code: def _obtain(
      self,
      dist: "Distribution",
      field: str,
      package_dir: Mapping[str, str],
  ):
      if field in self.dynamic_cfg:
          return self._expand_directive(
              f"tool.setuptools.dynamic.{field}",
              self.dynamic_cfg[field],
              package_dir,
          )
      self._ensure_previously_set(dist, field)
      return None
  
imports: import logging
  import os
  from contextlib import contextmanager
  from functools import partial
  from typing import TYPE_CHECKING, Callable, Dict, Mapping, Optional, Set, Union
  from ..errors import FileError, InvalidConfigError
  from ..warnings import SetuptoolsWarning
  from .None import expand
  from ._apply_pyprojecttoml import _PREVIOUSLY_DEFINED, _MissingDynamic
  from ._apply_pyprojecttoml import apply
label_desc: Use image node_id 12 for calling the _ConfigExpander obj's underlying member method code with example usage: obj._obtain(dist, field, package_dir) and returns: None, self