parameters: (self, expr, default=None)
docstring: ``default()`` for the sandboxed code.
  
          Try to evaluate an expression and return the result or a
          fallback-/default-value; the `default`-value is used
          if `expr` does not exist/is invalid/results in None.
  
          This is very useful for optional data.
  
          :Parameter:
              - expr: eval-expression
              - default: fallback-falue if eval(expr) fails or is None.
          :Returns:
              the eval-result or the "fallback"-value.
  
          :Note:      the eval-expression has to be quoted! (like in eval)
          :Example:   see module-docstring
docstring_summary: ``default()`` for the sandboxed code.
docstring_tokens: default () for the sandboxed code .
function: def f_default(self, expr, default=None):
          """``default()`` for the sandboxed code.
  
          Try to evaluate an expression and return the result or a
          fallback-/default-value; the `default`-value is used
          if `expr` does not exist/is invalid/results in None.
  
          This is very useful for optional data.
  
          :Parameter:
              - expr: eval-expression
              - default: fallback-falue if eval(expr) fails or is None.
          :Returns:
              the eval-result or the "fallback"-value.
  
          :Note:      the eval-expression has to be quoted! (like in eval)
          :Example:   see module-docstring
          """
          try:
              r = self.eval(expr, self.locals_ptr)
              if r is None:
                  return default
              return r
          #TODO: which exceptions should be catched here?
          except (NameError, LookupError, TypeError):
              return default
function_tokens: def f_default ( self , expr , default = None ) : try : r = self . eval ( expr , self . locals_ptr ) if r is None : return default return r #TODO: which exceptions should be catched here? except ( NameError , LookupError , TypeError ) : return default