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# modify-argument

Function argument modifications consist of a list of `modify-argument` nodes contained in modify-function, add-function or declare-function nodes. Nested remove-argument, rename to, remove-default-expression, replace-default-expression, replace-type, define-ownership, parent, reference-count, conversion-rule and and replace-value nodes specify the details of the modification.

    <modify-function>
        <modify-argument index="return | this | 1 ..." rename="..."
         invalidate-after-use = "true | false" pyi-type="...">
            // modifications
        </modify-argument>
    </modify-function>

Set the `index` attribute to “1” for the first argument, “2” for the second one and so on. Alternatively, set it to “return” or “this” if you want to modify the function’s return value or the object the function is called upon, respectively.

The optional `rename` attribute is used to rename a argument and use this new name in the generated code. This attribute can be used to enable the usage of `keyword arguments`.

The optional `pyi-type` attribute specifies the type to appear in the signature strings and `.pyi` files. The type string is determined by checking this attribute value, the replace-type modification and the C++ type. The attribute can be used for example to enclose a pointer return value within `Optional[]` to indicate that `None` can occur.

For the optional `invalidate-after-use` attribute, see Invalidation after use .

## Naming, type, default value modifications

### remove-argument

The `remove-argument` node removes the given argument from the function’s signature, and it is a child of the modify-argument node.

    <modify-argument>
        <remove-argument />
    </modify-argument>

### rename to

The `rename to` node is used to rename a argument and use this new name in the generated code, and it is a child of the modify-argument node.

    <modify-argument>
        <rename to='...' />
    </modify-argument>

Warning

This tag is deprecated, use the `rename` attribute from modify-argument tag instead.

### remove-default-expression

The `remove-default-expression` node disables the use of the default expression for the given argument, and it is a child of the modify-argument node.

    <modify-argument...>
        <remove-default-expression />
    </modify-argument>

### replace-default-expression

The `replace-default-expression` node replaces the specified argument with the expression specified by the `with` attribute, and it is a child of the modify-argument node.

    <modify-argument>
        <replace-default-expression with="..." />
    </modify-argument>

### replace-type

The `replace-type` node replaces the type of the given argument to the one specified by the `modified-type` attribute, and it is a child of the modify-argument node.

    <modify-argument>
        <replace-type modified-type="..." />
    </modify-argument>

If the new type is a class, the `modified-type` attribute must be set to the fully qualified name (including name of the package as well as the class name).

## Ownership/Reference modifications

### define-ownership

The `define-ownership` tag indicates that the function changes the ownership rules of the argument object, and it is a child of the modify-argument node.

    <modify-argument>
          <define-ownership class="target | native"
                            owner="target | c++ | default" />
    </modify-argument>

The `class` attribute specifies the class of function where to inject the ownership altering code (see Code Generation Terminology). The `owner` attribute specifies the new ownership of the object. It accepts the following values:

- target: the target language will assume full ownership of the object. The native resources will be deleted when the target language object is finalized.

- c++: The native code assumes full ownership of the object. The target language object will not be garbage collected.

- default: The object will get default ownership, depending on how it was created.

### reference-count

The `reference-count` tag dictates how an argument should be handled by the target language reference counting system (if there is any), it also indicates the kind of relationship the class owning the function being modified has with the argument (represented as lists of referred-to objects stored in the owner class). It is a child of the modify-argument node.

    <modify-argument>
          <reference-count action="add|remove|set|ignore" variable-name="..." />
    </modify-argument>

The `action` attribute specifies what should be done to the argument reference counting when the modified method is called. It accepts the following values:

- add: Adds the argument to the list of previous argument values stored under this `variable-name` or function signature and increments the argument reference counter.

- remove: Decrements the argument reference counter and removes it from the list of argument values stored under this `variable-name` or function signature.

- set: Decreases the reference count of the previously stored argument values under this `variable-name` or function signature and removes them. Stores the argument and increments the argument reference counter.

- ignore: does nothing with the argument reference counter (sounds worthless, but could be used in situations where the reference counter increase is mandatory by default).

The `variable-name` attribute specifies the name used for the variable that holds the reference(s). It defaults to the function signature.

For instance, in a model/view relation, a view receiving a model as argument for a setModel() method should increment the model’s reference counting, since the model should be kept alive as long as the view lives. Remember that our hypothetical view cannot become a parent of the model, since the said model could be used by other views as well.

### parent

The `parent` node lets you define the argument parent which will take ownership of argument and will destroy the C++ child object when the parent is destroyed (see Parent-child relationship). It is a child of the modify-argument node.

    <modify-argument index="1">
          <parent index="this" action="add | remove" />
    </modify-argument>

In the `index` argument you must specify the parent argument. The action add creates a parent link between objects, while remove will undo the parentage relationship.

## Other modifications

### conversion-rule

The `conversion-rule` node allows you to write customized code to convert the given argument between the target language and C++. It is then a child of the modify-argument node:

    <modify-argument index="2">
    <!-- for the second argument of the function -->
    <conversion-rule class="target | native">
        // the code
    </conversion-rule>
    </modify-argument>

The `class` attribute accepts one of the following values to define the conversion direction to be either `target-to-native` (Python to C++) or `native-to-target` (C++ to Python):

- `native`: Defines the conversion direction to be `target-to-native`.

    It is similar to the existing `<target-to-native>` element. See Conversion Rule Tag for more information.

- `target`: Defines the conversion direction to be `native-to-target`.

    It is similar to the existing `<native-to-target>` element. See Conversion Rule Tag for more information.

The conversions appear in the generated code (see Code Generation Terminology) as follows:

| Python Wrapper | C++ Wrapper
Parameters | `native` | `target`
Return value | `target` | `native`

This node is typically used in combination with the replace-type and remove-argument nodes. The given code is used instead of the generator’s conversion code.

An example for removing an argument might be a C++ function accepting a C-style array and a length parameter, which is modified to receive a Python list instead. The array parameter’s type would be modified to `PySequence` with `native` conversion rule; and the length parameter would be removed, specifying a native `native` conversion rule which would determine the value from the size of the Python sequence passed.

Writing %N in the code (where N is a number), will insert the name of the nth argument. Alternatively, %in and %out which will be replaced with the name of the conversion’s input and output variable, respectively. Note the output variable must be declared explicitly, for example:

    <conversion-rule class="native">
    bool %out = (bool) %in;
    </conversion-rule>

Note

Conversion rules for the C++ Wrapper only need to be specified if the function is virtual and overidable.

Note

You can also use the `conversion-rule` node to specify a conversion code which will be used instead of the generator’s conversion code everywhere for a given type.

### replace-value

The `replace-value` attribute lets you replace the return statement of a function with a fixed string. This attribute can only be used for the argument at `index` 0, which is always the function’s return value.

    <modify-argument index="0" replace-value="this"/>