在 python 中读取 YAML 文件时出错

Question

我有一个如下所示的yaml 文件：

%YAML 1.0
temp: !!opencv-matrix
 rows: 2
 cols: 23
 dt: f
 data: [ 3.35620789e+02, 3.64299591e+02, 3.95790131e+02,
   4.39863068e+02, 4.68664948e+02, 4.93518127e+02, 4.17159943e+02,
   4.21060364e+02, 3.99990234e+02, 4.17867157e+02, 4.34151215e+02,
   3.56201202e+02, 3.77741028e+02, 3.87051544e+02, 3.76879913e+02,
   4.42746796e+02, 4.52483917e+02, 4.73469604e+02, 4.52954742e+02,
   3.78402283e+02, 4.17679047e+02, 4.50588501e+02, 4.16388153e+02,
   9.05276794e+01, 9.21245193e+01, 1.02799362e+02, 9.93146744e+01,
   8.40704346e+01, 7.84236526e+01, 1.15820358e+02, 1.76747055e+02,
   1.61153061e+02, 1.68130676e+02, 1.58446228e+02, 1.07421455e+02,
   1.03407494e+02, 1.05380608e+02, 1.08374542e+02, 1.01048920e+02,
   9.76309204e+01, 9.83933716e+01, 1.02486870e+02, 1.71890350e+02,
   1.81417206e+02, 1.66303802e+02, 1.95539871e+02 ]

它基本上是一个opencv matrix，我在c++ 代码中创建了该文件。现在我想在python 中读取这个文件并且我有这个代码：

import yaml
with open("reference_3d.yml") as fin:
     rfr = yaml.load(fin.read())

但是当我运行代码时，它给了我这个错误：

Traceback (most recent call last):
File "scatter_plot.py", line 15, in <module>
rfr = yaml.load(fin.read())
File "/usr/local/lib/python2.7/site-packages/yaml/__init__.py", line 71, in  load
return loader.get_single_data()
File "/usr/local/lib/python2.7/site-packages/yaml/constructor.py", line 37, in get_single_data
node = self.get_single_node()
File "/usr/local/lib/python2.7/site-packages/yaml/composer.py", line 35, in  get_single_node
if not self.check_event(StreamEndEvent):
File "/usr/local/lib/python2.7/site-packages/yaml/parser.py", line 98, in check_event
self.current_event = self.state()
File "/usr/local/lib/python2.7/site-packages/yaml/parser.py", line 157, in parse_implicit_document_start
return self.parse_document_start()
File "/usr/local/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 '<scalar>'
in "<string>", line 2, column 2:
 temp: !!opencv-matrix
 ^

知道我应该如何解决这个错误吗？

Answer 1

这里的问题是您的 C++ 生成（无效）YAML 1.0，并且您尝试使用可以处理大部分 YAML 1.1 的 Python 解析器来解析它。

YAML 1.0 specification 没有太多示例，但文档指令指出可选的 c-ns-directive 应该出现在文档标题之后文档开始 (rules 48 and 49) 和规范clearly states that its form should be %YAML:1.0。因此，正确的 YAML 1.0 文档应以：

---
%YAML:1.0

但是，即使输出正确，PyYAML 也无法读取这个旧版 YAML 文件。

由于最新的 YAML 规范 (1.2) 是从 2009 年开始的，您能做的最好的事情就是将您的 C++ 程序和 Python 程序都切换到 1.2 兼容的库库。在yaml.org 上表明，对于 C++，这将是 yaml-cpp，对于 Python，这将是 ruamel.yaml（免责声明：我是该软件包的作者）。

YAML 文件看起来像（yaml-cpp 可能会更改标签，甚至可能更改转储的其余部分）：

%YAML 1.2
---
temp: !!opencv-matrix
 rows: 2
 cols: 23
 dt: f
 data: [ 3.35620789e+02, 3.64299591e+02, 3.95790131e+02,
   4.39863068e+02, 4.68664948e+02, 4.93518127e+02, 4.17159943e+02,
   4.21060364e+02, 3.99990234e+02, 4.17867157e+02, 4.34151215e+02,
   3.56201202e+02, 3.77741028e+02, 3.87051544e+02, 3.76879913e+02,
   4.42746796e+02, 4.52483917e+02, 4.73469604e+02, 4.52954742e+02,
   3.78402283e+02, 4.17679047e+02, 4.50588501e+02, 4.16388153e+02,
   9.05276794e+01, 9.21245193e+01, 1.02799362e+02, 9.93146744e+01,
   8.40704346e+01, 7.84236526e+01, 1.15820358e+02, 1.76747055e+02,
   1.61153061e+02, 1.68130676e+02, 1.58446228e+02, 1.07421455e+02,
   1.03407494e+02, 1.05380608e+02, 1.08374542e+02, 1.01048920e+02,
   9.76309204e+01, 9.83933716e+01, 1.02486870e+02, 1.71890350e+02,
   1.81417206e+02, 1.66303802e+02, 1.95539871e+02 ]

你可以这样做：

from ruamel import yaml

with open('reference_3d.yml') as fin:
    rfr = yaml.round_trip_load(fin)

print(rfr['temp']['data'][rfr['temp']['cols']-1])

获取第一个数据行的最后一个值（416.388153）

如果您不能或不愿意更改您的 C++ 程序，则只需使用 ruamel.yaml 并跳过 YAML 文件的第一行：

from ruamel import yaml

with open('reference_3d.yml') as fin:
    fin.readline()
    rfr = yaml.round_trip_load(fin)

print(rfr['temp']['data'][rfr['temp']['cols']-1])