组条目。好像要return一个"cursor"代替。我不知道那是什么。

    all_nodes = pymongo.MongoClient("mongodb://localhost")["provemath"]["nodes"].find(None)

    print('ALL NODES')
    for node in all_nodes:
        print(node)

    print('STILL NODES')
    for node in all_nodes:
        print(node)

输出为：

ALL NODES
{'_notes': [], '_examples': [], '_type': 'definition', '_plural': None, '_counterexamples': [], '_intuitions': [], '_id': 'unique', '_importance': 4, '_name': 'unique', '_dependencies': [], '_description': 'An occurence of a property is __unique__ if the property occurs exactly $ time.'}
{'_notes': ['Vertices are often drawn as a dot.  They are also called *nodes*.'], '_examples': [], '_type': 'definition', '_plural': '__vertices__', '_counterexamples': [], '_intuitions': [], '_id': 'vertex', '_importance': 4, '_name': 'vertex', '_dependencies': ['unique'], '_description': 'A __vertex__ is a fundamental unit used to create graphs.'}
{'_notes': ['It is possible that $a=b$.'], '_examples': [], '_type': 'definition', '_plural': None, '_counterexamples': [], '_intuitions': ['Edges are usually drawn as a line or curve connecting two vertices.  In the case that $a=b$, the edge is drawn as a small loop that connects $a$ to itself.'], '_id': 'edge', '_importance': 4, '_name': 'edge', '_dependencies': ['unique', 'vertex'], '_description': 'An __edge__ is a set ${a,b}$ where $a$ and $b$ are vertices.'}
{'_notes': [], '_examples': [], '_type': 'definition', '_plural': None, '_counterexamples': [], '_intuitions': [], '_id': 'loop', '_importance': 4, '_name': 'loop', '_dependencies': [], '_description': 'A __loop__ is an edge $e = {a,a} = {a}$.'}
STILL NODES

光标只能用一次吗？（或者还有其他事情发生？）如果我想要常规的数组行为，我将如何获得它？

Answer 1

是的。 Cursor 只适合一次使用。如mongodb manual

中所述

By default, the server will automatically close the cursor after 10 minutes of inactivity or if client has exhausted the cursor.

要使您输出为 python 列表，请使用

node_list = all_nodes[:]

或

node_list = [node for node in all_nodes]

另见 this question

当所有元素都被迭代时，pymongo 游标会发生什么？

What happens to a pymongo cursor when all its elements have been iterated?

pymongo

python-3.x