Python – Extension Programming with C

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Any code that you write using any compiled language like C, C++, or Java can be integrated or imported into another Python script. This code is considered as an “extension.” A Python extension module is nothing more than a normal C library. On Unix machines, these libraries usually end in .so (for shared object). On Windows machines, you typically see .dll (for dynamically linked library).Extending Python with C or C++ It is quite easy to add new built-in modules to Python, if you know how to program in C. Such extension modules can do two things that can’t be done directly in Python: they can implement new built-in object types, and they can call C library functions and system calls

Pre-Requisites for Writing Extensions

To start writing your extension, you are going to need the Python header files.

• On Unix machines, this usually requires installing a developer-specific package such as python2.5-dev.
• Windows users get these headers as part of the package when they use the binary Python installer.

Additionally, it is assumed that you have good knowledge of C or C++ to write any Python Extension using C programming.

First look at a Python Extension

For your first look at a Python extension module, you need to group your code into four part −

• The header file Python.h.
• The C functions you want to expose as the interface from your module.
• A table mapping the names of your functions as Python developers see them to C functions inside the extension module.
• An initialization function.

The Header File Python.h

You need include Python.h header file in your C source file, which gives you access to the internal Python API used to hook your module into the interpreter. Make sure to include Python.h before any other headers you might need. You need to follow the includes with the functions you want to call from Python.

Can you use Python with C?

To write Python modules in C, you’ll need to use the Python API, which defines the various functions, macros, and variables that allow the Python interpreter to call your C code. All of these tools and more are collectively bundled in the Python. h header file.

The C Functions

The signatures of the C implementation of your functions always takes one of the following three forms −

static PyObject *MyFunction( PyObject *self, PyObject *args );

static PyObject *MyFunctionWithKeywords(PyObject *self,
                                 PyObject *args,
                                 PyObject *kw);

static PyObject *MyFunctionWithNoArgs( PyObject *self );

Each one of the preceding declarations returns a Python object. There is no such thing as a void function in Python as there is in C. If you do not want your functions to return a value, return the C equivalent of Python’s None value. The Python headers define a macro, Py_RETURN_NONE, that does this for us. The names of your C functions can be whatever you like as they are never seen outside of the extension module. They are defined as static function. Your C functions usually are named by combining the Python module and function names together, as shown here −

static PyObject *module_func(PyObject *self, PyObject *args) {
   /* Do your stuff here. */
   Py_RETURN_NONE;
}

This is a Python function called func inside of the module module. You will be putting pointers to your C functions into the method table for the module that usually comes next in your source code.

The Method Mapping Table

This method table is a simple array of PyMethodDef structures. That structure looks something like this −

struct PyMethodDef {
   char *ml_name;
   PyCFunction ml_meth;
   int ml_flags;
   char *ml_doc;
};

Here is the description of the members of this structure −

• ml_name − This is the name of the function as the Python interpreter presents when it is used in Python programs.
• ml_meth − This must be the address to a function that has any one of the signatures described in previous seection.
• ml_flags − This tells the interpreter which of the three signatures ml_meth is using.
  • This flag usually has a value of METH_VARARGS.
  • This flag can be bitwise OR’ed with METH_KEYWORDS if you want to allow keyword arguments into your function.
  • This can also have a value of METH_NOARGS that indicates you do not want to accept any arguments.
• ml_doc − This is the docstring for the function, which could be NULL if you do not feel like writing one.

This table needs to be terminated with a sentinel that consists of NULL and 0 values for the appropriate members.

Example

For the above-defined function, we have following method mapping table −

static PyMethodDef <em>module</em>_methods[] = {
   { "<em>func</em>", (PyCFunction)<em>module_func</em>, METH_NOARGS, NULL },
   { NULL, NULL, 0, NULL }
};

The Initialization Function

The last part of your extension module is the initialization function. This function is called by the Python interpreter when the module is loaded. It is required that the function be named initModule, where Module is the name of the module. The initialization function needs to be exported from the library you will be building. The Python headers define PyMODINIT_FUNC to include the appropriate incantations for that to happen for the particular environment in which we’re compiling. All you have to do is use it when defining the function. Your C initialization function generally has the following overall structure −

PyMODINIT_FUNC init<em>Module</em>() {
   Py_InitModule3(<em>func</em>, <em>module</em>_methods, "docstring...");
}

Here is the description of Py_InitModule3 function −

• func − This is the function to be exported.
• module_methods − This is the mapping table name defined above.
• docstring − This is the comment you want to give in your extension.

Putting this all together looks like the following −

#include <Python.h>

static PyObject *<em>module_func</em>(PyObject *self, PyObject *args) {
   /* Do your stuff here. */
   Py_RETURN_NONE;
}

static PyMethodDef <em>module</em>_methods[] = {
   { "<em>func</em>", (PyCFunction)<em>module_func</em>, METH_NOARGS, NULL },
   { NULL, NULL, 0, NULL }
};

PyMODINIT_FUNC init<em>Module</em>() {
   Py_InitModule3(<em>func</em>, <em>module</em>_methods, "docstring...");
}

Example

A simple example that makes use of all the above concepts −

#include <Python.h>

static PyObject* helloworld(PyObject* self) {
   return Py_BuildValue("s", "Hello, Python extensions!!");
}

static char helloworld_docs[] =
   "helloworld( ): Any message you want to put here!!\n";

static PyMethodDef helloworld_funcs[] = {
   {"helloworld", (PyCFunction)helloworld, 
      METH_NOARGS, helloworld_docs},
      {NULL}
};

void inithelloworld(void) {
   Py_InitModule3("helloworld", helloworld_funcs,
                  "Extension module example!");
}

Here the Py_BuildValue function is used to build a Python value. Save above code in hello.c file. We would see how to compile and install this module to be called from Python script.

Building and Installing Extensions

The distutils package makes it very easy to distribute Python modules, both pure Python and extension modules, in a standard way. Modules are distributed in source form and built and installed via a setup script usually called setup.py as follows.

For the above module, you need to prepare following setup.py script −

from distutils.core import setup, Extension
setup(name='helloworld', version='1.0',  \
      ext_modules=[Extension('helloworld', ['hello.c'])])

Now, use the following command, which would perform all needed compilation and linking steps, with the right compiler and linker commands and flags, and copies the resulting dynamic library into an appropriate directory −

$ python setup.py install

On Unix-based systems, you’ll most likely need to run this command as root in order to have permissions to write to the site-packages directory. This usually is not a problem on Windows.

Importing Extensions

Once you installed your extension, you would be able to import and call that extension in your Python script as follows −

#!/usr/bin/python
import helloworld

print helloworld.helloworld()

This would produce the following result −

Hello, Python extensions!!

Passing Function Parameters

As you will most likely want to define functions that accept arguments, you can use one of the other signatures for your C functions. For example, following function, that accepts some number of parameters, would be defined like this −

static PyObject *<em>module_func</em>(PyObject *self, PyObject *args) {
   /* Parse args and do something interesting here. */
   Py_RETURN_NONE;
}

The method table containing an entry for the new function would look like this −

static PyMethodDef module_methods[] = {
   { "func", (PyCFunction)module_func, METH_NOARGS, NULL },
   { "func", module_func, METH_VARARGS, NULL },
   { NULL, NULL, 0, NULL }
};

You can use API PyArg_ParseTuple function to extract the arguments from the one PyObject pointer passed into your C function. The first argument to PyArg_ParseTuple is the args argument. This is the object you will be parsing. The second argument is a format string describing the arguments as you expect them to appear. Each argument is represented by one or more characters in the format string as follows.

static PyObject *<em>module_func</em>(PyObject *self, PyObject *args) {
   int i;
   double d;
   char *s;

   if (!PyArg_ParseTuple(args, "ids", &amp;i, &amp;d, &amp;s)) {
      return NULL;
   }
   
   /* Do something interesting here. */
   Py_RETURN_NONE;
}

Compiling the new version of your module and importing it enables you to invoke the new function with any number of arguments of any type −

module.func(1, s="three", d=2.0)
module.func(i=1, d=2.0, s="three")
module.func(s="three", d=2.0, i=1)

You can probably come up with even more variations.

The PyArg_ParseTuple Function

Here is the standard signature for PyArg_ParseTuple function −

int PyArg_ParseTuple(PyObject* tuple,char* format,...)

This function returns 0 for errors, and a value not equal to 0 for success. tuple is the PyObject* that was the C function’s second argument. Here format is a C string that describes mandatory and optional arguments.

Here is a list of format codes for PyArg_ParseTuple function −

Returning Values

Py_BuildValue takes in a format string much like PyArg_ParseTuple does. Instead of passing in the addresses of the values you are building, you pass in the actual values. Here’s an example showing how to implement an add function −

static PyObject *foo_add(PyObject *self, PyObject *args) {
   int a;
   int b;

   if (!PyArg_ParseTuple(args, "ii", &a, &b)) {
      return NULL;
   }
   return Py_BuildValue("i", a + b);
}

This is what it would look like if implemented in Python −

def add(a, b):
   return (a + b)

You can return two values from your function as follows, this would be cauptured using a list in Python.

static PyObject *foo_add_subtract(PyObject *self, PyObject *args) {
   int a;
   int b;

   if (!PyArg_ParseTuple(args, "ii", &a, &b)) {
      return NULL;
   }
   return Py_BuildValue("ii", a + b, a - b);
}

This is what it would look like if implemented in Python −

def add_subtract(a, b):
   return (a + b, a - b)

The Py_BuildValue Function

Here is the standard signature for Py_BuildValue function −

PyObject* Py_BuildValue(char* format,...)

Here format is a C string that describes the Python object to build. The following arguments of Py_BuildValue are C values from which the result is built. The PyObject* result is a new reference. Following table lists the commonly used code strings, of which zero or more are joined into string format.

Code {…} builds dictionaries from an even number of C values, alternately keys and values. For example, Py_BuildValue(“{issi}”,23,”zig”,”zag”,42) returns a dictionary like Python’s {23:’zig’,’zag’:42}.

Writing a Python Interface in C

In this tutorial, you’ll write a small wrapper for a C library function, which you’ll then invoke from within Python. Implementing a wrapper yourself will give you a better idea about when and how to use C to extend your Python module.

Understanding fputs()

fputs() is the C library function that you’ll be wrapping:

int fputs(const char *, FILE *)

This function takes two arguments:

1. const char * is an array of characters.
2. FILE * is a file stream pointer.

fputs() writes the character array to the file specified by the file stream and returns a non-negative value. If the operation is successful, then this value will denote the number of bytes written to the file. If there’s an error, then it returns EOF. You can read more about this C library function and its other variants in the manual page entry.

Writing the C Function for fputs()

This is a basic C program that uses fputs() to write a string to a file stream:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
    FILE *fp = fopen("write.txt", "w");
    fputs("Real Python!", fp);
    fclose(fp);
    return 1;
}

This snippet of code can be summarized as follows:

1. Open the file write.txt.
2. Write the string "Real Python!" to the file.

Note: The C code in this article should build on most systems. It has been tested on GCC without using any special flags. In the following section, you’ll write a wrapper for this C function.

Wrapping fputs()

It might seem a little weird to see the full code before an explanation of how it works. However, taking a moment to inspect the final product will supplement your understanding in the following sections. The code block below shows the final wrapped version of your C code:

 1#include <Python.h>
 2
 3static PyObject *method_fputs(PyObject *self, PyObject *args) {
 4    char *str, *filename = NULL;
 5    int bytes_copied = -1;
 6
 7    /* Parse arguments */
 8    if(!PyArg_ParseTuple(args, "ss", &str, &filename)) {
 9        return NULL;
10    }
11
12    FILE *fp = fopen(filename, "w");
13    bytes_copied = fputs(str, fp);
14    fclose(fp);
15
16    return PyLong_FromLong(bytes_copied);
17}

This code snippet references three object structures which are defined in Python.h:

1. PyObject
2. PyArg_ParseTuple()
3. PyLong_FromLong()

These are used for data type definition for the Python language. You’ll go through each of them now.

PyObject

PyObject is an object structure that you use to define object types for Python. All Python objects share a small number of fields that are defined using the PyObject structure. All other object types are extensions of this type.

PyObject tells the Python interpreter to treat a pointer to an object as an object. For instance, setting the return type of the above function as PyObject defines the common fields that are required by the Python interpreter in order to recognize this as a valid Python type.

Take another look at the first few lines of your C code:

 1static PyObject *method_fputs(PyObject *self, PyObject *args) {
 2    char *str, *filename = NULL;
 3    int bytes_copied = -1;
 4
 5    /* Snip */

In line 2, you declare the argument types you wish to receive from your Python code:

1. char *str is the string you want to write to the file stream.
2. char *filename is the name of the file to write to.

PyArg_ParseTuple()

PyArg_ParseTuple() parses the arguments you’ll receive from your Python program into local variables:

 1static PyObject *method_fputs(PyObject *self, PyObject *args) {
 2    char *str, *filename = NULL;
 3    int bytes_copied = -1;
 4
 5    /* Parse arguments */
 6    if(!PyArg_ParseTuple(args, "ss", &str, &filename)) {
 7        return NULL;
 8    }
 9
10    /* Snip */

If you look at line 6, then you’ll see that PyArg_ParseTuple() takes the following arguments:

• args are of type PyObject.
• "ss" is the format specifier that specifies the data type of the arguments to parse. (You can check out the official documentation for a complete reference.)
• &str and &filename are pointers to local variables to which the parsed values will be assigned.

PyArg_ParseTuple() evaluates to false on failure. If it fails, then the function will return NULL and not proceed any further.

fputs()

As you’ve seen before, fputs() takes two arguments, one of which is the FILE * object. Since you can’t parse a Python textIOwrapper object using the Python API in C, you’ll have to use a workaround:

 1static PyObject *method_fputs(PyObject *self, PyObject *args) {
 2    char *str, *filename = NULL;
 3    int bytes_copied = -1;
 4
 5    /* Parse arguments */
 6    if(!PyArg_ParseTuple(args, "ss", &str, &filename)) {
 7        return NULL;
 8    }
 9
10    FILE *fp = fopen(filename, "w");
11    bytes_copied = fputs(str, fp);
12    fclose(fp);
13
14    return PyLong_FromLong(bytes_copied);
15}

Here’s a breakdown of what this code does:

• In line 10, you’re passing the name of the file that you’ll use to create a FILE * object and pass it on to the function.
• In line 11, you call fputs() with the following arguments:
  • str is the string you want to write to the file.
  • fp is the FILE * object you defined in line 10.

You then store the return value of fputs() in bytes_copied. This integer variable will be returned to the fputs() invocation within the Python interpreter.

PyLong_FromLong(bytes_copied)

PyLong_FromLong() returns a PyLongObject, which represents an integer object in Python. You can find it at the very end of your C code:

 1static PyObject *method_fputs(PyObject *self, PyObject *args) {
 2    char *str, *filename = NULL;
 3    int bytes_copied = -1;
 4
 5    /* Parse arguments */
 6    if(!PyArg_ParseTuple(args, "ss", &str, &filename)) {
 7        return NULL;
 8    }
 9
10    FILE *fp = fopen(filename, "w");
11    bytes_copied = fputs(str, fp);
12    fclose(fp);
13
14    return PyLong_FromLong(bytes_copied);
15}

Line 14 generates a PyLongObject for bytes_copied, the variable to be returned when the function is invoked in Python. You must return a PyObject* from your Python C extension module back to the Python interpreter.

Writing the Init Function

You’ve written the code that makes up the core functionality of your Python C extension module. However, there are still a few extra functions that are necessary to get your module up and running. You’ll need to write definitions of your module and the methods it contains, like so:

static PyMethodDef FputsMethods[] = {
    {"fputs", method_fputs, METH_VARARGS, "Python interface for fputs C library function"},
    {NULL, NULL, 0, NULL}
};


static struct PyModuleDef fputsmodule = {
    PyModuleDef_HEAD_INIT,
    "fputs",
    "Python interface for the fputs C library function",
    -1,
    FputsMethods
};

These functions include meta information about your module that will be used by the Python interpreter. Let’s go through each of the structs above to see how they work.

PyMethodDef

In order to call the methods defined in your module, you’ll need to tell the Python interpreter about them first. To do this, you can use PyMethodDef. This is a structure with 4 members representing a single method in your module. Ideally, there will be more than one method in your Python C extension module that you want to be callable from the Python interpreter. This is why you need to define an array of PyMethodDef structs:

static PyMethodDef FputsMethods[] = {
    {"fputs", method_fputs, METH_VARARGS, "Python interface for fputs C library function"},
    {NULL, NULL, 0, NULL}
};

Each individual member of the struct holds the following info:

• "fputs" is the name the user would write to invoke this particular function.
• method_fputs is the name of the C function to invoke.
• METH_VARARGS is a flag that tells the interpreter that the function will accept two arguments of type PyObject*:
  1. self is the module object.
  2. args is a tuple containing the actual arguments to your function. As explained previously, these arguments are unpacked using PyArg_ParseTuple().
     
• The final string is a value to represent the method docstring.

PyModuleDef

Just as PyMethodDef holds information about the methods in your Python C extension module, the PyModuleDef struct holds information about your module itself. It is not an array of structures, but rather a single structure that’s used for module definition:

static struct PyModuleDef fputsmodule = {
    PyModuleDef_HEAD_INIT,
    "fputs",
    "Python interface for the fputs C library function",
    -1,
    FputsMethods
};

There are a total of 9 members in this struct, but not all of them are required. In the code block above, you initialize the following five:

1. PyModuleDef_HEAD_INIT is a member of type PyModuleDef_Base, which is advised to have just this one value.
2. "fputs" is the name of your Python C extension module.
3. The string is the value that represents your module docstring. You can use NULL to have no docstring, or you can specify a docstring by passing a const char * as shown in the snippet above. It is of type Py_ssize_t. You can also use PyDoc_STRVAR() to define a docstring for your module.
4. -1 is the amount of memory needed to store your program state. It’s helpful when your module is used in multiple sub-interpreters, and it can have the following values:
   • A negative value indicates that this module doesn’t have support for sub-interpreters.
   • A non-negative value enables the re-initialization of your module. It also specifies the memory requirement of your module to be allocated on each sub-interpreter session.
     
5. FputsMethods is the reference to your method table. This is the array of PyMethodDef structs you defined earlier.

For more information, check out the official Python documentation on PyModuleDef.

PyMODINIT_FUNC

Now that you’ve defined your Python C extension module and method structures, it’s time to put them to use. When a Python program imports your module for the first time, it will call PyInit_fputs():

PyMODINIT_FUNC PyInit_fputs(void) {
    return PyModule_Create(&fputsmodule);
}

PyMODINIT_FUNC does 3 things implicitly when stated as the function return type:

1. It implicitly sets the return type of the function as PyObject*.
2. It declares any special linkages.
3. It declares the function as extern “C.” In case you’re using C++, it tells the C++ compiler not to do name-mangling on the symbols.

PyModule_Create() will return a new module object of type PyObject *. For the argument, you’ll pass the address of the method structure that you’ve already defined previously, fputsmodule.

Note: In Python 3, your init function must return a PyObject * type. However, if you’re using Python 2, then PyMODINIT_FUNC declares the function return type as void.

Putting It All Together

Now that you’ve written the necessary parts of your Python C extension module, let’s take a step back to see how it all fits together. The following diagram shows the components of your module and how they interact with the Python interpreter:

When you import your Python C extension module, PyInit_fputs() is the first method to be invoked. However, before a reference is returned to the Python interpreter, the function makes a subsequent call to PyModule_Create(). This will initialize the structures PyModuleDef and PyMethodDef, which hold meta information about your module. It makes sense to have them ready since you’ll make use of them in your init function.

Once this is complete, a reference to the module object is finally returned to the Python interpreter. The following diagram shows the internal flow of your module:

The module object returned by PyModule_Create() has a reference to the module structure PyModuleDef, which in turn has a reference to the method table PyMethodDef. When you call a method defined in your Python C extension module, the Python interpreter uses the module object and all of the references it carries to execute the specific method. (While this isn’t exactly how the Python interpreter handles things under the hood, it’ll give you an idea of how it works.)

Similarly, you can access various other methods and properties of your module, such as the module docstring or the method docstring. These are defined inside their respective structures.

Now you have an idea of what happens when you call fputs() from the Python interpreter. The interpreter uses your module object as well as the module and method references to invoke the method. Finally, let’s take a look at how the interpreter handles the actual execution of your Python C extension module:

Once method_fputs() is invoked, the program executes the following steps:

1. Parse the arguments you passed from the Python interpreter with PyArg_ParseTuple()
2. Pass these arguments to fputs(), the C library function that forms the crux of your module
3. Use PyLong_FromLong to return the value from fputs()

To see these same steps in code, take a look at method_fputs() again:

 1static PyObject *method_fputs(PyObject *self, PyObject *args) {
 2    char *str, *filename = NULL;
 3    int bytes_copied = -1;
 4
 5    /* Parse arguments */
 6    if(!PyArg_ParseTuple(args, "ss", &str, &filename)) {
 7        return NULL;
 8    }
 9
10    FILE *fp = fopen(filename, "w");
11    bytes_copied = fputs(str, fp);
12    fclose(fp);
13
14    return PyLong_FromLong(bytes_copied);
15}

To recap, your method will parse the arguments passed to your module, send them on to fputs(), and return the results.

Packaging Your Python C Extension Module

Before you can import your new module, you first need to build it. You can do this by using the Python package distutils.

You’ll need a file called setup.py to install your application. For this tutorial, you’ll be focusing on the part specific to the Python C extension module. For a full primer, check out How to Publish an Open-Source Python Package to PyPI.

A minimal setup.py file for your module should look like this:

from distutils.core import setup, Extension

def main():
    setup(name="fputs",
          version="1.0.0",
          description="Python interface for the fputs C library function",
          author="<your name>",
          author_email="your_email@gmail.com",
          ext_modules=[Extension("fputs", ["fputsmodule.c"])])

if __name__ == "__main__":
    main()

The code block above shows the standard arguments that are passed to setup(). Take a closer look at the last positional argument, ext_modules. This takes a list of objects of the Extensions class. An object of the Extensions class describes a single C or C++ extension module in a setup script. Here, you pass two keyword arguments to its constructor, namely:

• name is the name of the module.
• [filename] is a list of paths to files with the source code, relative to the setup script.

Building Your Module

Now that you have your setup.py file, you can use it to build your Python C extension module. It’s strongly advised that you use a virtual environment to avoid conflicts with your Python environment.

Navigate to the directory containing setup.py and run the following command:

$ python3 setup.py install

This command will compile and install your Python C extension module in the current directory. If there are any errors or warnings, then your program will throw them now. Make sure you fix these before you try to import your module.

By default, the Python interpreter uses clang for compiling the C code. If you want to use gcc or any other C compiler for the job, then you need to set the CC environment variable accordingly, either inside the setup script or directly on the command line. For instance, you can tell the Python interpreter to use gcc to compile and build your module this way:

$ CC=gcc python3 setup.py install

However, the Python interpreter will automatically fall back to gcc if clang is not available.

Running Your Module

Now that everything is in place, it’s time to see your module in action! Once it’s successfully built, fire up the interpreter to test run your Python C extension module:>>>

>>> import fputs
>>> fputs.__doc__
'Python interface for the fputs C library function'
>>> fputs.__name__
'fputs'
>>> # Write to an empty file named `write.txt`
>>> fputs.fputs("Real Python!", "write.txt")
13
>>> with open("write.txt", "r") as f:
>>>     print(f.read())
'Real Python!'

Your function performs as expected! You pass a string "Real Python!" and a file to write this string to, write.txt. The call to fputs() returns the number of bytes written to the file. You can verify this by printing the contents of the file.

Also recall how you passed certain arguments to the PyModuleDef and PyMethodDef structures. You can see from this output that Python has used these structures to assign things like the function name and docstring.

With that, you have a basic version of your module ready, but there’s a lot more that you can do! You can improve your module by adding things like custom exceptions and constants.

Raising Exceptions

Python exceptions are very different from C++ exceptions. If you want to raise Python exceptions from your C extension module, then you can use the Python API to do so. Some of the functions provided by the Python API for exception raising are as follows:

You can use any of these to raise an exception. However, which to use and when depends entirely on your requirements. The Python API has all the standard exceptions pre-defined as PyObject types.

Raising Exceptions From C Code

While you can’t raise exceptions in C, the Python API will allow you to raise exceptions from your Python C extension module. Let’s test this functionality by adding PyErr_SetString() to your code. This will raise an exception whenever the length of the string to be written is less than 10 characters:

 1static PyObject *method_fputs(PyObject *self, PyObject *args) {
 2    char *str, *filename = NULL;
 3    int bytes_copied = -1;
 4
 5    /* Parse arguments */
 6    if(!PyArg_ParseTuple(args, "ss", &str, &fd)) {
 7        return NULL;
 8    }
 9
10    if (strlen(str) < 10) {
11        PyErr_SetString(PyExc_ValueError, "String length must be greater than 10");
12        return NULL;
13    }
14
15    fp = fopen(filename, "w");
16    bytes_copied = fputs(str, fp);
17    fclose(fp);
18
19    return PyLong_FromLong(bytes_copied);
20}

Here, you check the length of the input string immediately after you parse the arguments and before you call fputs(). If the string passed by the user is shorter than 10 characters, then your program will raise a ValueError with a custom message. The program execution stops as soon as the exception occurs.

Note how method_fputs() returns NULL after raising the exception. This is because whenever you raise an exception using PyErr_*(), it automatically sets an internal entry in the exception table and returns it. The calling function is not required to subsequently set the entry again. For this reason, the calling function returns a value that indicates failure, usually NULL or -1. (This should also explain why there was a need to return NULL when you parse arguments in method_fputs() using PyArg_ParseTuple().)

Raising Custom Exceptions

You can also raise custom exceptions in your Python C extension module. However, things are a bit different. Previously, in PyMODINIT_FUNC, you were simply returning the instance returned by PyModule_Create and calling it a day. But for your custom exception to be accessible by the user of your module, you need to add your custom exception to your module instance before you return it:

static PyObject *StringTooShortError = NULL;

PyMODINIT_FUNC PyInit_fputs(void) {
    /* Assign module value */
    PyObject *module = PyModule_Create(&fputsmodule);

    /* Initialize new exception object */
    StringTooShortError = PyErr_NewException("fputs.StringTooShortError", NULL, NULL);

    /* Add exception object to your module */
    PyModule_AddObject(module, "StringTooShortError", StringTooShortError);

    return module;
}

As before, you start off by creating a module object. Then you create a new exception object using PyErr_NewException. This takes a string of the form module.classname as the name of the exception class that you wish to create. Choose something descriptive to make it easier for the user to interpret what has actually gone wrong.

Next, you add this to your module object using PyModule_AddObject. This takes your module object, the name of the new object being added, and the custom exception object itself as arguments. Finally, you return your module object.

Now that you’ve defined a custom exception for your module to raise, you need to update method_fputs() so that it raises the appropriate exception:

 1static PyObject *method_fputs(PyObject *self, PyObject *args) {
 2    char *str, *filename = NULL;
 3    int bytes_copied = -1;
 4
 5    /* Parse arguments */
 6    if(!PyArg_ParseTuple(args, "ss", &str, &fd)) {
 7        return NULL;
 8    }
 9
10    if (strlen(str) < 10) {
11        /* Passing custom exception */
12        PyErr_SetString(StringTooShortError, "String length must be greater than 10");
13        return NULL;
14    }
15
16    fp = fopen(filename, "w");
17    bytes_copied = fputs(str, fp);
18    fclose(fp);
19
20    return PyLong_FromLong(bytes_copied);
21}

After building the module with the new changes, you can test that your custom exception is working as expected by trying to write a string that is less than 10 characters in length:>>>

>>> import fputs
>>> # Custom exception
>>> fputs.fputs("RP!", fp.fileno())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
fputs.StringTooShortError: String length must be greater than 10

When you try to write a string with fewer than 10 characters, your custom exception is raised with a message explaining what went wrong.

Defining Constants

There are cases where you’ll want to use or define constants in your Python C extension module. This is quite similar to how you defined custom exceptions in the previous section. You can define a new constant and add it to your module instance using PyModule_AddIntConstant():

PyMODINIT_FUNC PyInit_fputs(void) {
    /* Assign module value */
    PyObject *module = PyModule_Create(&fputsmodule);

    /* Add int constant by name */
    PyModule_AddIntConstant(module, "FPUTS_FLAG", 64);

    /* Define int macro */
    #define FPUTS_MACRO 256

    /* Add macro to module */
    PyModule_AddIntMacro(module, FPUTS_MACRO);

    return module;
}

This Python API function takes the following arguments:

• The instance of your module
• The name of the constant
• The value of the constant

You can do the same for macros using PyModule_AddIntMacro():

PyMODINIT_FUNC PyInit_fputs(void) {
    /* Assign module value */
    PyObject *module = PyModule_Create(&fputsmodule);

    /* Add int constant by name */
    PyModule_AddIntConstant(module, "FPUTS_FLAG", 64);

    /* Define int macro */
    #define FPUTS_MACRO 256

    /* Add macro to module */
    PyModule_AddIntMacro(module, FPUTS_MACRO);

    return module;
}

This function takes the following arguments:

• The instance of your module
• The name of the macro that has already been defined

Note: If you want to add string constants or macros to your module, then you can use PyModule_AddStringConstant() and PyModule_AddStringMacro(), respectively.

Open up the Python interpreter to see if your constants and macros are working as expected:>>>

>>> import fputs
>>> # Constants
>>> fputs.FPUTS_FLAG
64
>>> fputs.FPUTS_MACRO
256

Here, you can see that the constants are accessible from within the Python interpreter.

Testing Your Module

You can test your Python C extension module just as you would any other Python module. This can be demonstrated by writing a small test function for pytest:

import fputs

def test_copy_data():
    content_to_copy = "Real Python!"
    bytes_copied = fputs.fputs(content_to_copy, 'test_write.txt')

    with open('test_write.txt', 'r') as f:
        content_copied = f.read()

    assert content_copied == content_to_copy

In the test script above, you use fputs.fputs() to write the string "Real Python!" to an empty file named test_write.txt. Then, you read in the contents of this file and use an assert statement to compare it to what you had originally written. You can run this test suite to make sure your module is working as expected:

$ pytest -q
test_fputs.py                                                 [100%]
1 passed in 0.03 seconds

For a more in-depth introduction, check out Getting Started With Testing in Python.

Considering Alternatives

In this tutorial, you’ve built an interface for a C library function to understand how to write Python C extension modules. However, there are times when all you need to do is invoke some system calls or a few C library functions, and you want to avoid the overhead of writing two different languages. In these cases, you can use Python libraries such as ctypes or cffi. These are Foreign Function libraries for Python that provide access to C library functions and data types. Though the community itself is divided as to which library is best, both have their benefits and drawbacks. In other words, either would make a good choice for any given project, but there are a few things to keep in mind when you need to decide between the two:

• The ctypes library comes included in the Python standard library. This is very important if you want to avoid external dependencies. It allows you to write wrappers for other languages in Python.
• The cffi library is not yet included in the standard library. This might be a dealbreaker for your particular project. In general, it’s more Pythonic in nature, but it doesn’t handle preprocessing for you.

For more information on these libraries, check out Extending Python With C Libraries and the “ctypes” Module and Interfacing Python and C: The CFFI Module.

Note: Apart from ctypes and cffi, there are various other tools available. For instance, you can also use swig and boost::Py.