code: def from_unixtime(unixtime, format=None) -> Expression:
      """
      Converts unix timestamp (seconds since '1970-01-01 00:00:00' UTC) to datetime string the given
      format. The default format is "yyyy-MM-dd HH:mm:ss".
      """
      if format is None:
          return _unary_op("fromUnixtime", unixtime)
      else:
          return _binary_op("fromUnixtime", unixtime, format)
  
imports: from typing import Union
  from pyflink import add_version_doc
  from pyflink.java_gateway import get_gateway
  from pyflink.table.expression import Expression, _get_java_expression, TimePointUnit, JsonOnNull
  from pyflink.table.types import _to_java_data_type, DataType
  from pyflink.table.udf import UserDefinedFunctionWrapper
  from pyflink.util.java_utils import to_jarray, load_java_class
label_desc: Use image node_id 28 for calling a global function with example usage: from_unixtime(unixtime, format) and returns: _unary_op, _binary_op