[Bug]: Malformed Numeric Scalars Crash SafeLoader with ValueError

[tylzh97]

Summary

Numeric constructors accept malformed scalars (0x_, !!float +_, !!float 1::3) and pass them straight into int/float, bubbling uncaught ValueError.

poc:

import yaml
yaml.safe_load("0x_")
yaml.safe_load("!!float +_")
yaml.safe_load("!!float 1::3")

Traceback:

$ uv run python 
Python 3.10.18 (main, Sep  2 2025, 14:19:37) [Clang 20.1.4 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import yaml
>>> yaml.safe_load("0x_")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/__init__.py", line 125, in safe_load
    return load(stream, SafeLoader)
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/__init__.py", line 81, in load
    return loader.get_single_data()
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/constructor.py", line 51, in get_single_data
    return self.construct_document(node)
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/constructor.py", line 55, in construct_document
    data = self.construct_object(node)
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/constructor.py", line 100, in construct_object
    data = constructor(self, node)
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/constructor.py", line 250, in construct_yaml_int
    return sign*int(value[2:], 16)
ValueError: invalid literal for int() with base 16: ''

Suggested Fix

• Validate prefixes and segments before conversion.
• Wrap numeric parsing in try/except and raise ConstructorError.