code: def at(a):
      """
      Access a single value for a row/column label pair.
  
      Similar to ``loc``, in that both provide label-based lookups. Use
      ``at`` if you only need to get or set a single value in a DataFrame
      or Series.
  
      Raises
      ------
      KeyError
          If 'label' does not exist in DataFrame.
  
      See Also
      --------
      DataFrame.iat : Access a single value for a row/column pair by integer
          position.
      DataFrame.loc : Access a group of rows and columns by label(s).
      Series.at : Access a single value using a label.
  
      Examples
      --------
      >>> import mars.dataframe as md
      >>> df = md.DataFrame([[0, 2, 3], [0, 4, 1], [10, 20, 30]],
      ...                   index=[4, 5, 6], columns=['A', 'B', 'C'])
      >>> df.execute()
          A   B   C
      4   0   2   3
      5   0   4   1
      6  10  20  30
  
      Get value at specified row/column pair
  
      >>> df.at[4, 'B'].execute()
      2
  
      # Set value at specified row/column pair
      #
      # >>> df.at[4, 'B'] = 10
      # >>> df.at[4, 'B']
      # 10
  
      Get value within a Series
  
      >>> df.loc[5].at['B'].execute()
      4
      """
      return DataFrameAt(a)
  
imports: import numpy
  from .loc import DataFrameLoc
label_desc: Use image node_id 1 for calling a global function with example usage: at(a) and returns: DataFrameAt