How to add internal whitespace to YAML sequence with Python ruamel

Question

I'm using ruamel.yaml to format YAML documents in Python:

#!/usr/bin/env python
import sys

from ruamel.yaml import *

yaml = YAML()

numbers = CommentedSeq()
numbers.fa.set_flow_style()
numbers.extend([1, 2, 3])

root = CommentedMap()
root['numbers'] = numbers
yaml.dump(root, sys.stdout)

I followed the advice in this accepted answer to format the sequence inline. This outputs the following:

numbers: [1, 2, 3]

How can I add internal whitespace, and output this instead:

numbers: [ 1, 2, 3 ]

Answer 1

In ruamel.yaml>=0.17.33 you can set the indicators on the Emitter class variable to influence this:

import sys

from ruamel.yaml import YAML, CommentedSeq

yaml = YAML()
yaml.Emitter.flow_seq_start = '[ '      # defaults to '['
yaml.Emitter.flow_seq_end = ' ]'        # defaults to ']'
yaml.Emitter.flow_seq_separator = ' ,'  # defaults to ','

numbers = CommentedSeq()
numbers.fa.set_flow_style()
numbers.extend([1, 2, 3])

root = dict(numbers=numbers)

yaml.dump(root, sys.stdout)

which gives:

numbers: [ 1 , 2 , 3 ]

In ruamel.yaml<=0.17.32 the bracket open and bracket closed were hardcoded in methods in the emitter, so the only thing you can do is provide alternative methods for those

import sys

from ruamel.yaml import *

class MyEmitter(ruamel.yaml.emitter.RoundTripEmitter):
    flow_start = '[ '   # these class attributes don't exist in RoundTripEmitter
    flow_end = ' ]'

    def expect_flow_sequence(self, force_flow_indent = False) -> None:
        if force_flow_indent:
            self.increase_indent(flow=True, sequence=True)
        ind = self.indents.seq_flow_align(
            self.best_sequence_indent, self.column, force_flow_indent
        )
        self.write_indicator(' ' * ind + self.flow_start, True, whitespace=True)
        if not force_flow_indent:
            self.increase_indent(flow=True, sequence=True)
        self.flow_context.append('[')
        self.state = self.expect_first_flow_sequence_item

    def expect_first_flow_sequence_item(self) -> None:
        if isinstance(self.event, SequenceEndEvent):
            self.indent = self.indents.pop()
            popped = self.flow_context.pop()
            assert popped == '['
            self.write_indicator(self.flow_end, False)
            if self.event.comment and self.event.comment[0]:
                # eol comment on empty flow sequence
                self.write_post_comment(self.event)
            elif self.flow_level == 0:
                self.write_line_break()
            self.state = self.states.pop()
        else:
            if self.canonical or self.column > self.best_width:
                self.write_indent()
            self.states.append(self.expect_flow_sequence_item)
            self.expect_node(sequence=True)

    def expect_flow_sequence_item(self) -> None:
        if isinstance(self.event, SequenceEndEvent):
            self.indent = self.indents.pop()
            popped = self.flow_context.pop()
            assert popped == '['
            if self.canonical:
                self.write_indicator(',', False)
                self.write_indent()
            self.write_indicator(self.flow_end, False)
            if self.event.comment and self.event.comment[0]:
                # eol comment on flow sequence
                self.write_post_comment(self.event)
            else:
                self.no_newline = False
            self.state = self.states.pop()
        else:
            self.write_indicator(',', False)
            if self.canonical or self.column > self.best_width:
                self.write_indent()
            self.states.append(self.expect_flow_sequence_item)
            self.expect_node(sequence=True)


yaml = YAML()
yaml.Emitter = MyEmitter

numbers = CommentedSeq()
numbers.fa.set_flow_style()
numbers.extend([1, 2, 3])

root = dict(numbers=numbers)

yaml.dump(root, sys.stdout)

which gives:

numbers: [ 1, 2, 3 ]