class_docstr: Perform a database (SQL) merge operation between two DataFrame objects
  using either columns as keys or their row indexes
class_docstr_tok: ["Perform","a","database","(","SQL",")","merge","operation","between","two","DataFrame","objects","using","either","columns","as","keys","or","their","row","indexes"]
code: def _indicator_pre_merge(self, left, right):
      columns = left.columns.union(right.columns)
  
      for i in ["_left_indicator", "_right_indicator"]:
          if i in columns:
              raise ValueError(
                  "Cannot use `indicator=True` option when "
                  "data contains a column named {name}".format(name=i)
              )
      if self.indicator_name in columns:
          raise ValueError(
              "Cannot use name of an existing column for indicator column"
          )
  
      left = left.copy()
      right = right.copy()
  
      left["_left_indicator"] = 1
      left["_left_indicator"] = left["_left_indicator"].astype("int8")
  
      right["_right_indicator"] = 2
      right["_right_indicator"] = right["_right_indicator"].astype(
          "int8"
      )
  
      return left, right
  
imports: import copy
  import string
  import warnings
  import numpy
  from pandas._libs import hashtable, join, lib
  import pandas.compat
  from pandas.compat import filter, lzip, map, range, zip
  from pandas.errors import MergeError
  from pandas.util._decorators import Appender, Substitution
  from pandas.core.dtypes.common import ensure_float64, ensure_int64, ensure_object, is_array_like, is_bool, is_bool_dtype, is_categorical_dtype, is_datetime64_dtype, is_datetime64tz_dtype, is_datetimelike, is_dtype_equal, is_extension_array_dtype, is_float_dtype, is_int64_dtype, is_integer, is_integer_dtype, is_list_like, is_number, is_numeric_dtype, is_object_dtype, needs_i8_conversion
  from pandas.core.dtypes.missing import isnull, na_value_for_dtype
  from pandas import Categorical, DataFrame, Index, MultiIndex, Series, Timedelta
  import pandas.core.algorithms
  from pandas.core.arrays.categorical import _recode_for_categories
  import pandas.core.common
  from pandas.core.frame import _merge_doc
  from pandas.core.internals import concatenate_block_managers, items_overlap_with_suffix
  import pandas.core.sorting
  from pandas.core.sorting import is_int64_overflow_possible
label_desc: Use image node_id 3 for calling the _MergeOperation obj's underlying member method code with example usage: obj._indicator_pre_merge(left, right) and returns: left,  right