python-2.7 - 在 ruamel.yaml 往返解析中保留空消息

Question

使用 ruamel.yaml，YAML 往返解析的输出

a: {b: }

是

a: {b: !!null ''}

有什么方法可以保留空消息，或覆盖None代表以输出如上所述的空消息？

Answer 1

这不是微不足道的，我不确定下面介绍的解决方案是否没有不良副作用。

不平凡的原因与多方面有关：

• 您正在使用可读性较差的流样式

  a: {b: } 
  

  而不是块样式：

  a:
      b:
  

  后者往返无变化

• 键的空值b加载为None，我无法在 ruamel.yaml 中对其进行子类化，因此无法将样式信息附加到该值，并且您必须依赖“默认”发射器（在您的情况下不会不要做你想做的）。

• 这个

  a: {b:} 
  

  和你的完全不同

  a: {b: } 
  

  目前发射器为了安全而将标签信息插入流中。

使用该背景信息，您可以将表示的样式强制为None空字符串，并基于该 hack 发射器：

import sys
import ruamel.yaml

yaml_str = """\
a: {b: }
"""

class MyEmitter(ruamel.yaml.emitter.Emitter):
    def choose_scalar_style(self):
        # override selection for 'null'
        if self.analysis is None:
            self.analysis = self.analyze_scalar(self.event.value)
        if self.event.style == '"' or self.canonical:
            return '"'
        if (not self.event.style or self.event.style == '?') and \
           (self.event.implicit[0] or not self.event.implicit[2]):
            if (not (self.simple_key_context and
                     (self.analysis.empty or self.analysis.multiline)) and
                (self.flow_level and self.analysis.allow_flow_plain or
                    (not self.flow_level and self.analysis.allow_block_plain))):
                return ''
        if (self.event.style == '') and self.event.tag == 'tag:yaml.org,2002:null' and \
           (self.event.implicit[0] or not self.event.implicit[2]):
            if self.flow_level and not self.analysis.allow_flow_plain:
                return ''
        self.analysis.allow_block = True
        if self.event.style and self.event.style in '|>':
            if (not self.flow_level and not self.simple_key_context and
                    self.analysis.allow_block):
                return self.event.style
        if not self.event.style and self.analysis.allow_double_quoted:
            if "'" in self.event.value or '\n' in self.event.value:
                return '"'
        if not self.event.style or self.event.style == '\'':
            if (self.analysis.allow_single_quoted and
                    not (self.simple_key_context and self.analysis.multiline)):
                return '\''
        return '"'

    def process_scalar(self):
        # if style '' and tag is 'null' insert empty space
        if self.analysis is None:
            self.analysis = self.analyze_scalar(self.event.value)
        if self.style is None:
            self.style = self.choose_scalar_style()
        split = (not self.simple_key_context)
        if self.sequence_context and not self.flow_level:
            self.write_indent()
        if self.style == '"':
            self.write_double_quoted(self.analysis.scalar, split)
        elif self.style == '\'':
            self.write_single_quoted(self.analysis.scalar, split)
        elif self.style == '>':
            self.write_folded(self.analysis.scalar)
        elif self.style == '|':
            self.write_literal(self.analysis.scalar)
        elif self.event.tag == 'tag:yaml.org,2002:null':
            self.stream.write(u' ')  # not sure if this doesn't break other things
        else:
            self.write_plain(self.analysis.scalar, split)
        self.analysis = None
        self.style = None
        if self.event.comment:
            self.write_post_comment(self.event)

class MyRepresenter(ruamel.yaml.representer.RoundTripRepresenter):
    def represent_none(self, data):
        if len(self.represented_objects) == 0 and not self.serializer.use_explicit_start:
            # this will be open ended (although it is not yet)
            return self.represent_scalar(u'tag:yaml.org,2002:null', u'null')
        return self.represent_scalar(u'tag:yaml.org,2002:null', u'', style='')

MyRepresenter.add_representer(type(None),
                                     MyRepresenter.represent_none)


yaml = ruamel.yaml.YAML()
yaml.Emitter = MyEmitter
yaml.Representer = MyRepresenter
data = yaml.load(yaml_str)
yaml.dump(data, sys.stdout)

这使：

a: {b: }