Note: Everything said on this page applies only to extension types, defined with the cdef class statement. It doesn't apply to classes defined with the Python class statement, where the normal Python rules apply.
The __new__ method is where you should perform basic C-level initialisation of the object, including allocation of any C data structures that your object will own. You need to be careful what you do in the __new__ method, because the object may not yet be a valid Python object when it is called. Therefore, you must not invoke any Python operations which might touch the object; in particular, do not try to call any of its methods.
Unlike the corresponding method in Python, your __new__ method is not responsible for creating the object. By the time your __new__ method is called, memory has been allocated for the object and any C attributes it has have been initialised to 0 or null. (Any Python attributes have also been initialised to None, but you probably shouldn't rely on that.) Your __new__ method is guaranteed to be called exactly once.
Note that the first parameter of the __new__ method is the object to be initialised, not the class of the object as it is in Python.
Any initialisation which cannot safely be done in the __new__ method should be done in the __init__ method. By the time __init__ is called, the object is a fully valid Python object and all operations are safe. Under some circumstances it is possible for __init__ to be called more than once or not to be called at all, so your other methods should be designed to be robust in such situations.
Keep in mind that any arguments passed to the constructor will be passed to the __new__ method as well as the __init__ method. If you anticipate subclassing your extension type in Python, you may find it useful to give the __new__ method * and ** arguments so that it can accept and ignore extra arguments. Otherwise, any Python subclass which has an __init__ with a different signature will have to override __new__ as well as __init__ -- something that the writer of a Python class doesn't normally expect to have to do.
You need to be careful what you do in a __dealloc__ method. By the time your __dealloc__ method is called, the object may already have been partially destroyed and may not be in a valid state as far as Python is concerned, so you should avoid invoking any Python operations which might touch the object. In particular, don't call any other methods of the object or do anything which might cause the object to be resurrected. It's best if you stick to just deallocating C data.
You don't need to worry about deallocating Python attributes of your object, because that will be done for you by Pyrex after your __dealloc__ method returns.
An extension class can also have a __del__ method, which works in the same way as the __del__ method of a Python class. You shouldn't free any C data structures in a __del__ method, however; you should do that in the __dealloc__ method instead. Also, keep in mind that the presence of a __del__ method on any object participating in a reference cycle will prevent the cycle from being reclaimed by the garbage collector.
If an extension type has both a __del__ and a __dealloc__ method, __del__ is called first, and then __dealloc__.
This means that you can't rely on the first parameter of these methods being "self", and you should test the types of both operands before deciding what to do. If you can't handle the combination of types you've been given, you should return NotImplemented.
This also applies to the in-place arithmetic method __ipow__. It doesn't apply to any of the other in-place methods (__iadd__, etc.) which always take self as the first argument.
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You don't have to declare your method as taking these parameter types.
If you declare different types, conversions will be performed as necessary.
| Name | Parameters | Return type | Description |
| General | |||
| __new__ | self, ... | Basic initialisation (no direct Python equivalent) | |
| __init__ | self, ... | Further initialisation | |
| __dealloc__ | self | Basic deallocation (no direct Python equivalent) | |
| __cmp__ | x, y | int | 3-way comparison |
| __richcmp__ | x, y, int op | object | Rich comparison (no direct Python equivalent) |
| __str__ | self | object | str(self) |
| __repr__ | self | object | repr(self) |
| __hash__ | self | int | Hash function |
| __call__ | self, ... | object | self(...) |
| __iter__ | self | object | Return iterator for sequence |
| __getattr__ | self, name | object | Get attribute |
| __setattr__ | self, name, val | Set attribute | |
| __delattr__ | self, name | Delete attribute | |
| Arithmetic operators | |||
| __add__ | x, y | object | binary + operator |
| __sub__ | x, y | object | binary - operator |
| __mul__ | x, y | object | * operator |
| __div__ | x, y | object | / operator for old-style division |
| __floordiv__ | x, y | object | // operator |
| __truediv__ | x, y | object | / operator for new-style division |
| __mod__ | x, y | object | % operator |
| __divmod__ | x, y | object | combined div and mod |
| __pow__ | x, y, z | object | ** operator or pow(x, y, z) |
| __neg__ | self | object | unary - operator |
| __pos__ | self | object | unary + operator |
| __abs__ | self | object | absolute value |
| __nonzero__ | self | int | convert to boolean |
| __invert__ | self | object | ~ operator |
| __lshift__ | x, y | object | << operator |
| __rshift__ | x, y | object | >> operator |
| __and__ | x, y | object | & operator |
| __or__ | x, y | object | | operator |
| __xor__ | x, y | object | ^ operator |
| Numeric conversions | |||
| __int__ | self | object | Convert to integer |
| __long__ | self | object | Convert to long integer |
| __float__ | self | object | Convert to float |
| __oct__ | self | object | Convert to octal |
| __hex__ | self | object | Convert to hexadecimal |
| In-place arithmetic operators | |||
| __iadd__ | self, x | object | += operator |
| __isub__ | self, x | object | -= operator |
| __imul__ | self, x | object | *= operator |
| __idiv__ | self, x | object | /= operator for old-style division |
| __ifloordiv__ | self, x | object | //= operator |
| __itruediv__ | self, x | object | /= operator for new-style division |
| __imod__ | self, x | object | %= operator |
| __ipow__ | x, y, z | object | **= operator |
| __ilshift__ | self, x | object | <<= operator |
| __irshift__ | self, x | object | >>= operator |
| __iand__ | self, x | object | &= operator |
| __ior__ | self, x | object | |= operator |
| __ixor__ | self, x | object | ^= operator |
| Sequences and mappings | |||
| __len__ | self | int | len(self) |
| __getitem__ | self, x | object | self[x] |
| __setitem__ | self, x, y | self[x] = y | |
| __delitem__ | self, x | del self[x] | |
| __getslice__ | self, int i, int j | object | self[i:j] |
| __setslice__ | self, int i, int j, x | self[i:j] = x | |
| __delslice__ | self, int i, int j | del self[i:j] | |
| __contains__ | self, x | int | x in self |
| Iterators | |||
| __next__ | self | object | Get next item (called next in Python) |
| Buffer interface (no Python equivalents - see note 1) | |||
| __getreadbuffer__ | self, int i, void **p | ||
| __getwritebuffer__ | self, int i, void **p | ||
| __getsegcount__ | self, int *p | ||
| __getcharbuffer__ | self, int i, char **p | ||
| Descriptor objects (no Python equivalents - see note 2) | |||
| __get__ | self, instance, class | object | Get value of attribute |
| __set__ | self, instance, value | Set value of attribute | |
| __delete__ | self, instance | Delete attribute | |
Note 1: The buffer interface is intended for use by C code and is not directly accessible from Python. It is described in the Python/C API Reference Manual under sections 6.6 and 10.6.
Note 2: Descriptor objects are part of the support mechanism for new-style Python classes. See the discussion of descriptors in the Python documentation. See also PEP 252, "Making Types Look More Like Classes", and PEP 253, "Subtyping Built-In Types".
Note 3: If your type defines a __new__ method, any method called new that you define will be overwritten with the system-supplied new at module import time.