Commit 2e59a1a4 authored by gabrieldemarmiesse's avatar gabrieldemarmiesse

Transfer of the dynamic attributes into the userguide.

parent 3143d776
...@@ -39,14 +39,14 @@ interface to them. ...@@ -39,14 +39,14 @@ interface to them.
.. _readonly: .. _readonly:
Attributes Static Attributes
============ =================
Attributes of an extension type are stored directly in the object's C struct. Attributes of an extension type are stored directly in the object's C struct.
The set of attributes is fixed at compile time; you can't add attributes to an The set of attributes is fixed at compile time; you can't add attributes to an
extension type instance at run time simply by assigning to them, as you could extension type instance at run time simply by assigning to them, as you could
with a Python class instance. (You can subclass the extension type in Python with a Python class instance. (You can subclass the extension type in Python
and add attributes to instances of the subclass, however.) and add attributes to instances of the subclass, see :ref:`dynamic_attributes`.)
There are two ways that attributes of an extension type can be accessed: by There are two ways that attributes of an extension type can be accessed: by
Python attribute lookup, or by direct access to the C struct from Cython code. Python attribute lookup, or by direct access to the C struct from Cython code.
...@@ -76,6 +76,51 @@ and the depth attribute readable but not writable. ...@@ -76,6 +76,51 @@ and the depth attribute readable but not writable.
Python access, not direct access. All the attributes of an extension type Python access, not direct access. All the attributes of an extension type
are always readable and writable by C-level access. are always readable and writable by C-level access.
.. _dynamic_attributes:
Dynamic Attributes
==================
It is not possible to add attributes to an extension type at runtime by default.
You have two ways of avoiding this limitation, both add an overhead when
a method is called from Python code.
The first workaround is making a child Python class and is preferred way of
keeping the static attributes of an extension
type while enabling the use of dynamic attributes::
cdef class Animal:
cdef int number_of_legs
def __cinit__(self, int number_of_legs):
self.number_of_legs = number_of_legs
class ExtendableAnimal(Animal): # Note that we use class, not cdef class
pass
dog = ExtendableAnimal(4)
dog.has_tail = True
Declaring a ``__dict__`` attribute is the second way of enabling dynamic attributes
and can have a significant performance penalty compared to subclassing,
especially when using ``cpdef`` class methods::
cdef class Animal:
cdef int number_of_legs
cdef dict __dict__
def __cinit__(self, int number_of_legs):
self.number_of_legs = number_of_legs
dog = Animal(4)
dog.has_tail = True
Type declarations Type declarations
=================== ===================
......
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