code: def _convert_expression(expr):
      """Convert an object to an expression.
  
      Thus function converts an object to an expression (a unicode string) and
      checks to make sure it isn't empty after conversion. This is used to
      convert operators to their string representation for recursive calls to
      :func:`~pandas.eval`.
  
      Parameters
      ----------
      expr : object
          The object to be converted to a string.
  
      Returns
      -------
      s : unicode
          The string representation of an object.
  
      Raises
      ------
      ValueError
        * If the expression is empty.
      """
      s = pprint_thing(expr)
      _check_expression(s)
      return s
  
file_path: catboost/contrib/python/pandas/py2/pandas/core/computation/eval.py
imports: import tokenize
  import warnings
  from pandas.compat import string_types
  from pandas.util._validators import validate_bool_kwarg
  from pandas.core.computation.engines import _engines
  from pandas.core.computation.scope import _ensure_scope
  from pandas.io.formats.printing import pprint_thing
label_desc: Use image node_id 5 for calling a global function with example usage: _convert_expression(expr) and returns: s