How can I control what scalar form PyYAML uses for my data?

Question

I've got an object with a short string attribute, and a long multi-line string attribute. I want to write the short string as a YAML quoted scalar, and the multi-line string as a literal scalar:

my_obj.short = "Hello" my_obj.long = "Line1\nLine2\nLine3" 

I'd like the YAML to look like this:

short: "Hello" long: |   Line1   Line2   Line3 

How can I instruct PyYAML to do this? If I call yaml.dump(my_obj), it produces a dict-like output:

{long: 'line1      line2      line3      ', short: Hello} 

(Not sure why long is double-spaced like that...)

Can I dictate to PyYAML how to treat my attributes? I'd like to affect both the order and style.

Answer 1

Falling in love with @lbt's approach, I got this code:

import yaml  def str_presenter(dumper, data):   if len(data.splitlines()) > 1:  # check for multiline string     return dumper.represent_scalar('tag:yaml.org,2002:str', data, style='|')   return dumper.represent_scalar('tag:yaml.org,2002:str', data)  yaml.add_representer(str, str_presenter)  # to use with safe_dump: yaml.representer.SafeRepresenter.add_representer(str, str_presenter) 

It makes every multiline string be a block literal.

I was trying to avoid the monkey patching part. Full credit to @lbt and @J.F.Sebastian.

Answer 2

Based on Any yaml libraries in Python that support dumping of long strings as block literals or folded blocks?

import yaml from collections import OrderedDict  class quoted(str):     pass  def quoted_presenter(dumper, data):     return dumper.represent_scalar('tag:yaml.org,2002:str', data, style='"') yaml.add_representer(quoted, quoted_presenter)  class literal(str):     pass  def literal_presenter(dumper, data):     return dumper.represent_scalar('tag:yaml.org,2002:str', data, style='|') yaml.add_representer(literal, literal_presenter)  def ordered_dict_presenter(dumper, data):     return dumper.represent_dict(data.items()) yaml.add_representer(OrderedDict, ordered_dict_presenter)  d = OrderedDict(short=quoted("Hello"), long=literal("Line1\nLine2\nLine3\n"))  print(yaml.dump(d)) 

Output

short: "Hello" long: |   Line1   Line2   Line3