What does isinstance mean combined with ScalarNode, SequenceNode, and MappingNode?

Question

I have googled without success so I would like what does isinstance means combined with yaml ScalarNode, SequenceNode and MappingNode? (I know already what isinstance is)

For example

if isinstance(v,yaml.ScalarNode):
  #do something
elif isinstance(v,yaml.SequenceNode):
  #something else
elif isinstance(v, yaml.MappingNode):
  #another thing

Answer 1

Node types are a part of YAML's Representation data structure. YAML defines its (de)serialization pipeline as follows:

The Representation is a, potentially cyclic, graph of nodes. In it, anchors and aliases have been resolved. In PyYAML, you typically use subgraphs of this data structure to implement custom constructors and representers that generate your native objects, as indicated by the arrows in the diagram.

A ScalarNode is a node representing a single scalar in the YAML source or output. A single scalar can be a plain scalar (eg foo ), a quoted scalar ( 'foo' or "foo" ), or a block scalar (starting with | or > ). The scalar content, with escape sequences, newlines, indentation already processed, is available in the field .value as string. This is even true for values that are by default constructed into non-strings. For example, true will by default generate a boolean value, but as ScalarNode, it contains the value "true" as string.

SequenceNode is a node representing a sequence. The value field of a SequenceNode contains a list of nodes that correspond to the items in the sequence.

MappingNode is a node representing a mapping. The value field of a MappingNode contains a list of tuples, where each tuple consists of the key node and the value node.

All nodes have a field tag that contains the resolved tag of the node. ie a ScalarNode with value true would typically have the tag yaml.org,2002:bool . The resolved tag depends on the loader you use, for example if you use PyYAML's BaseLoader , true will resolve to a normal string, which is yaml.org,2002:str . In any case, if there was an explicit tag on the node (eg !!str , that tag will be in the tag field.

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Coming back to the question, this kind of code is typically used in custom constructors. They get a node as input and are to produce a native value and return it. Usually, a custom constructor expects a specific kind of node but if you want to do proper error reporting, you still want to check whether you actually got the kind of node you need. For this, you use the code you posted.