This section describes a new set of facilities introduced in Pyrex 0.8
for making C declarations, functions and extension types in one Pyrex module available
for use in another Pyrex module. These facilities are closely modelled on
the Python import mechanism, and can be thought of as a compile-time version
Sharing Declarations Between Pyrex Modules
Definition and Implementation files
A Pyrex module can be split into two parts: a definition file with
a .pxd suffix, containing C declarations that are to be available
to other Pyrex modules, and an implementation file with a .pyx
suffix, containing everything else. When a module wants to use something
declared in another module's definition file, it imports it using the cimport statement.
What a Definition File contains
A definition file can contain:
It cannot contain any non-extern C variable declarations.
- Any kind of C type declaration.
- extern C function or variable declarations.
- Declarations of C functions defined in the module.
- The definition part of an extension type (see below).
It cannot contain the implementations of any C or Python functions, or
any Python class definitions, or any executable statements.
NOTE: You don't need to (and shouldn't) declare anything in a
declaration file public in order to make it available to other Pyrex
modules; its mere presence in a definition file does that. You only need a
public declaration if you want to make something available to external C code.
What an Implementation File contains
An implementation file can contain any kind of Pyrex statement, although
there are some restrictions on the implementation part of an extension type
if the corresponding definition file also defines that type (see below).
The cimport statement
The cimport statement is used in a definition or implementation
file to gain access to names declared in another definition file. Its syntax
exactly parallels that of the normal Python import statement:
cimport module [, module...]
from module cimport name
[as name] [, name [as name]
Here is an example. The file on the left is a definition file which exports
a C data type. The file on the right is an implementation file which imports
and uses it.
|cdef enum otherstuff:
sausage, eggs, lettuce
cdef struct spamdish:
from dishes cimport spamdish
cdef void prepare(spamdish *d):
d.oz_of_spam = 42
d.filler = dishes.sausage
cdef spamdish d
print "%d oz spam, filler no. %d" % \
It is important to understand that the cimport statement can only
be used to import C data types, C functions and variables, and extension
types. It cannot be used to import any Python objects, and (with one exception)
it doesn't imply any Python import at run time. If you want to refer to any
Python names from a module that you have cimported, you will have to include
a regular import statement for it as well.
The exception is that when you use cimport to import an extension
type, its type object is imported at run time and made available by the
name under which you imported it. Using cimport to import extension
types is covered in more detail below.
Search paths for definition files
When you cimport a module called modulename, the Pyrex
compiler searches for a file called modulename.pxd along the search
path for include files, as specified by -I command line options.
Also, whenever you compile a file modulename.pyx, the corresponding
definition file modulename.pxd is first searched for along the
same path, and if found, it is processed before processing the .pyx
Using cimport to resolve naming
The cimport mechanism provides a clean and simple way to solve the problem
of wrapping external C functions with Python functions of the same name.
All you need to do is put the extern C declarations into a .pxd file for
an imaginary module, and cimport that module. You can then refer to the C
functions by qualifying them with the name of the module. Here's an example:
|cdef extern from "lunch.h":
def eject_tomato(float speed):
You don't need any c_lunch.pyx file, because the only things
defined in c_lunch.pxd are extern C entities. There won't be any
actual c_lunch module at run time, but that doesn't matter; the c_lunch.pxd file has done its job of providing an additional namespace at compile time.
Sharing C Functions
functions defined at the top level of a module can be made available
via cimport by putting headers for them in the .pxd file, for example,
cdef float cube(float)
from volume cimport cube
def menu(description, size):
print description, ":", cube(size), \
"cubic metres of spam"
menu("Main course", 3)
cdef float cube(float x):
return x * x * x
Sharing Extension Types
An extension type can be made available via cimport by splitting its definition into two parts, one in
a definition file and the other in the corresponding implementation file.
The definition part of the extension type can only declare C attributes
and C methods, not Python methods, and it must declare all of that
type's C attributes and C methods.
The implementation part must implement all of the C methods declared in
the definition part, and may not add any further C attributes or methods. It may also
define Python methods.
Here is an example of a module which defines and exports an extension
type, and another module which uses it.
|cdef class Shrubbery:
cdef int width
cdef int length
|cdef class Shrubbery:
def __cinit__(self, int w, int l):
self.width = w
self.length = l
return Shrubbery(3, 7)
cdef Shrubbing.Shrubbery sh
sh = Shrubbing.standard_shrubbery()
print "Shrubbery size is %d x %d" % (sh.width, sh.length)
Some things to note about this example:
If you are exporting an extension type that has a base class, the
base class must be declared in the definition part. Repeating the base
class in the implementation part is not necessary, but if you do, it
must match the base class in the definition part.
- There is a cdef class Shrubbery declaration in both Shrubbing.pxd
and Shrubbing.pyx. When the Shrubbing module is compiled, these two declarations
are combined into one.
- In Landscaping.pyx, the cimport Shrubbing declaration
allows us to refer to the Shrubbery type as Shrubbing.Shrubbery.
But it doesn't bind the name Shrubbing in Landscaping's module namespace
at run time, so to access Shrubbing.standard_shrubbery we also
need to import Shrubbing.
you have two structs, unions or extension types defined in different
.pxd files, and they need to refer to each other, there is a potential
for problems with circular imports. These problems can be avoided by
placing forward declarations of all the structs, unions and extension
types defined in the .pxd file before the first cimport statement.
|cdef struct Spam|
from blarg cimport Eggs
cdef struct Spam:
|cdef struct Eggs|
from foo cimport Spam
cdef struct Eggs:
the forward declarations weren't present, a circular import problem
would occur, analogous to that which arises in Python when two modules
try to import names from each
other. Placing the forward declarations before the cimport statements ensures that all type names are known to the Pyrex compiler sufficiently far in advance.
that any .pyx file is free to cimport anything it wants from any .pxd
file without needing this precaution. It's only when two .pxd files
import each other that circular
import issues arise.
Back to the Language Overview