parameters: (geob, field)
return_statement: return field, None
docstring: Check if a field given by user contains
      join fields and external field.
docstring_summary: Check if a field given by user contains
      join fields and external field.
docstring_tokens: Check if a field given by user contains join fields and external field .
function: def check_ext_field(geob, field):
      """
      Check if a field given by user contains
      join fields and external field.
      """
      l = field.split(':', 1)
  
      if len(l) <= 1:
          return field, None
  
      f, ext_f = l
  
      if geob.hasJoin(f):
          return f, ext_f
  
      # In case of multiple join fields
      f = tuple(f.split(SPLIT))
  
      if geob.hasJoin(f):
          return f, ext_f
  
      return field, None
function_tokens: def check_ext_field ( geob , field ) : l = field . split ( ':' , 1 ) if len ( l ) <= 1 : return field , None f , ext_f = l if geob . hasJoin ( f ) : return f , ext_f # In case of multiple join fields f = tuple ( f . split ( SPLIT ) ) if geob . hasJoin ( f ) : return f , ext_f return field , None