Fix AttributeError when node.id is accessed on non-Node objects

[veeceey]

Summary

Fixes #907 by safely accessing node.id and node.start_mark attributes in error handling code.

Problem

When constructor methods like construct_scalar(), construct_sequence(), etc. receive a non-Node object (e.g., None, a type object, or any arbitrary Python object), the error handling code unconditionally accesses node.id and node.start_mark, which causes an AttributeError before the intended ConstructorError can be raised.

Solution

Use getattr() to safely access these attributes:

• For node.id: fall back to type(node).__name__ to still provide useful type information in the error message
• For node.start_mark: fall back to None since position information isn't available for non-Node objects

Testing

Tested with the reproduction case from the issue:

import yaml

class Null:
    pass

obj = yaml.loader.SafeLoader("")
ret = obj.construct_yaml_float(Null)

Before: AttributeError: type object 'Null' has no attribute 'id'
After: ConstructorError: expected a scalar node, but found type

Also verified that normal YAML parsing continues to work correctly.

[veeceey]

Manual Test Results

I've verified the fix works correctly with various test cases:

Test 1: Original reproduction case (Null type)

import yaml

class Null:
    pass

obj = yaml.loader.SafeLoader("")
ret = obj.construct_yaml_float(Null)

Result: ✅ ConstructorError: expected a scalar node, but found type (previously raised AttributeError)

Test 2: None object

obj = yaml.loader.SafeLoader("")
ret = obj.construct_scalar(None)

Result: ✅ ConstructorError: expected a scalar node, but found NoneType

Test 3: String instead of SequenceNode

obj = yaml.loader.SafeLoader("")
ret = obj.construct_sequence("not a node")

Result: ✅ ConstructorError: expected a sequence node, but found str

Test 4: Custom object instead of MappingNode

class CustomObject:
    pass

obj = yaml.loader.SafeLoader("")
ret = obj.construct_mapping(CustomObject())

Result: ✅ ConstructorError: expected a mapping node, but found CustomObject

Test 5: Integer instead of MappingNode

obj = yaml.loader.SafeLoader("")
ret = obj.construct_pairs(42)

Result: ✅ ConstructorError: expected a mapping node, but found int

Test 6: Normal YAML parsing

data = yaml.safe_load("key: value")

Result: ✅ Works correctly, returns {'key': 'value'}

All tests pass. The fix ensures proper ConstructorError exceptions are raised instead of AttributeError when invalid objects are passed to constructor methods, while maintaining backward compatibility for normal YAML parsing.

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.