如何解析包含多个文档的YAML文件？

Question

这是我的解析代码:

import yaml

def yaml_as_python(val):
    """Convert YAML to dict"""
    try:
        return yaml.load_all(val)
    except yaml.YAMLError as exc:
        return exc

with open('circuits-small.yaml','r') as input_file:
    results = yaml_as_python(input_file)
    print results
    for value in results:
         print value

以下是该文件的示例:

ingests:
  - timestamp: 1970-01-01T00:00:00.000Z
    id: SwitchBank_35496721
    attrs:
      Feeder: Line_928
      Switch.normalOpen: 'true'
      IdentifiedObject.description: SwitchBank
      IdentifiedObject.mRID: SwitchBank_35496721
      PowerSystemResource.circuit: '928'
      IdentifiedObject.name: SwitchBank_35496721
      IdentifiedObject.aliasName: SwitchBank_35496721
    loc: vector [43.05292, -76.126800000000003, 0.0]
    kind: SwitchBank
  - timestamp: 1970-01-01T00:00:00.000Z
    id: UndergroundDistributionLineSegment_34862802
    attrs:
      Feeder: Line_928
      status: de-energized
      IdentifiedObject.description: UndergroundDistributionLineSegment
      IdentifiedObject.mRID: UndergroundDistributionLineSegment_34862802
      PowerSystemResource.circuit: '928'
      IdentifiedObject.name: UndergroundDistributionLineSegment_34862802
    path:
    - vector [43.052942000000002, -76.126716000000002, 0.0]
    - vector [43.052585000000001, -76.126515999999995, 0.0]
    kind: UndergroundDistributionLineSegment
  - timestamp: 1970-01-01T00:00:00.000Z
    id: UndergroundDistributionLineSegment_34806014
    attrs:
      Feeder: Line_928
      status: de-energized
      IdentifiedObject.description: UndergroundDistributionLineSegment
      IdentifiedObject.mRID: UndergroundDistributionLineSegment_34806014
      PowerSystemResource.circuit: '928'
      IdentifiedObject.name: UndergroundDistributionLineSegment_34806014
    path:
    - vector [43.05292, -76.126800000000003, 0.0]
    - vector [43.052928999999999, -76.126766000000003, 0.0]
    - vector [43.052942000000002, -76.126716000000002, 0.0]
    kind: UndergroundDistributionLineSegment
... 
ingests:
  - timestamp: 1970-01-01T00:00:00.000Z
    id: OverheadDistributionLineSegment_31168454

在追溯中,请注意它开始出现问题...

Traceback (most recent call last):
  File "convert.py", line 29, in <module>
    for value in results:
  File "/Users/conduce-laptop/anaconda2/lib/python2.7/site-packages/yaml/__init__.py", line 82, in load_all
    while loader.check_data():
  File "/Users/conduce-laptop/anaconda2/lib/python2.7/site-packages/yaml/constructor.py", line 28, in check_data
    return self.check_node()
  File "/Users/conduce-laptop/anaconda2/lib/python2.7/site-packages/yaml/composer.py", line 18, in check_node
    if self.check_event(StreamStartEvent):
  File "/Users/conduce-laptop/anaconda2/lib/python2.7/site-packages/yaml/parser.py", line 98, in check_event
    self.current_event = self.state()
  File "/Users/conduce-laptop/anaconda2/lib/python2.7/site-packages/yaml/parser.py", line 174, in parse_document_start
    self.peek_token().start_mark)
yaml.parser.ParserError: expected '<document start>', but found '<block mapping start>'
  in "circuits-small.yaml", line 42, column 1

我想要的是它将每个文档解析为一个单独的对象,也许所有这些文档都在同一个列表中,或者几乎任何可以与PyYAML模块一起使用的文件.我相信...它实际上是有效的YAML所以我很惊讶它没有自动处理它.

Answer 1

错误消息非常具体,文档需要以文档开始标记开头.您的第一个文档没有这样的标记,尽管它有一个文档结束标记.在您明确结束第一个文档后,...您无法再在PyYAML中使用没有文档边界标记的文档,您必须使用以下命令启动它---:

文件的结尾应如下所示:

    kind: UndergroundDistributionLineSegment
...
---
ingests:
  - timestamp: 1970-01-01T00:00:00.000Z
    id: OverheadDistributionLineSegment_31168454

您可以从第一个文档中省略显式文档开始标记,但是您需要为每个后续文档包含一个开始标记.文档结束标记是可选的.

如果您无法完全控制输入,则使用.load_all()不安全.通常没有理由承担这种风险,您应该使用.safe_load_all()并扩展它SafeLoader来处理YAML可能包含的任何特定标记.

除此之外,您应该在文档开始指示符(您还应该添加到第一个文档)之前使用显式版本指令启动YAML文档:

%YAML 1.1
---

这是为了YAML文件的未来编辑器的好处,因为您使用的是PyYAML,它仅支持(大部分)YAML 1.1而不支持YAML 1.2规范(2009年形式).替代方案当然是将您的YAML解析器升级到例如ruamel.yaml,这也会警告您使用不安全的load_all()(免责声明:我是该解析器的作者).ruamel.yaml在显式的文档结束标记(允许@flyx指出)之后,不允许你有一个裸文档,这是一个错误.