# An introduction to PyObjC

## Preface

PyObjC is a bridge between Python and Objective-C. It allows Python scripts to use and extend existing Objective-C class libraries; most importantly the Cocoa libraries by Apple.

This document describes how to use Objective-C class libraries from Python scripts and how to interpret the documentation of those libraries from the point of view of a Python programmer.

## First Steps

When dealing with the Objective-C runtime, there are certain patterns you need to learn when writing Python code. If you’re not already an Objective-C programmer, some of them will seem strange or even “un-pythonic” at first. However, you will get used to it, and the way that PyObjC works is quite compliant with the PEP 20 (the Zen of Python, `import this`). In fact, Ronald is Dutch ;)

With no further ado, here are the three most important things you must know before embarking on any PyObjC voyage:

### Underscores, and lots of them

Objective-C objects communicate with each other by sending messages. The syntax for messages is somewhere in-between Python’s positional and keyword arguments. Specificlaly, Objective-C message dispatch uses positional arguments, but parts of the message name (called “selector” in Objective-C terminology) are interleaved with the arguments.

An Objective-C message looks like this:

>     [someObject doSomething:arg1 withSomethingElse:arg2];

The selector (message name) for the above snippet is this (note the colons):

>     doSomething:withSomethingElse:

In order to have a lossless and unambiguous translation between Objective-C messages and Python methods, the Python method name equivalent is simply the selector with colons replaced by underscores. Since each colon in an Objective-C selector is a placeholder for an argument, the number of underscores in the PyObjC-ified method name is the number of arguments that should be given.

The PyObjC translation of the above selector is (note the underscores):

>     doSomething_withSomethingElse_

The message dispatch, translated to PyObjC, looks like this:

>     someObject.doSomething_withSomethingElse_(arg1, arg2)

Methods that take one argument will have a trailing underscore.

It may take a little while to get used to, but PyObjC does not ever rename selectors. The trailing underscore will seem strange at first, especially for cases like this:

>     # note the trailing underscore
>     someObject.setValue_(aValue)

There are a few additional rules regarding message dispatch, see the Overview of the bridge for the complete rundown.

### Two-phase instantiation

Objective-C, being a low-level runtime, separates the two concepts required to instantiate an object.

allocation:

  Reserve a chunk of memory large enough to hold the new object, and make sure that all of its declared instance variables are set to “zero” (this means nil pointers to objects, 0 for integers, etc.).

initialization:

  Fill in the blank slate allocated by the allocation phase.

In Objective-C, the convention is for allocation to be performed by a class method called `alloc`, and initialization is done with method beginning with the word `init`. For example, here is the syntax for instantiating an `NSObject`:

>     myObject = NSObject.alloc().init()

And here is an example for creating an `NSData` instance given a few bytes:

>     myData = NSData.alloc().initWithBytes_length_('the bytes', 9)

You must also follow this convention when subclassing Objective-C classes. When initializing, an object must always (directly or indirectly) call the designated initializer of its `super`. The designated initializer is the “most basic” initializer through which all initialization eventually ends up. The designated initializer for `NSObject` is `init`. To find the designated initializer for other classes, consult the documentation for that class. Here is an example of an `NSObject` subclass with a customized initialization phase:

>      1class MyClass(NSObject):
>      2
>      3   def init(self):
>      4       """
>      5       Designated initializer for MyClass
>      6       """
>      7       # ALWAYS call the super's designated initializer.
>      8       # Also, make sure to re-bind "self" just in case it
>      9       # returns something else, or even None!
>     10       self = objc.super(MyClass, self).init()
>     11       if self is None: return None
>     12
>     13       self.myVariable = 10
>     14
>     15       # Unlike Python's __init__, initializers MUST return self,
>     16       # because they are allowed to return any object!
>     17       return self
>     18
>     19
>     20class MyOtherClass(MyClass):
>     21
>     22   def initWithOtherVariable_(self, otherVariable):
>     23       """
>     24       Designated initializer for MyOtherClass
>     25       """
>     26       self = objc.super(MyOtherClass, self).init()
>     27       if self is None: return None
>     28
>     29       self.otherVariable = otherVariable
>     30       return self
>     31
>     32myInstance = MyClass.alloc().init()
>     33myOtherInstance = MyOtherClass.alloc().initWithOtherVariable_(20)

Many Objective-C classes provide class methods that perform two-phase instantiation for you in one step. Several examples of this are:

>      1# This is equivalent to:
>      2#
>      3#   myObject = NSObject.alloc().init()
>      4#
>      5myObject = NSObject.new()
>      6
>      7# This is equivalent to:
>      8#
>      9#   myDict = NSDictionary.alloc().init()
>     10#
>     11myDict = NSDictionary.dictionary()
>     12
>     13# This is equivalent to:
>     14#
>     15#   myString = NSString.alloc().initWithString_(u'my string')
>     16#
>     17myString = NSString.stringWithString_(u'my string')

### Objective-C uses accessors everywhere

Unlike Python, Objective-C convention says to use accessors rather than directly accessing instance variables of other objects. This means that in order to access an instance variable `value` of an object `valueContainer` you will have to use the following syntax:

>      1# Getting
>      2#
>      3# notice the method call
>      4#
>      5myValue = valueContainer.value()
>      6
>      7# Setting
>      8#
>      9# notice the naming convention and trailing underscore
>     10#
>     11valueContainer.setValue_(myNewValue)

When writing your own classes from Python, this is a bit harder since Python only has one namespace for all attributes, even methods. If you choose to implement accessors from Python, then you will have to name the instance variable something else:

>      1class MyValueHolder(NSObject):
>      2
>      3    def initWithValue_(self, value):
>      4        self = objc.super(MyValueHolder, self).init()
>      5        # It's recommended not to use typical Python convention here,
>      6        # as instance variables prefixed with underscores are reserved
>      7        # by the Objective-C runtime.  It still works if you use
>      8        # underscores, however.
>      9        self.ivar_value = value
>     10        return self
>     11
>     12    def value(self):
>     13        return self.ivar_value
>     14
>     15    def setValue_(self, value):
>     16        self.ivar_value = value

It’s also possible to use Key-Value Coding in some cases, which eliminates the need for writing most accessors, but only in scenarios where the rest of the code is using it.

## Objective-C for PyObjC users

It is recommended that you take the time to understand a little bit about Objective-C before jumping into PyObjC development. The class libraries that you will be using from Cocoa are not documented in Python, and their documentation will be confusing without a grasp on the semantics and syntax of Objective-C.

Objective-C is an object-oriented programming language implemented as a superset of C that borrows heavily in concept and syntax from Smalltalk. It features single inheritance with dynamic dispatch and (in theory) multiple root classes. This is basically the same as Python with single inheritance.

An important difference between Python and Objective-C is that the latter is not a pure object-oriented language. Some values are not objects, but values of plain C types, such as `int` and `double`. These basic C types can be used as the types of arguments and the return value of methods.

Object allocation and initialization are explicit and separate actions in Objective-C. The former is done by the class-method `alloc`, while the latter is done by instance methods whose name customarily starts with `init`.

Objective-C code looks just like plain C code, with some easily recognizable Smalltalk-like extensions for the object-oriented parts of the language. An example class declaration (usually found in `.h` files) and implementation (usually found in `.m` files) are listed below. Class declarations are easily recognized as blocks of code between `@interface` and `@end`, and similarly the implementation is between `@implementation` and `@end`. An expression enclosed in brackets in Objective-C is called a message, and is the equivalent to an instance method invocation in Python. For example, this Objective-C code:

>     [aMutableArray addObject:@"constant string"];

Is equivalent in intent to the following in Python:

>     aList.append(u"constant string")

Objective-C messages have three components: a target, a selector, and zero or more arguments. The target, `aMutableArray`, is the object or class receiving the message. The selector, `addObject:` uniquely identifies the kind of message that is being sent. Finally, the arguments, `@"constant string"` are used by the implementation of the method upon receipt of the message. The syntax of Objective-C message dispatch is deceptively similar to keyword arguments in Python, but they are actually quite different. Objective-C messages can not have default arguments, and all arguments are passed in a specific order. The components of a selector may not be reordered. Syntactically, one argument must be interleaved at every colon in the selector. The message:

>     [anArray indexOfObject:someObject inRange:someRange]

Target:

  `anArray`

Selector:

  `indexOfObject:inRange:`

Arguments:

  `someObject`, `someRange`

As documented later, the straightforward translation of such a message to Python is:

>     anArray.indexOfObject_inRange_(someObject, someRange)

This may be awkward and “unpythonic” at first, however this syntax is necessary to preserve the semantics of Objective-C message dispatch.

A class declaration:

>      1@interface MyClass : MySuperClass
>      2{
>      3    id  anInstanceVariable;
>      4    int anotherInstanceVariable;
>      5}
>      6
>      7// A class method that returns an initialized instance of this class.
>      8// Similar to an implementation of __call__ on the metaclass.
>      9+instanceWithObject:(id)anObject andInteger:(int)anInteger;
>     10
>     11// An instance method, the designated initializer for MyClass.
>     12// Similar to an implementation of __new__ on MyClass.
>     13-initWithObject:(id)anObject andInteger:(int)anInteger;
>     14
>     15// An accessor, instance variables (attributes) are in a separate
>     16// namespace and are considered "private" in Objective-C.  Conventionally,
>     17// there is nothing similar to this in Python.
>     18-(int)anotherInstanceVariable;
>     19@end

A class implementation:

>      1@implementation MyClass
>      2
>      3// Note that a type is not declared for the return value.  Undeclared types
>      4// are assumed to be "id", which means any kind of instance.
>      5+instanceWithObject:(id)anObject andInteger:(int)anInteger
>      6{
>      7    // 1. Create an instance of MyClass.
>      8    // 2. Initialize it with its designated initializer
>      9    //    "initWithObject:andInteger:".
>     10    // 3. Autorelease it, so that it does not leak memory.
>     11    // 4. Return the new instance.
>     12    //
>     13    // NOTE:
>     14    //   By convention,initializers (such as +new, -init, -copy)
>     15    //   are the only methods that should return retained objects.
>     16    //
>     17    // NOTE:
>     18    //   Since this is a class method, "self" refers to the class!
>     19    //
>     20    // Very roughly similar to:
>     21    //   return self.__new__(anObject, anInteger)
>     22    return [[[self alloc] initWithObject:anObject andInteger:anInteger] autorelease];
>     23}
>     24
>     25// Note that a type is not declared for the return value.  Undeclared types
>     26// are assumed to be "id", which means any kind of instance.
>     27-initWithObject:(id)anObject andInteger:(int)anInteger
>     28{
>     29    // Call the designated initializer of the superclass.
>     30    // Similar to:
>     31    //     self = super(MyClass, self).__new__()
>     32    self = [super init];
>     33
>     34    // Bail if initialization of the superclass failed.
>     35    // Similar to:
>     36    //     if self is None:
>     37    //         return None
>     38    if (!self) {
>     39        return nil;
>     40    }
>     41
>     42    // Set the instance variable (attribute).  The argument must be
>     43    // retained, since it will persist as long as the instance does.
>     44    // Similar to:
>     45    //     # Reference counting is automatic in Python
>     46    //     self.anInstanceVariable = anObject
>     47    anInstanceVariable = [anObject retain];
>     48
>     49    // Set the other instance variable.  Note that since anInteger is
>     50    // a primitive "C" type, not an object, no reference counting takes
>     51    // place.
>     52    // Similar to:
>     53    //     # Everything is an object in Python
>     54    //     self.anotherInstanceVariable = anInteger
>     55    anotherInstanceVariable = anInteger;
>     56
>     57    // Like __new__ in Python, initializers in Objective-C must
>     58    // explicitly return self.  Note that this is different from
>     59    // __init__.
>     60    // Similar to:
>     61    //     return self
>     62    return self;
>     63}
>     64
>     65
>     66// an accessor, instance variables (attributes) are in a separate
>     67// namespace and are considered "private"
>     68-(int)anotherInstanceVariable
>     69{
>     70    return anotherInstanceVariable;
>     71}
>     72
>     73// Since objects were retained as instance variables on this object,
>     74// they must be freed when the object is.  This is similar to an
>     75// implementation of __del__ in Python.  Since Objective-C has no
>     76// cyclic garbage collection, this isn't discouraged like it is in
>     77// Python.
>     78-(void)dealloc
>     79{
>     80    // Very roughly similar to:
>     81    //     del self.instanceVariable
>     82    [instanceVariable release];
>     83
>     84    // Very roughly similar to:
>     85    //     objc.super(MyClass, self).__del__()
>     86    [super dealloc];
>     87}
>     88
>     89@end

Objective-C also features exceptions, but they are typically only used for disaster recovery, not error handling, so you will not encounter them very often. Read The Objective-C Programming Language if you want to know more about exceptions in Objective-C.

One thing to keep in mind when translating Objective-C snippets to Python is that any message can be sent to `nil`, and the return value of that message will be `nil`. PyObjC translates `nil` to `None` when crossing the bridge, so any such attempt will raise an `AttributeError`.

For more information about Objective-C see:

- The Objective-C Programming Language at Apple.

> The link is not correct, but the actual document is not online at the moment.

## Overview of the bridge

### Classes

Objective-C classes are visible as (new-style) Python classes and can be subclassed just like normal Python classes. All the usual introspection mechanisms work as well, as do `__slots__` and descriptors. The major differences between normal Python classes and Objective-C classes are the way that instances are created and initialized, and the fact that Objective-C selectors look strange when translated to Python methods.

Multiple inheritance may be used when subclassing an Objective-C class, so long as the Objective-C class is the first base class and there is only one Objective-C base class. The Objective-C runtime does not support multiple inheritance. These mix-in classes should not contain different implementations for Objective-C methods. To achieve this behavior, Categories should be used instead.

Another thing to keep in mind is that the names of Objective-C classes must be globally unique per process, including across Python modules. That is, it is not possible to have two Python modules that define a class with the same name. It is conventional to choose class names with a short prefix that uniquely identify your project or company. For example, Apple uses `NS` as the prefix for all classes in the Cocoa libraries. Note that the `NS` prefix made much more sense when it was called NeXTstep, but persists to this day for compatibility reasons.

As described in Objective-C for PyObjC users the creation of Objective-C objects is a two-stage process. To initialize objects, first call a class method to allocate the memory (typically `alloc`), and then call an initializer (typically starts with `init`). Some classes have class methods which perform this behind the scenes, especially classes that create cached, immutable, or singleton instances.

### Messages and Functions

Objective-C methods are bridged to Python methods. Because Objective-C message dispatch syntax can not be translated directly to Python, a few simple translations must take place. The rules for these translations are:

1. Replace all colons in the selector with underscores:

   > - `someMethod:withFoo:andBar:` translates to `someMethod_withFoo_andBar_`

2. If the result of the first step is `class` or `raise` (which are Python keywords), append two underscores:

   > - `class` translates to `class__`

   > - `raise` translates to `raise__`

3. Use this translated selector as a normal Python method. The arguments must be passed in the same order, and the number of arguments passed will normally be equal to the number of underscores in the method name; exceptions to this rule and the behavior of “result” are mentioned below.

       1result = [someObject someMethod:firstArg withFoo:foo andBar:bar];

   translates to

       1result = someObject.someMethod_withFoo_andBar_(firstArg, foo, bar)

Note that it is currently not possible to support methods with a variable number of arguments from Python. These selectors must be wrapped by custom Objective-C code in order to be accessible by Python.

Wrapped/bridged methods (and functions) have the same number of arguments as the corresponding Objective-C method or function, unless otherwise noted in the documentation (Notes on supported APIs and classes on macOS for Cocoa on macOS).

Most methods or functions that take or return pointers to values will be an exception to this rule if it is callable from Python at all. In Objective-C terminology, there are three kinds of pointers that can be used in a method:

`in`:

  Used to pass data by reference to the function. This is not a special case from Python.

  Instead of a regular value you may also pass in the value `objc.NULL`, when you do that the Objective-C method will receive a NULL pointer instead of a pointer to your value.

`out`:

  Used to pass data from the function (e.g. an additional return value).

  Pass in either `None` or `objc.NULL` for output arguments to the method. If the value is `objc.NULL` the Objective-C code will receive a NULL pointer for this argument, otherwise it will receive a valid pointer.

`inout`:

  A combination of in and out (a value is passed by reference, and mutated upon return). Unlike `out`, these arguments remain in the argument list, and thus do not have an effect on the number of arguments a method expects. See below for notes on how `inout` arguments change the return value.

  Instead of a regular value you may also pass in the value `objc.NULL`, when you do that the Objective-C method will receive a NULL pointer instead of a pointer to your value.

In order to determine what the return value of such an exceptional message will look like, you must “make a list” of the return values with the following rules:

1. If the return type of the method or function is not `void`, add it to the list.

2. For each argument in the method or function, add it to the list if it is `out` or `inout`. When `objc.NULL` was used as the argument value it will also be used as the result value.

After creating this list, you will have one of three cases:

Empty:

  The return value of this call will always be `None`.

One element:

  The return value of this call will correspond to the one element of the list.

More than one element:

  The return value of this call will be a tuple in the same order as the list.

The rules for pass by reference arguments may look quite complicated, but it turns out this is very straightforward when working with them.

As an example of a method with two output arguments, `NSMatrix` implements a selector named `getNumberOfRows:columns:` with the following signature:

>     -(void)getNumberOfRows:(int *)rowCount columns:(int *)columnCount

This method is used from Python like this:

>     rowCount, columnCount = matrix.getNumberOfRows_columns_(None, None)

When a function or method has an array of values and the length of that array as arguments, `None` may be passed as the length to specify that the length of the given sequence should be used.

Python’s `array.array` type may be used to represent a C array if the typestr and size match what is expected by the selector. Numeric, numarray, and other third party array types are not supported at the moment.

When defining methods in an Objective-C subclass, the bridge must provide type signatures for each method to the Objective-C runtime. The default type signature is for all arguments as well as the return value to be objects (just like with normal Python methods). If there is no return statement in the implementation, then the return value will be void. The bridge will automatically pick a better signature when it has more information available. Specifically, a method overrides an existing method, the bridge will assume you want to use the same method signature. Furthermore, if the method is implemented in an (informal) protocol known to the bridge it will use the signature from the corresponding method in that signature.

The end result is that it is rarely necessary to explicitly add information about the signature of methods. For the two most common cases where this is necessary, we have provided convenience decorators (used like `staticmethod` or `classmethod`):

`objc.accessor`:

  Use this to wrap a Key-Value Coding or Key-Value Observing compliant accessor.

`PyObjCTools.AppHelper.endSheetMethod`:

  Use this to wrap the implementation of a sheet’s “didEndSelector” callback.

For complete control of the mapping to Objective-C you can use the function `objc.selector` to create custom descriptors. See the documentation of the `objc` module for the arguments you can use with this function. It is normally used like this:

>     1    class MyObject(NSObject):
>     2
>     3            # -(void)someMethod:(float)arg
>     4            def someMethod_(self, arg):
>     5                    pass
>     6
>     7            someMethod_ = objc.selector(someMethod_, signature='v@:f')

In Python 2.4 or later there is a decorator for this purpose:

>     1    class MyObject(NSObject):
>     2
>     3            @objc.signature('v@:f')
>     4            def someMethod_(self, arg):
>     5                    pass

### Reference counting

The Cocoa libraries, and most (if not all) other class libraries for Objective-C use explicit reference counting to manage memory. The methods `retain`, `release` and `autorelease` are used to manage these reference counts. You won’t have to manage reference counts in Python, the bridge does all that work for you (but see Notes on supported APIs and classes on macOS for some advanced issues).

The only reasons reference counts are mentioned at all are to tell you about ignoring them, and more importantly to introduce you to some issues w.r.t. reference counting.

It turns out that Cocoa uses a primitive form of `weak references`. Those are not true `weak references` as in Python, but use-cases where an object stores a reference to another object without increasing the reference count for that other object. The bridge cannot solve the issues this introduces for you, which means that you get hard crashes when you’re not careful when dealing with those `weak references`.

The basic rule to deal with weak references is: make sure objects stays alive as long as someone might have a weak reference to them. Due to the way the bridge works, this means that you must make sure that you don’t create weak references from Objective-C to a plain Python object. The Python object stays alive, but the proxy object as seen by the Objective-C code is actually an autoreleased object that will be cleaned up unless the Objective-C code increases its reference count.

The document Notes on supported APIs and classes on macOS contains information about classes that work with weak references. The most important are notification centers and `NSOutlineView`, to be exact: the outline view stores weak references to the objects return by the method `outlineView:child:ofItem:` of its data source. The easiest way to avoid crashes with outline views is to make sure that your model for the view uses subclasses of `NSObject` to represent the nodes in the outline view.

Another gotcha is that `obj.setDelegate_()` often does not retain the delegate, so a reference should be maintained elsewhere.

### Protocols

Cocoa defines a number of formal and informal protocols that specify methods that should be implemented by a class if it is to be used in a specific role, such as the data source for an `NSTableView`.

Those protocols are represented by instances of `objc.informal_protocol`, and `objc.formal_protocol`. The only ones that have to care about these objects are the maintainers of wrappers around Objective-C frameworks: they have to keep these protocol wrappers up-to-date.

PyObjC will automatically use the information in the `informal_protocol` objects to add the right method signatures to methods, and to warn about classes that partially implement a protocol.

See PyObjC protocol support for more information.

### Cocoa Bindings

In macOS 10.3 Apple introduced Cocoa Bindings, a method to make it easier to create and use Controller objects using Key-Value Observing and Key-Value Coding. In order to create accessors compatible with this, you must use `objc.accessor` to create an appropriate selector descriptor.

PyObjC automaticly emits the right Key-Value Observing notifications when you set attributes on an Objective-C class. This is however not supported for pure python objects. You should therefore use `NSMutableArray` instances instead of Python lists for instance variables that will be observed and contain a sequence of values (and simularly for `NSMutableDictionary` instead of `dict`).

NOTE: Key-Value Observing is not supported for “pure” python objects, that is instances of classes that don’t inherit from `NSObject`. Adding such support is not possible adding a KVO-like interface to the Python interpreter.

### Categories

Objective-C has a mechanism for modularizing a class definition. It is possible to add methods to an existing class in a separate compilation unit and even a separate library. This mechanism is named categories and is used to enhance existing classes, for splitting classes in several parts and to document informal protocols.

An example of a category definition:

>     1    @interface NSObject (MyCategory)
>     2    - (NSSize)objectFootprint;
>     3    @end

This declares an additional category on `NSObject`. This category contains a single method.

The function `objc.classAddMethods` can be used to get the same effect in Python:

>     1    def objectFootprint(self):
>     2            pass
>     3
>     4    objc.classAddMethods(NSObject, [objectFootprint])

This is not very clear, PyObjC therefore also provides the following mechanism, implemented on top of `objc.classAddMethods`:

>     1    class NSObject(objc.Category(NSObject)):
>     2            def objectFootprint(self):
>     3                    pass

To make it clear that `objc.Category` performs a special task the name in the class definition must be the same as the `__name__` of the argument to `objc.Category`.

## Accessing Python objects from Objective-C

All Python objects can be accessed from Objective-C through proxy objects. Whenever a Python object crosses the line from Python to Objective-C a proxy object is created (of class `OC_PythonObject`, a subclass of `NSProxy`). This proxy object will forward all method calls from Objective-C to Python, and will return the results back to Objective-C.

See the section ‘Method protocol’ for a description of how PyObjC translates between Python and Objective-C method calls.

A number of Python types/classes are treated specially:

- Python numbers (`int`, `float`, `long`) are translated into `NSNumber` instances. Their identity is not preserved across the bridge.

- Python `str` is proxied using `OC_PythonString`, a subclass of `NSString`. A Python `str` may be used anywhere a `NSString` is expected, but `unicode` should be used whenever possible. `OC_PythonString` will use the default encoding of `NSString`, which is normally MacRoman but could be something else.

- Python `unicode` is proxied using `OC_PythonUnicode`, a subclass of `NSString`. A Python `unicode` may be used anywhere a `NSString` is expected.

- Python `dict` is proxied using `OC_PythonDictionary`, a subclass of `NSMutableDictionary`. A Python `dict` may be used anywhere an `NSDictionary` is expected.

- Python `list` and `tuple` are proxied using `OC_PythonArray`, a subclass of `NSMutableArray`. Python `list` or `tuple` objects may be used anywhere an `NSArray` is expected.

- Python objects that implement the Python buffer API, except for `str` and `unicode`, are proxied using `OC_PythonData`, a `NSData` subclass. Objects that implement the Python buffer API such as `buffer`, `array.array`, `mmap.mmap`, etc. may be used anywhere a `NSData` is expected.

These special cases allow for more transparent bridging between Python and Objective-C.

## Cocoa for Python programmers

Cocoa frameworks are mapped onto Python packages with the same name; that is the classes, constants and functions from the AppKit framework are available after you import `AppKit` in your Python script.

These helper modules contain only functions, constants and classes that wrap items in the corresponding framework. All utility functions and classes are located in the `PyObjCTools` package and `objc` module. Note that it is possible to use `pydoc` (or the `help()`) function with the framework wrappers, but that this is not very useful for the entire module due to the size of these modules.

This makes it easier to find documentation for an item: if you import it from the wrapper module for an Objective-C framework the documentation for that item can be found in the documentation for the framework; otherwise the item is documented in the PyObjC documentation.

PyObjC includes a number of examples that show how to use Cocoa from Python. The PyObjC Example index contains an overview of those examples.

More information on Cocoa programming can be found at:

- Cocoa documentation at the Apple developer website

- Cocoa examples at the Apple developer website

- Your local bookstore or library

## Notes on specific tasks

### Working with threads

Most of Cocoa, and thus PyObjC, requires an `NSAutoreleasePool` in order to function properly. PyObjC does this automatically on the first thread it is imported from, but other threads will require explicit `NSAutoreleasePool` management. The following practice for working with `NSAutoreleasePool` is recommended:

>     1    pool = NSAutoreleasePool.alloc().init()
>     2    ...
>     3    del pool

Typically this will be done at the beginning and end of the thread. It is important to use `del` before rebinding the `pool` local to another `NSAutoreleasePool` instance, otherwise it will not have the intended effect.

For long running threads and tight loops, it can also be useful to use this pattern in the body of the loop in order to optimize memory usage. For example, `NSRunLoop` will be create a new `NSAutoreleasePool` at the beginning of each run loop iteration and release it at the end.

Warning

Autorelease pools are thread global state and will not be shared amongst threads, every thread needs to create its own pool(s).

Warning

It is possible to create nested pools (that is, create a new pool when there already is an active pool), but nested pools must be released in reverse order. The pools form a stack that must be cleaned up by popping pools from the top. When you don’t do this you can end up with unexpected behavior, including hard crashes.

### Finalizers

In normal Python, there are two methods for writing finalizers: implementing `__del__`, and using `weakref.ref` callbacks. Generally, `__del___` is discouraged as it does not allow the object to participate in cyclic garbage collection and create uncollectible garbage if not implemented properly. `weakref.ref` callbacks avoid this restriction as they do not provide a real reference to the object.

In Objective-C, there is no cyclic garbage collection, so all Objective-C objects (including subclasses from Python) are already subject to these restrictions. When subclassing an Objective-C class, you may implement `dealloc` or `__del__`. If you implement `dealloc`, ensure that you call the super `dealloc` at the end. If you implement both `__del__` and `dealloc`, the order in which they are called is undefined.

It is not currently possible to create a `weakref.ref` for any Objective-C object. It is probably technically possible to do, but tricky, so it may eventually be implemented in a future version of PyObjC (especially if a future Objective-C runtime supports it).

### Copying

It is possible for a Python subclass of an Objective-C class to implement the `NSCopying` protocol. Some care must be taken when the superclass already implements the protocol.

Some `NSCopying` compliant Objective-C classes copy the template object manually. In those cases the Python subclass must also copy the additional ivars manually.

Other `NSCopying` compliant Objective-C classes use a convenience function that creates a shallow copy of the object and all of its ivars. In those cases the Python subclass will not have to explicitly copy all of the ivars. However, the ivars in the copy will refer to the same objects as the original, and will thus share some state. As with shallow copies in Python, if any of the ivars refer to mutable objects (`list`, `dict`, etc.) it may be desirable to explicitly make shallow or deep copies of the mutable ivars.

NOTE: PyObjC might introduce a helper class when you inherit from a class that implements `NSCopying` as an internal implementation detail. External code should not rely on the existance of this class.

NOTE2: `SomeClass.copyWithZone_` should not be implemented unless a superclass already implements `copyWithZone:`, or else the behavior will be undefined (memory corruption, crashes, etc.).

## Building applications

There currently is one recommended way for building applications with PyObjC: use py2app.

### “py2app” : setup.py

The PyObjC installer includes a copy of the `py2app` package. This package offers a way to build distutils scripts for building (standalone) applications and plugin bundles.

An example `setup.py` script:

>     1from distutils.core import setup
>     2import py2app
>     3
>     4setup(
>     5    app = ["iClass.py"],
>     6    data_files = ["English.lproj"],
>     7)

During development you typically invoke it from the command line like this:

>     $ python setup.py py2app -A

This will build an application bundle in a folder named `dist` in the current folder. The `-A` option tells `py2app` to add symbolic links for data folders and files and an Alias to your main script, allowing you quickly rebuild the application without doing a full dependency scan, with the additional bonus that you can edit them without rebuild. To build a standalone application, simply do not use the `-A` option. Note that if you are using a version of Python shipped with your operating system, it will not be included in the application. Otherwise, your application will include stripped down version of the Python runtime that you ran setup.py with.

For more information about `py2app` usage, read through some of the `setup.py` scripts used by the examples in the Examples folder. On any `setup.py` script that imports `py2app`, you can use the following command to see the list of options:

>     $ python setup.py py2app --help