pretty output with pyyaml

Question

I have a python project where I'd like to use YAML (pyYaml 3.11), particularly because it is "pretty" and easy for users to edit in a text editor if and when necessary. My problem, though, is if I bring the YAML into a python application (as I will need to) and edit the contents (as I will need to) then writing the new document is typically not quite as pretty as what I started with.

The pyyaml documentation is pretty poor - does not even document the parameters to the dump function. I found http://dpinte.wordpress.com/2008/10/31/pyaml-dump-option/. However, I'm still missing the information I need. (I started to look at the source, but it doesn't seem the most inviting. If I don't get the solution here, then that's my only recourse.)

I start with a document that looks like this:

- color green :
     inputs :
        - port thing :
            widget-hint : filename
            widget-help : Select a filename
        - port target_path : 
            widget-hint : path
            value : 'thing' 
     outputs:
        - port value:
             widget-hint : string
     text : |
            I'm lost and I'm found
            and I'm hungry like the wolf.

After loading into python (yaml.safe_load( s )), I try a couple ways of dumping it out:

>>> print yaml.dump( d3, default_flow_style=False, default_style='' )
- color green:
    inputs:
    - port thing:
        widget-help: Select a filename
        widget-hint: filename
    - port target_path:
        value: thing
        widget-hint: path
    outputs:
    - port value:
        widget-hint: string
    text: 'I''m lost and I''m found

      and I''m hungry like the wolf.

      '

>>> print yaml.dump( d3, default_flow_style=False, default_style='|' )
- "color green":
    "inputs":
    - "port thing":
        "widget-help": |-
          Select a filename
        "widget-hint": |-
          filename
    - "port target_path":
        "value": |-
          thing
        "widget-hint": |-
          path
    "outputs":
    - "port value":
        "widget-hint": |-
          string
    "text": |
      I'm lost and I'm found
      and I'm hungry like the wolf.

Ideally, I would like "short strings" to not use quotes, as in the first result. But I would like multi-line strings to be written as blocks, as with the second result. I guess fundamentally, I'm trying to minimize an explosion of unnecessary quotes in the file which I perceive would make it much more annoying to edit in a text editor.

Does anyone have any experience with this?

Answer 1

If you can use ruamel.yaml (disclaimer: I am the author of this enhanced version of PyYAML) you can do:

import ruamel.yaml

yaml_str = """\
- color green :
     inputs :
        - port thing :
            widget-hint : filename
            widget-help : Select a filename
        - port target_path :
            widget-hint : path
            value : 'thing'
     outputs:
        - port value:
             widget-hint : string
     text : |
            I'm lost and I'm found
            and I'm hungry like the wolf.
"""

data = ruamel.yaml.round_trip_load(yaml_str)
res = ""
for line in ruamel.yaml.round_trip_dump(data, indent=5, block_seq_indent=3).splitlines(True):
    res += line[3:]
print(res)

you get:

- color green:
       inputs:
          - port thing:
                 widget-hint: filename
                 widget-help: Select a filename
          - port target_path:
                 widget-hint: path
                 value: thing
       outputs:
          - port value:
                 widget-hint: string
       text: |
            I'm lost and I'm found
            and I'm hungry like the wolf.

Not exactly what you did start out with (but after this round-trip it is stable). With more recent versions of ruamel.yaml you can set both the indent and the relative indent of a sequence - within that indent. The latter however also influences your top-level sequence, hence the post processing.

Important (to me) things that are preserved: comments, anchors, mapping merges, and literal scalars ( multi-line using |)

Answer 2

Try the pyaml pretty printer. It gets closer, though it does put quotes around short strings with spaces in them:

>>> print pyaml.dump(d3)
- 'color green':
    inputs:
      - 'port thing':
          widget-help: 'Select a filename'
          widget-hint: filename
      - 'port target_path':
          value: thing
          widget-hint: path
    outputs:
      - 'port value':
          widget-hint: string
    text: |
      I'm lost and I'm found
      and I'm hungry like the wolf.