Instantiating Objective-C objects

Changed in version 10.4: The Pythonic interface for instantiation classes was added.

Objective-C uses a two step object instantiation process, similar to Python’s __new__ and __init__ methods, but with explicit invocation of the methods alloc and init. By default this is mirrored in the Python proxies:

some_value = NSObject.alloc().init()

some_button = NSButton.alloc().initWithFrame_(some_frame)

This looks very foreign in Python, therefore PyObjC also supports a condensed version of this by directly calling the class:

some_value = NSObject()

some_button = NSButton(frame=some_frame)

The generic rules for instantiation objects through calling the class are:

  • Calling the class without arguments is supported unless the init method has been marked with NS_UNAVAILABLE in Apple’s SDK.

  • Every instance selector of the Objective-C class with a name starting with init adds a possible set of keyword arguments using the following algorithm:

    1. Strip initWith or init from the start of the selector.

    2. Lowercase the first character of the result.

    3. All segments are now keyword only arguments, in that order.

    For example, given -[SomeClass initWithX:y:z], the following invocation is valid: SomeClass(x=1, y=2, z=3). Using the keywords in a different order is not valid.

  • Some classes may have additional sets of keyword arguments that will be documented in the framework notes. In general those will be based on factory class methods for which there is no equivalent using the alloc().init...(...) pattern.

For classes in system frameworks the possible init methods are For classes in system frameworks the possibly init method are registered using framework data.

For classes implemented in Python the possible init methods are detected automatically. Just implement one or more Objective-C style init methods to add sets of keyword arguments for __new__ (and don’t implement __new__ or __init__ in the Python class).

Set init to None to require using one or more keyword arguments, that is:

class MyObject(NSObject):
    init = None  # Calling MyObject() is not allowed

    def initWithX_y_(self, x_value, y_value):
        self = super.init()
        self.x = x_value
        self.y = y_value
        return self

value = MyObject(x=42, y=24)