# Converting values between Python and Objective-C

## Introduction

PyObjC provides transparant conversion or proxying of values between Python and Objective-C. In general this works as expected, this document provides a detailed guide to how values are converted or proxied.

## Basic C types

The Objective-C language not only has classes and objects, but also has the basic C types which are not classes. PyObjC converts between those and the corresponding Python type.

The C type ‘char’ does not have a unambigous meaning in C, it is used for a number of tasks. In the table below the various tasks have been represented separately: booleans (`BOOL`), representing characters in text (`char`) and represeting small integers (`int8_t`). PyObjC uses metadata in the framework wrappers to know when to use which representation.

C type | Python 2.x | Python 3.x
--- | --- | ---
`int8_t` | `int` | `int`
`BOOL` | `bool` | `bool`
`char` | `str` of len(1) | `bytes` of len(1)
`unsigned char` | `int` | `int`
`short` | `int` | `int`
`unsigned short` | `int` | `int`
`int` | `int` | `int`
`unsigned int` | `int` | `int`
`int` | `int` | `int`
`unsigned int` | `int` or `long` | `int`
`long` | `int` | `int`
`unsigned long` | `int` or `long` | `int`
`long long` | `int` or `long` | `int`
`unsigned long long` | `int` or `long` | `int`
`float` | `float` | `float`
`double` | `float` | `float`

PyObjC does range checking when converting values to C, and will raise `ValueError` when the input value is out of range.

PyObjC will accept negative values when converting a Python numeric value to an unsigned integer value. This is done due to limitations in the metadata creation process, sometimes constant values that are used with unsigned integer arguments are represented as negative values in the metadata files. This feature will be fixed in a future version of PyObjC and users should therefore not rely on being able to convert negative values to an unsigned integer type.

## Compound C types

### Arrays

C Arrays are represented a lists where all elements are of the right basic type (as described earlier).

### Structs

C structs are by default represented as Python tuples, and you can always use tuples of the right arity to pass values to a function.

The framework wrappers also provide wrapper types that provide a nicer interface, those wrappers can be used with indexed access (like tuples), but also have named attributes. The wrapper types are mutable, and are comparible with mutable `namedtuple` objects.

### Unions

PyObjC cannot convert to and from C union types at the moment.

## Classes and instances

TBD

## Functions and methods

TBD