## evaluate(field, expression) {#field_comparisonexpression}

Evaluates comparison expression.


```python
def evaluate(self, field: aspose.words.fields.Field, expression: aspose.words.fields.ComparisonExpression):
    ...
```

| Parameter | Type | Description |
| --- | --- | --- |
| field | [Field](../../field/) |  |
| expression | [ComparisonExpression](../../comparisonexpression/) |  |

### Remarks

The implementation should return ``None`` to indicate that the default evaluation should be performed.



### Examples

Shows how to implement custom evaluation for the IF and COMPARE fields (ComparisonExpressionEvaluator).

```python
class ComparisonExpressionEvaluator(aw.fields.IComparisonExpressionEvaluator):

    def __init__(self, result):
        self.m_invocations = []
        self.m_result = result
        if self.m_result != None:
            print(self.m_result.error_message)
            print(self.m_result.result)

    def evaluate(self, field, expression):
        self.m_invocations.append([expression.left_expression, expression.comparison_operator, expression.right_expression])
        return self.m_result

    def assert_invocations_count(self, expected):
        self.assertEqual(expected, len(self.m_invocations))
        return self

    def assert_invocation_arguments(self, invocation_index, expected_left_expression, expected_comparison_operator, expected_right_expression):
        arguments = self.m_invocations[invocation_index]
        self.assertEqual(expected_left_expression, arguments[0])
        self.assertEqual(expected_comparison_operator, arguments[1])
        self.assertEqual(expected_right_expression, arguments[2])
        return self
```

### See Also

* module [aspose.words.fields](../../)
* class [IComparisonExpressionEvaluator](../)