Cython 0.19’s New Directives for Auto Encoding and Decoding

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Well, have I got news for ya! Cython 0.19 has introduced two new directives that will make your life easier: c_string_type and c_string_encoding.

Before we dive into the details, let’s take a moment to appreciate how much of a pain in the butt it was to manually encode and decode strings before these bad boys came along. Remember when you had to write code like this?

# Import the struct module to use for converting data types
import struct

# Define a function called my_c_function that takes in an input
def my_c_function(input):
    # Convert the input string into a C-style byte array using the 'utf8' encoding
    input_bytes = bytes(input, 'utf8')
    
    # Call the C function with the byte array as an argument and store the result in a variable
    c_result = my_c_function_ptr(input_bytes)
    
    # Decode the result back into a Python string using the 'Q' format for the C function's return type
    output = struct.unpack('Q', c_result)[0]
    
    # Convert the integer output into a byte array
    output = output.tobytes()
    
    # Decode the byte array back into a Python string using the 'utf8' encoding
    output = bytes(output, 'utf8')
    
    # Return the decoded output string
    return output

Ugh! Who has time for all that? Well, not anymore, my friends. With Cython’s new directives, you can write code like this:

# Import necessary modules
import cython # Importing Cython module
from libc.string cimport strlen, strcpy # Importing specific functions from the libc.string module

# Set up context to enable the new directives
@cython.context('') 

# Define a function with input parameter
def my_new_function(input):
    # Declare and initialize a pointer to the input string
    cdef char* input_ptr = <char*>input 
    # Get the length of the input string using the strlen function
    cdef int input_length = strlen(input_ptr)
    
    # Call a C function with the input string as an argument
    result_buf = (c_int * 8)() # Create a buffer to store the result
    my_c_function_ptr(input_ptr, len(input), <void*>result_buf) # Call the C function with the input string and buffer as arguments
    
    # Convert the C-style byte array to a Python string using automatic encoding and decoding
    output = <bytes>result_buf[0] + result_buf[1:8].cast('U') # Convert the first element of the buffer to bytes and concatenate it with the remaining elements converted to UTF-8 encoded string
    
    return output # Return the output string

That’s right! No more manual encoding or decoding. Just let Cython handle it for you automatically using its new c_string_type and c_string_encoding directives. And the best part? It works with any C strings that contain text, not just those in a specific encoding like UTF-8.

So how do these directives work exactly? Well, let’s say your C code uses ASCII or ISO 8859-1 encoded strings (which is pretty common). You can set the c_string_type to ‘unicode’ and the c_string_encoding to ‘latin1’, like so:

# Import necessary libraries
import cython # Importing the cython library
from libc.string cimport strlen, strcpy # Importing specific functions from the libc.string library

# Set up context to enable the new directives
@cython.context('') # Using the context decorator to enable the new directives

# Define a function with input parameter
def my_new_function(input):
    # Define a pointer to the input string and get its length
    cdef char* input_ptr = <char*>input # Using the cdef keyword to define a C variable
    cdef int input_length = strlen(input_ptr) # Using the cdef keyword to define a C variable

    # Call C function with the byte array as an argument
    result_buf = (c_int * 8)() # Creating a C-style byte array with 8 elements
    my_c_function_ptr(input_ptr, len(input), <void*>result_buf) # Calling a C function with the input string and the byte array as arguments

    # Convert C-style byte array to Python string using automatic encoding and decoding
    output = <bytes>result_buf[0] + result_buf[1:8].cast('U') # Converting the C-style byte array to a Python string using automatic encoding and decoding
    # Note: The 'Q' format is used for the C function's return type and the output is assumed to be a UTF-8 encoded string

    return output.decode('latin1') # Decoding the output string using automatic encoding from Cython
    # Note: The output is decoded using the 'latin1' encoding, which was set in the c_string_encoding directive in the context decorator.

And that’s it! No more manual decoding or messing around with struct.unpack(). Just let Cython handle the heavy lifting for you, and enjoy your newfound speed and efficiency.

So what are you waiting for? Go ahead and give these directives a try in your next project. Your Python strings will thank you!

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