Prettily dump 2D array to YAML in Python

Question

Suppose I have a 2D array defined like so:

import sys

from ruamel.yaml import YAML

yaml = YAML()
yaml.version = (1,2)

def main():
    data = {
        "foo": [
            [1,2,3],
            [4,5,6]
        ]
    }
    yaml.dump(data, sys.stdout)

if __name__ == '__main__':
    main()

I would like to output a "readable", valid YAML file with each "row" on a separate line:

"foo":
- [1,2,3]
- [4,5,6]

or even

"foo": [
  [1,2,3],
  [4,5,5]
]

I've looked into ruamel.yaml but the default behavior is each column on a separate line which, while valid, is not easily readable:

%YAML 1.2
---
foo:
- - 1
  - 2
  - 3
- - 4
  - 5
  - 6

Answer 1

When you set the attribute .default_flow_style to None (instead of the default value False ), your leaf-nodes will be represented in flow style:

import sys
import ruamel.yaml


yaml = ruamel.yaml.YAML()
yaml.version = (1,2)
yaml.default_flow_style = None

def main():
    data = {
        "foo": [
            [1,2,3],
            [4,5,6]
        ]
    }
    yaml.dump(data, sys.stdout)

if __name__ == '__main__':
    main()

which gives:

%YAML 1.2
---
foo:
- [1, 2, 3]
- [4, 5, 6]

But the above works for the whole file.

If you want an individual lists to be represented as flow-style sequences in YAML, you should make them of the type CommentedSeq and then you can set the attribute per object . That is also the way ruamel.yaml "knows" how to preserve the style of a sequence when round-tripping:

import sys
import ruamel.yaml

yaml = ruamel.yaml.YAML()
yaml.version = (1,2)

a = ruamel.yaml.comments.CommentedSeq([1,2,3])
a.fa.set_flow_style()


def main():
    data = {
        "foo": [
        a,
            [4,5,6]
        ]
    }
    yaml.dump(data, sys.stdout)

if __name__ == '__main__':
    main()

giving:

%YAML 1.2
---
foo:
- [1, 2, 3]
- - 4
  - 5
  - 6