ruamel.yaml.round_trip_dump expected 4 but found 2 (indentation) - yaml

Question

Code:
#!/usr/bin/env python2
import sys
import ruamel.yaml
yaml_str = '''
---
project: ''
project_lead: &ptl
name: ''
company: ''
email: ''
id: ''
timezone: ''
primary_contact: *ptl
committers:
- <<: *ptl
- name: ''
email: ''
company: ''
id: ''
timezone: ''
- name: ''
email: ''
company: ''
id: ''
timezone: ''
'''
DATA = ruamel.yaml.round_trip_load(yaml_str, version=None, preserve_quotes=True)
indent = 4
block_seq_indent = 2
ruamel.yaml.round_trip_dump(DATA, sys.stdout, default_flow_style=True, indent=indent,
block_seq_indent=block_seq_indent)
Output:
project: ''
project_lead: &ptl
name: ''
company: ''
email: ''
id: ''
timezone: ''
primary_contact: *ptl
committers:
- <<: *ptl
- name: ''
email: ''
company: ''
id: ''
timezone: ''
- name: ''
email: ''
company: ''
id: ''
timezone: ''
ISSUE:
foo.yaml|10 col 3 error| wrong indentation: expected 4 but found 2 (indentation)
If I increase
block_seq_indent = 4
the values following it (email, company, id, timezone) don't follow its indentation.
Is my source YAML 'wrong'? Can I work around this? The ruamel module is great, and I'm able to add values programmatically, but I can't seem to get valid YAML in my output.

Answer 1

Your input has four positions indent on the (nested) block mapping that is the value for project_lead, and it has six indent on the sequence that is the value for committers, with an offset for the dash of four (in sequences the indent is counted to the beginning of the sequence element).
This does indeed go wrong if you use the old API as you do, but with the new API (and the appropriate values), you can do:
import sys
import ruamel.yaml
from cStringIO import StringIO
yaml_str = '''\
---
project: ''
project_lead: &ptl
name: ''
company: ''
email: ''
id: ''
timezone: ''
primary_contact: *ptl
committers:
- <<: *ptl
- name: ''
email: ''
company: ''
id: ''
timezone: ''
- name: ''
email: ''
company: ''
id: ''
timezone: ''
'''
yaml = ruamel.yaml.YAML()
yaml.preserve_quotes = True
yaml.indent(mapping=4, sequence=6, offset=4)
yaml.explicit_start = True
data = yaml.load(yaml_str)
yaml.dump(data, sys.stdout)
buf = StringIO()
yaml.dump(data, buf)
assert buf.getvalue() == yaml_str
without getting an error.
Please note:
the default_flow_style=True has no effect in your example, they only would affect new mappings and lists, and I am not sure you want to do that for all added values.
I added yaml.explicit_start = True to get the leading ---
Your example starts with an empty newline (directly after the ''', that is as if you did yaml_str = '\n---\nproject: .....'). You cannot really get that output with ruamel.yaml() from a data structure, hence the backslash in my yaml_str.
although this might be considered a bug in the old API, I have no intention of fixing it.