Specific formatting of sequences in YAML - ruamel.yaml

Question

Is it possible to output sequences with ruamel.yaml in the following format:
-
key1: 1
key2: 2
key3: 3
instead of standard
- key1: 1
key2: 2
key3: 3
...and this
- skills:
- Python
- Perl
instead of standard...
- skills:
- Python
- Perl
The second example is what yaml.indent(sequence = 4, offset = 2) should be for. But then the top-level list also gets indented:
- skills:
- Python
- Perl

Answer 1

These are essentially two questions: the first on how to prevent
getting the default compact representation and the second on using
different indentations for different sequences.
To start with the first: you can achieve that using the .compact() method (make sure you
have ruamel.yaml>=0.15.73 installed):
import sys
import ruamel.yaml
yaml_str = """\
-
key1: 1
key2: 2
key3: 3
"""
yaml = ruamel.yaml.YAML()
yaml.compact(seq_map=False)
data = yaml.load(yaml_str)
yaml.dump(data, sys.stdout)
which gives:
-
key1: 1
key2: 2
key3: 3
You can also provide seq_seq=False as argument to .compact() if you don't want compact
sequences within sequences (looking like - - abc)
The second is not possible out of the box, as indenting (and
compacting for that matter) is always applied to all sequences
resp. mappings, even to a root level sequence.
The best is to use a transform function:
import sys
import ruamel.yaml
yaml_str = """\
- skills:
- Python
- Perl
"""
def dedent2(s):
return ''.join([x[2:] if x.startswith(' ') else x for x in s.splitlines(True)])
yaml = ruamel.yaml.YAML()
yaml.indent(sequence=4, offset=2)
data = yaml.load(yaml_str)
yaml.dump(data, sys.stdout, transform=dedent2)
which gives:
- skills:
- Python
- Perl