如何从PyYAML异常中获取详细信息？

Question

我想优雅地通知用户他们的混乱YAML文件存在缺陷.第288行python-3.4.1/lib/python-3.4/yaml/scanner.py是报告常见解析错误并通过抛出异常来处理它的地方:

raise ScannerError("while scanning a simple key", key.mark,
                   "could not found expected ':'", self.get_mark())

我正在努力报道它.

try:
    parsed_yaml = yaml.safe_load(txt)

except yaml.YAMLError as exc:
    print ("scanner error 1")
    if hasattr(exc, 'problem_mark'):
        mark = exc.problem_mark
        print("Error parsing Yaml file at line %s, column %s." %
                                            (mark.line, mark.column+1))
    else:
        print ("Something went wrong while parsing yaml file")
    return

这给了

$ yaml_parse.py
scanner error 1
Error parsing Yaml file line 1508, column 9.

但是如何获取错误文本以及其他内容key.mark和其他标记？

更有用的是,我如何检查PyYaml源来解决这个问题？ScannerError类似乎忽略了参数(来自scanner.py第32行):

class ScannerError(MarkedYAMLError):
     pass

Answer 1

该类ScannerError没有定义方法（该pass语句的工作方式类似于无操作。这使得它的功能与其基类相同MarkedYAMLError，并且基类存储数据。来自error.py：

class MarkedYAMLError(YAMLError):
    def __init__(self, context=None, context_mark=None,
                 problem=None, problem_mark=None, note=None):
        self.context = context
        self.context_mark = context_mark
        self.problem = problem
        self.problem_mark = problem_mark
        self.note = note

    def __str__(self):
        lines = []
        if self.context is not None:
            lines.append(self.context)
        if self.context_mark is not None  \
           and (self.problem is None or self.problem_mark is None
                or self.context_mark.name != self.problem_mark.name
                or self.context_mark.line != self.problem_mark.line
                or self.context_mark.column != self.problem_mark.column):
            lines.append(str(self.context_mark))
        if self.problem is not None:
            lines.append(self.problem)
        if self.problem_mark is not None:
            lines.append(str(self.problem_mark))
        if self.note is not None:
            lines.append(self.note)
        return '\n'.join(lines)

如果您从一个文件开始txt.yaml：

hallo: 1
bye

和一个test.py：

import ruamel.yaml as yaml
txt = open('txt.yaml')
data = yaml.load(txt, yaml.SafeLoader)

你会得到不那么描述性的错误：

...
ruamel.yaml.scanner.ScannerError: while scanning a simple key
  in "txt.yaml", line 2, column 1
could not find expected ':'
  in "txt.yaml", line 3, column 1

但是，如果您更改第二行test.py：

import ruamel.yaml as yaml
txt = open('txt.yaml').read()
data = yaml.load(txt, yaml.SafeLoader)

你会得到更有趣的错误描述：

...
ruamel.yaml.scanner.ScannerError: while scanning a simple key
  in "<byte string>", line 2, column 1:
    bye
    ^
could not find expected ':'
  in "<byte string>", line 3, column 1:

    ^

这种差异是因为get_mark()(in reader.py) 如果不处理流，则有更多上下文可以指向：

def get_mark(self):
    if self.stream is None:
        return Mark(self.name, self.index, self.line, self.column,
                    self.buffer, self.pointer)
    else:
        return Mark(self.name, self.index, self.line, self.column,
                    None, None)

该数据进入context_mark属性。当您想为错误提供更多上下文时请查看该内容。但如上所示，仅当您从缓冲区（而不是流）解析 YAML 输入时才有效。

搜索 YAML 源是一项艰巨的任务，各种类的所有方法都附加到它们作为父类的 Loader 或 Dumper。追踪这一点的最佳帮助是使用grepon def method_name(，因为至少方法名称都是独特的（因为它们必须如此才能发挥作用）。

_{在上面，我使用了名为ruamel.yaml的 PyYAML 增强版本，出于此答案的目的，它们应该具有相同的工作原理。}

Answer 2

根据@ Anthon的回答,这段代码非常有效:

try:
    import yaml
except:
    print ('Fatal error:  Yaml library not available')
    quit()

f = open ('y.yml')
txt = f.read()

try:
    yml = yaml.load(txt, yaml.SafeLoader)

except yaml.YAMLError as exc:
    print ("Error while parsing YAML file:")
    if hasattr(exc, 'problem_mark'):
        if exc.context != None:
            print ('  parser says\n' + str(exc.problem_mark) + '\n  ' +
                str(exc.problem) + ' ' + str(exc.context) +
                '\nPlease correct data and retry.')
        else:
            print ('  parser says\n' + str(exc.problem_mark) + '\n  ' +
                str(exc.problem) + '\nPlease correct data and retry.')
    else:
        print ("Something went wrong while parsing yaml file")
    return

# make use of `yml`

具有轻度破坏数据的示例输出:

$ yaml_parse.py
Error while parsing YAML file:
  parser says
  in "<unicode string>", line 1525, column 9:
      - name: Curve 1
            ^
  could not found expected ':' while scanning a simple key
Please correct data and retry.

$ yaml_parse.py
Error while parsing YAML file:
  parser says
  in "<unicode string>", line 1526, column 10:
        curve: title 1
             ^
  mapping values are not allowed here
Please correct data and retry.