code: def validate_how(how: str) -> str:
      """Check the given how for join is valid."""
      if how == "full":
          warnings.warn(
              "Warning: While pandas-on-Spark will accept 'full', you should use 'outer' "
              + "instead to be compatible with the pandas merge API",
              UserWarning,
          )
      if how == "outer":
          # 'outer' in pandas equals 'full' in Spark
          how = "full"
      if how not in ("inner", "left", "right", "full"):
          raise ValueError(
              "The 'how' parameter has to be amongst the following values: ",
              "['inner', 'left', 'right', 'outer']",
          )
      return how
  
imports: import functools
  from contextlib import contextmanager
  import os
  from typing import Any, Callable, Dict, Iterator, List, Optional, Tuple, Union, TYPE_CHECKING, cast, no_type_check, overload
  import warnings
  import pandas
  from pandas.api.types import is_list_like
  from pyspark.sql import functions, Column, DataFrame, SparkSession
  from pyspark.sql.types import DoubleType
  from pyspark.sql.utils import is_remote, get_dataframe_class
  from pyspark.errors import PySparkTypeError
  from pyspark import pandas
  from pyspark.pandas._typing import Axis, Label, Name, DataFrameOrSeries
  from pyspark.pandas.typedef.typehints import as_spark_type
label_desc: Use image node_id 16 for calling a global function with example usage: validate_how(how) and returns: how