Parsing YAML, get line numbers even in ordered maps

Question

I need to get the line numbers of certain keys of a YAML file.

Please note, this answer does not solve the issue: I do use ruamel.yaml, and the answers do not work with

Answer 1

This issue is not that you are using !omap and that it doesn't give you the line-numbers as with "normal" mappings. That should be clear from the fact that you get 4 from doing print(data['key1']['key4'].lc.line) (where key4 is a key in the outer !omap).

As this answers indicates,

you can access the property lc on collection items

The value for data['key1']['key4'] is a collection item (another !omap), but the value for data['key1']['key2'] is not a collection item but a, built-in, python string, which has no slot to store the lc attribute.

To get an .lc attribute on a non-collection like a string you have to subclass the RoundTripConstructor, to use something like the classes in scalarstring.py (with __slots__ adjusted to accept the lc attribute and then transfer the line information available in the nodes to that attribute and then set the line, column information:

import sys
import ruamel.yaml

yaml_str = """
key1: !!omap
  - key2: item2
  - key3: item3
  - key4: !!omap
    - key5: 'item5'
    - key6: |
        item6
"""

class Str(ruamel.yaml.scalarstring.ScalarString):
    __slots__ = ('lc')

    style = ""

    def __new__(cls, value):
        return ruamel.yaml.scalarstring.ScalarString.__new__(cls, value)

class MyPreservedScalarString(ruamel.yaml.scalarstring.PreservedScalarString):
    __slots__ = ('lc')

class MyDoubleQuotedScalarString(ruamel.yaml.scalarstring.DoubleQuotedScalarString):
    __slots__ = ('lc')

class MySingleQuotedScalarString(ruamel.yaml.scalarstring.SingleQuotedScalarString):
    __slots__ = ('lc')

class MyConstructor(ruamel.yaml.constructor.RoundTripConstructor):
    def construct_scalar(self, node):
        # type: (Any) -> Any
        if not isinstance(node, ruamel.yaml.nodes.ScalarNode):
            raise ruamel.yaml.constructor.ConstructorError(
                None, None,
                "expected a scalar node, but found %s" % node.id,
                node.start_mark)

        if node.style == '|' and isinstance(node.value, ruamel.yaml.compat.text_type):
            ret_val = MyPreservedScalarString(node.value)
        elif bool(self._preserve_quotes) and isinstance(node.value, ruamel.yaml.compat.text_type):
            if node.style == "'":
                ret_val = MySingleQuotedScalarString(node.value)
            elif node.style == '"':
                ret_val = MyDoubleQuotedScalarString(node.value)
            else:
                ret_val = Str(node.value)
        else:
            ret_val = Str(node.value)
        ret_val.lc = ruamel.yaml.comments.LineCol()
        ret_val.lc.line = node.start_mark.line
        ret_val.lc.col = node.start_mark.column
        return ret_val


yaml = ruamel.yaml.YAML()
yaml.Constructor = MyConstructor

data = yaml.load(yaml_str)
print(data['key1']['key4'].lc.line)
print(data['key1']['key2'].lc.line)
print(data['key1']['key4']['key6'].lc.line)

Please note that the output of the last call to print is 6, as the literal scalar string starts with the |.

If you also want to dump data, you'll need to make a Representer aware of those My.... types.