✏ EDIT: composer.py

__all__ = ['Composer', 'ComposerError']

from .error import MarkedYAMLError
from .events import *
from .nodes import *

class ComposerError(MarkedYAMLError):
    pass

class Composer:

def __init__(self):
        self.anchors = {}

def check_node(self):
        # Drop the STREAM-START event.
        if self.check_event(StreamStartEvent):
            self.get_event()

# If there are more documents available?
        return not self.check_event(StreamEndEvent)

def get_node(self):
        # Get the root node of the next document.
        if not self.check_event(StreamEndEvent):
            return self.compose_document()

def get_single_node(self):
        # Drop the STREAM-START event.
        self.get_event()

# Compose a document if the stream is not empty.
        document = None
        if not self.check_event(StreamEndEvent):
            document = self.compose_document()

# Ensure that the stream contains no more documents.
        if not self.check_event(StreamEndEvent):
            event = self.get_event()
            raise ComposerError("expected a single document in the stream",
                    document.start_mark, "but found another document",
                    event.start_mark)

# Drop the STREAM-END event.
        self.get_event()

return document

def compose_document(self):
        # Drop the DOCUMENT-START event.
        self.get_event()

# Compose the root node.
        node = self.compose_node(None, None)

# Drop the DOCUMENT-END event.
        self.get_event()

self.anchors = {}
        return node

def compose_node(self, parent, index):
        if self.check_event(AliasEvent):
            event = self.get_event()
            anchor = event.anchor
            if anchor not in self.anchors:
                raise ComposerError(None, None, "found undefined alias %r"
                        % anchor, event.start_mark)
            return self.anchors[anchor]
        event = self.peek_event()
        anchor = event.anchor
        if anchor is not None:
            if anchor in self.anchors:
                raise ComposerError("found duplicate anchor %r; first occurence"
                        % anchor, self.anchors[anchor].start_mark,
                        "second occurence", event.start_mark)
        self.descend_resolver(parent, index)
        if self.check_event(ScalarEvent):
            node = self.compose_scalar_node(anchor)
        elif self.check_event(SequenceStartEvent):
            node = self.compose_sequence_node(anchor)
        elif self.check_event(MappingStartEvent):
            node = self.compose_mapping_node(anchor)
        self.ascend_resolver()
        return node

def compose_scalar_node(self, anchor):
        event = self.get_event()
        tag = event.tag
        if tag is None or tag == '!':
            tag = self.resolve(ScalarNode, event.value, event.implicit)
        node = ScalarNode(tag, event.value,
                event.start_mark, event.end_mark, style=event.style)
        if anchor is not None:
            self.anchors[anchor] = node
        return node

def compose_sequence_node(self, anchor):
        start_event = self.get_event()
        tag = start_event.tag
        if tag is None or tag == '!':
            tag = self.resolve(SequenceNode, None, start_event.implicit)
        node = SequenceNode(tag, [],
                start_event.start_mark, None,
                flow_style=start_event.flow_style)
        if anchor is not None:
            self.anchors[anchor] = node
        index = 0
        while not self.check_event(SequenceEndEvent):
            node.value.append(self.compose_node(node, index))
            index += 1
        end_event = self.get_event()
        node.end_mark = end_event.end_mark
        return node

def compose_mapping_node(self, anchor):
        start_event = self.get_event()
        tag = start_event.tag
        if tag is None or tag == '!':
            tag = self.resolve(MappingNode, None, start_event.implicit)
        node = MappingNode(tag, [],
                start_event.start_mark, None,
                flow_style=start_event.flow_style)
        if anchor is not None:
            self.anchors[anchor] = node
        while not self.check_event(MappingEndEvent):
            #key_event = self.peek_event()
            item_key = self.compose_node(node, None)
            #if item_key in node.value:
            #    raise ComposerError("while composing a mapping", start_event.start_mark,
            #            "found duplicate key", key_event.start_mark)
            item_value = self.compose_node(node, item_key)
            #node.value[item_key] = item_value
            node.value.append((item_key, item_value))
        end_event = self.get_event()
        node.end_mark = end_event.end_mark
        return node

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