# Specification Files

A SIP specification consists of some C/C++ type and function declarations and some directives. The declarations may contain annotations which provide SIP with additional information that cannot be expressed in C/C++. SIP does not implement a full C/C++ parser.

## Syntax Definition

The following is a semi-formal description of the syntax of a specification file.

    specification ::= {module-statement}

    module-statement ::= [module-directive | statement]

    module-directive ::= [
            %CompositeModule |
            %Copying |
            %DefaultDocstringFormat |
            %DefaultDocstringSignature |
            %DefaultEncoding |
            %DefaultMetatype |
            %DefaultSupertype |
            %ExportedHeaderCode |
            %ExportedTypeHintCode |
            %Extract |
            %Feature |
            %HideNamespace |
            %Import |
            %Include |
            %InitialisationCode |
            %License |
            %MappedType |
            %Module |
            %ModuleCode |
            %ModuleHeaderCode |
            %Platforms |
            %PreInitialisationCode |
            %PostInitialisationCode |
            %Timeline |
            %TypeHintCode |
            %UnitCode |
            %UnitPostIncludeCode |
            %VirtualErrorHandler |
            mapped-type-template]

    statement :: [class-statement | function | variable]

    class-statement :: [
            %If |
            class |
            class-template |
            enum |
            namespace |
            opaque-class |
            operator |
            struct |
            union |
            typedef |
            exception]

    class ::= class name [: super-classes] [class-annotations]
            { {class-line} };

    super-classes ::= [public | protected | private] name
            [, super-classes]

    class-line ::= [
            class-statement |
            %BIGetBufferCode |
            %BIReleaseBufferCode |
            %ConvertToSubClassCode |
            %ConvertToTypeCode |
            %Docstring |
            %FinalisationCode |
            %GCClearCode |
            %GCTraverseCode |
            %InstanceCode |
            %PickleCode |
            %TypeCode |
            %TypeHeaderCode |
            %TypeHintCode |
            constructor |
            destructor |
            method |
            static-method |
            virtual-method |
            special-method |
            operator |
            virtual-operator |
            class-variable |
            public: |
            public Q_SLOTS: |
            public slots: |
            protected: |
            protected Q_SLOTS: |
            protected slots: |
            private: |
            private Q_SLOTS: |
            private slots: |
            Q_SIGNALS: |
            signals:]

    constructor ::= [explicit] name ( [argument-list] )
            [noexcept] [function-annotations]
            [c++-constructor-signature] ; [%Docstring]
            [%MethodCode]

    c++-constructor-signature ::= [( [argument-list] )]

    destructor ::= [virtual] ~ name () [noexcept] [= 0]
            [function-annotations] ; [%MethodCode]
            [%VirtualCatcherCode]

    method ::= [Q_SIGNAL] [Q_SLOT] type name (
            [argument-list] ) [const] [final] [noexcept]
            [= 0] [function-annotations] [c++-signature] ;
            [%Docstring] [%MethodCode]

    c++-signature ::= [ type ( [argument-list] )]

    static-method ::= static function

    virtual-method ::= [Q_SIGNAL] [Q_SLOT] virtual type name
            ( [argument-list] ) [const] [final]
            [noexcept] [= 0] [function-annotations] [c++-signature]
            ; [%MethodCode] [%VirtualCatcherCode]
            [%VirtualCallCode]

    special-method ::= type special-method-name
            ( [argument-list] ) [function-annotations] ;
            [%MethodCode]

    special-method-name ::= [__abs__ | __add__ | __and__ |
            __aiter__ | __anext__ | __await__ | __bool__ |
            __call__ | __contains__ | __delattr__ |
            __delitem__ | __div__ | __eq__ | __float__ |
            __floordiv__ | __ge__ | __getattr__ |
            __getattribute__ | __getitem__ | __gt__ |
            __hash__ | __iadd__ | __iand__ | __idiv__ |
            __ifloordiv__ | __ilshift__ | __imatmul__ |
            __imod__ | __imul__ | __index__ | __int__ |
            __invert__ | __ior__ | __irshift__ | __isub__ |
            __iter__ | __itruediv__ | __ixor__ | __le__ |
            __len__ | __lshift__ | __lt__ | __matmul |
            __mod__ | __mul__ | __ne__ | __neg__ |
            __next__ | __or__ | __pos__ | __repr__ |
            __rshift__ | __setattr__ | __setitem__ | __str__ |
            __sub__ | __truediv__ | __xor__]

    operator ::= operator-type
            ( [argument-list] ) [const] [final]
            [noexcept] [function-annotations] ;
            [%MethodCode]

    virtual-operator ::= virtual operator-type
            ( [argument-list] ) [const] [final]
            [noexcept] [= 0] [function-annotations] ;
            [%MethodCode] [%VirtualCatcherCode]
            [%VirtualCallCode]

    operatator-type ::= [ operator-function | operator-cast ]

    operator-function ::= type operator operator-name

    operator-cast ::= operator type

    operator-name ::= [+ | - | * | / | % | & |
            | | ^ | << | >> | += | -= | *= |
            /= | %= | &= | |= | ^= | <<= | >>= |
            ~ | () | [] | < | <= | == | != |
            > | >>= | =]

    class-variable ::= [static] variable

    class-template :: = template < type-list > class

    mapped-type-template :: = template < type-list >
            %MappedType

    enum ::= enum [enum-key] [name] [enum-annotations] { {enum-line} };

    enum-key ::= [class | struct]

    enum-line ::= [%If | name [enum-annotations] ,

    function ::= type name ( [argument-list] ) [noexcept]
            [function-annotations] ; [%Docstring]
            [%MethodCode]

    namespace ::= namespace name [{ {namespace-line} }] ;

    namespace-line ::= [%TypeHeaderCode | statement]

    opaque-class ::= class scoped-name ;

    struct ::= struct name { {class-line} };

    union ::= union name { {class-line} };

    typedef ::= typedef [typed-name | function-pointer]
            typedef-annotations ;

    variable::= typed-name [variable-annotations] ;
            [%AccessCode] [%GetCode]
            [%SetCode]

    exception ::= %Exception exception-name [exception-base]
            { [%TypeHeaderCode] %RaiseCode };

    exception-name ::= scoped-name

    exception-base ::= ( [exception-name | python-exception] )

    python-exception ::= [SIP_ArithmeticError | SIP_AssertionError |
            SIP_AttributeError | SIP_BaseException |
            SIP_BlockingIOError | SIP_BrokenPipeError |
            SIP_BufferError | SIP_ChildProcessError |
            SIP_ConnectionAbortedError | SIP_ConnectionError |
            SIP_ConnectionRefusedError | SIP_ConnectionResetError |
            SIP_EnvironmentError | SIP_EOFError | SIP_Exception |
            SIP_FileExistsError | SIP_FileNotFoundError |
            SIP_FloatingPointError | SIP_GeneratorExit |
            SIP_ImportError | SIP_IndentationError |
            SIP_IndexError | SIP_InterruptedError | SIP_IOError |
            SIP_IsADirectoryError | SIP_KeyboardInterrupt |
            SIP_KeyError | SIP_LookupError | SIP_MemoryError |
            SIP_NameError | SIP_NotADirectoryError |
            SIP_NotImplementedError | SIP_OSError |
            SIP_OverflowError | SIP_PermissionError |
            SIP_ProcessLookupError | SIP_ReferenceError |
            SIP_RuntimeError | SIP_StandardError |
            SIP_StopIteration | SIP_SyntaxError | SIP_SystemError |
            SIP_SystemExit | SIP_TabError | SIP_TimeoutError |
            SIP_TypeError | SIP_UnboundLocalError |
            SIP_UnicodeDecodeError | SIP_UnicodeEncodeError |
            SIP_UnicodeError | SIP_UnicodeTranslateError |
            SIP_ValueError | SIP_VMSError | SIP_WindowsError |
            SIP_ZeroDivisionError | SIP_Warning |
            SIP_BytesWarning | SIP_DeprecationWarning |
            SIP_FutureWarning | SIP_ImportWarning |
            SIP_PendingDeprecationWarning | SIP_ResourceWarning |
            SIP_RuntimeWarning | SIP_SyntaxWarning |
            SIP_UnicodeWarning | SIP_UserWarning]

    argument-list ::= argument [, argument-list] [, ...]

    argument ::= type [name] [argument-annotations] [default-value]

    default-value ::= = expression

    expression ::= [value | value binary-operator expression]

    value ::= [unary-operator] simple-value

    simple-value ::= [scoped-name | function-call | real-value |
            integer-value | boolean-value | string-value |
            character-value]

    typed-name::= type name

    function-pointer::= type (* name )( [type-list] )

    type-list ::= type [, type-list]

    function-call ::= scoped-name ( [value-list] )

    value-list ::= value [, value-list]

    real-value ::= a floating point number

    integer-value ::= a number

    boolean-value ::= [true | false]

    string-value ::= " {character} "

    character-value ::= ' character '

    unary-operator ::= [! | ~ | - | + | * | &]

    binary-operator ::= [- | + | * | / | & | |]

    argument-annotations ::= see Argument Annotations

    class-annotations ::= see Class Annotations

    enum-annotations ::= see Enum Annotations

    function-annotations ::= see Function Annotations

    typedef-annotations ::= see Typedef Annotations

    variable-annotations ::= see Variable Annotations

    type ::= [const] base-type {*} [&]

    type-list ::= type [, type-list]

    base-type ::= [scoped-name | template |
            struct scoped-name |
            union scoped-name |
            char | signed char | unsigned char | wchar_t |
            int | unsigned | unsigned int | size_t |
            short | unsigned short |
            long | unsigned long |
            long long | unsigned long long |
            float | double |
            bool |
            void |
            Py_hash_t |
            Py_ssize_t |
            PyObject |
            SIP_PYBUFFER |
            SIP_PYCALLABLE |
            SIP_PYDICT |
            SIP_PYENUM |
            SIP_PYLIST |
            SIP_PYOBJECT |
            SIP_PYSLICE |
            SIP_PYTUPLE |
            SIP_PYTYPE]

    scoped-name ::= name [:: scoped-name]

    template ::= scoped-name < type-list >

    dotted-name ::= name [. dotted-name]

    name ::= _A-Za-z {_A-Za-z0-9}

Here is a short list of differences between C++ and the subset supported by SIP that might trip you up.

> - SIP does not support the use of `[]` in types. Use pointers instead.

> - A global `operator` can only be defined if its first argument is a class or a named enum that has been wrapped in the same module.

> - Variables declared outside of a class are effectively read-only.

## Variable Numbers of Arguments

SIP supports the use of `...` as the last part of a function signature. Any remaining arguments are collected as a Python tuple.

## Additional SIP Types

SIP supports a number of additional data types that can be used in Python signatures.

SIP_PYBUFFER

This is a `PyObject *` that implements the Python buffer protocol.

SIP_PYCALLABLE

This is a `PyObject *` that is a Python callable object.

SIP_PYDICT

This is a `PyObject *` that is a Python dictionary object.

SIP_PYENUM

This is a `PyObject *` that is a Python enum object.

SIP_PYLIST

This is a `PyObject *` that is a Python list object.

SIP_PYOBJECT

This is a `PyObject *` of any Python type. The type `PyObject *` can also be used.

SIP_PYSLICE

This is a `PyObject *` that is a Python slice object.

SIP_PYTUPLE

This is a `PyObject *` that is a Python tuple object.

SIP_PYTYPE

This is a `PyObject *` that is a Python type object.

## Python API vs. C/C++ API

It is important to understand that a SIP specification describes the Python API, i.e. the API available to the Python programmer when they `import` the generated module. It does not have to accurately represent the underlying C/C++ library. There is nothing wrong with omitting functions that make little sense in a Python context, or adding functions implemented with handwritten code that have no C/C++ equivalent. It is even possible (and sometimes necessary) to specify a different super-class hierarchy for a C++ class. All that matters is that the generated code compiles properly.

In most cases the Python API matches the C/C++ API. In some cases handwritten code (see `%MethodCode`) is used to map from one to the other without SIP having to know the details itself. However, there are a few cases where SIP generates a thin wrapper around a C++ method or constructor (see Generated Derived Classes) and needs to know the exact C++ signature. To deal with these cases SIP allows two signatures to be specified. For example:

    class Klass
    {
    public:
        // The Python signature is a tuple, but the underlying C++ signature
        // is a 2 element array.
        Klass(SIP_PYTUPLE) [(int *)];
    %MethodCode
            int iarr[2];

            if (PyArg_ParseTuple(a0, "ii", &iarr[0], &iarr[1]))
            {
                // Note that we use the SIP generated derived class
                // constructor.
                Py_BEGIN_ALLOW_THREADS
                sipCpp = new sipKlass(iarr);
                Py_END_ALLOW_THREADS
            }
    %End
    };

## Namespaces

SIP implements C++ namespaces as a Python class which cannot be instantiated. The contents of the namespace, including nested namespaces, are implemented as attributes of the class.

The namespace class is created in the module that SIP is parsing when it first sees the namespace defined. If a function (for example) is defined in a namespace that is first defined in another module then the function is added to the namespace class in that other module.

Say that we have a file `a.sip` that defines a module `a_module` as follows:

    %Module a_module

    namespace N
    {
        void hello();
    };

We also have a file `b.sip` that defines a module `b_module` as follows:

    %Module b_module

    %Import a.sip

    namespace N
    {
        void bye();
    };

When SIP parses `b.sip` it first sees the `N` namespace defined in module `a_module`. Therefore it places the `bye()` function in the `N` Python class in the `a_module`. It does not create an `N` Python class in the `b_module`. Consequently the following code will call the `bye()` function:

    import a_module
    import b_module
    a_module.N.bye()

While this reflects the C++ usage it may not be obvious to the Python programmer who might expect to call the `bye()` function using:

    import b_module
    b_module.N.bye()

In order to achieve this behavior make sure that the `N` namespace is first defined in the `b_module`. The following version of `b.sip` does this:

    %Module b_module

    namespace N;

    %Import a.sip

    namespace N
    {
        void bye();
    };

Alternatively you could just move the `%Import` directive so that it is at the end of the file.