1.1 --- a/lib/operator/core.py	Tue Nov 29 17:25:54 2016 +0100
     1.2 +++ b/lib/operator/core.py	Tue Nov 29 17:27:31 2016 +0100
     1.3 @@ -24,7 +24,7 @@
     1.4  
     1.5  from native import _is as is_, _is_not as is_not
     1.6  
     1.7 -def binary_op(a, b, left_accessor, right_accessor):
     1.8 +def binary_op(a, b, left_accessor, right_accessor, default=None):
     1.9  
    1.10      """
    1.11      A single parameterised function providing the binary operator mechanism for
    1.12 @@ -57,11 +57,14 @@
    1.13              return result
    1.14  
    1.15      # Where no methods were available, or if neither method could support the
    1.16 -    # operation, raise an exception.
    1.17 +    # operation, raise an exception or provide a default result.
    1.18  
    1.19 -    raise TypeError
    1.20 +    if default is None:
    1.21 +        raise TypeError
    1.22 +    else:
    1.23 +        return default
    1.24  
    1.25 -def unary_op(a, accessor):
    1.26 +def unary_op(a, accessor, default=None):
    1.27  
    1.28      """
    1.29      A single parameterised function providing the unary operator mechanism for
    1.30 @@ -81,9 +84,12 @@
    1.31              return result
    1.32  
    1.33      # Where no method was available, or if the method could not support the
    1.34 -    # operation, raise an exception.
    1.35 +    # operation, raise an exception or provide a default result.
    1.36  
    1.37 -    raise TypeError
    1.38 +    if default is None:
    1.39 +        raise TypeError
    1.40 +    else:
    1.41 +        return default
    1.42  
    1.43  def augassign(a, b, augmented_accessor, left_accessor, right_accessor):
    1.44