Python JIT Compilers – Just in time compilation

Python, known for its simplicity and ease of use, has gained immense popularity among developers. However, its interpreted nature often comes at the cost of performance. This is where Just-in-Time (JIT) compilation steps in as a game-changer. In this article, we’ll explore the concept of JIT compilation, its benefits, and delve into popular Python JIT compilers such as PyPy and Numba.

Basics of Just-in-Time Compilation:

Just-in-Time (JIT) compilation is a dynamic compilation technique that bridges the gap between interpreted languages and compiled languages. Unlike traditional ahead-of-time (AOT) compilation, which converts the entire codebase into machine code before execution, JIT compilation takes a different approach.

It compiles the code on-the-fly, converting sections of the code into machine code right before they are executed, thus optimizing performance.

JIT compilation offers several advantages.

Firstly, it combines the flexibility of an interpreted language with the performance of a compiled language. It allows the interpreter to make intelligent optimizations based on runtime information, leading to faster execution. Additionally, JIT compilation enables dynamic code generation and adaptive optimizations, resulting in improved performance for specific code paths.

Keep in mind however, that not all JIT Compilers are created equal. A “bad” JIT compiler will simply remove the interpreter overhead. A “good” JIT compiler will also optimize the code heavily and achieve significant performance gains.

Understanding Python’s Execution Model:

To grasp the importance of JIT compilers in Python, it’s crucial to understand Python’s execution model. Python is an interpreted language, which means it translates the source code into bytecode, which is then executed by the Python interpreter. This interpretation process introduces overhead and can limit performance, especially for computationally intensive tasks.

Another factor affecting Python’s performance is the Global Interpreter Lock (GIL). The GIL ensures thread safety by allowing only one thread to execute Python bytecode at a time. While this simplifies memory management, it can restrict parallel execution and limit the performance gains from multi-core processors.

The GIL has been a very controversial and highly debated topic amongst the Python community, with many looking to remove it for better multi-threaded performance. However, these proposals have not been accepted yet, primarily because the single-core performance of Python degrades when you remove GIL.

Some JIT compilers can optionally disable the GIL (e.g Numba). There are also some alternative implementations of CPython (not to be confused with JIT compilers) called Jython and IronPython which do not have the GIL. We will not be discussing these however in this article.

Introduction to Python JIT Compilers:

Python JIT compilers offer a solution to the performance limitations of interpreted execution. These compilers dynamically analyze and optimize the code at runtime, resulting in significant speedups. Let’s take a closer look at some popular Python JIT compilers:

PyPy JIT Compiler:

PyPy is a fast, compliant, and highly compatible alternative to the standard CPython interpreter. It utilizes a Just-in-Time compiler to improve performance. PyPy analyzes the Python bytecode and translates it into machine code on the fly. This process eliminates much of the interpretation overhead, leading to faster execution speeds.

One of PyPy’s key advantages is its ability to handle code with tight loops and intensive numerical computations efficiently. By applying optimizations such as loop unrolling and just-in-time specialization, PyPy can often outperform CPython by a significant margin. PyPy also provides a rich set of libraries and supports popular Python frameworks.

To install PyPy, you can visit the official PyPy website ( and follow the installation instructions for your operating system. Once installed, you can execute Python code using the PyPy interpreter.

def calculate_sum(limit):
    total = 0
    for i in range(1, limit + 1):
        total += i
    return total

limit = 10_000_000
result = calculate_sum(limit)
print(f"The sum of numbers from 1 to {limit} is: {result}")

Save the above code into a file named

To execute the script using PyPy, ensure you have PyPy installed on your system. Then, open a terminal or command prompt and navigate to the directory containing the file. Run the following command:


You should observe that PyPy executes the code faster than CPython, especially for larger values of the limit variable. This speedup is a result of PyPy’s just-in-time compilation and optimizations.

It’s worth noting that PyPy is compatible with most Python code and libraries. However, in some cases, due to differences in implementation details, certain extensions or modules may not work correctly with PyPy. If you encounter any issues, refer to the PyPy documentation for guidance on compatibility and troubleshooting.

Numba JIT Compiler:

Numba is a just-in-time compiler specifically designed for numerical and scientific Python code. It leverages the Low-Level Virtual Machine (LLVM) compiler infrastructure to generate optimized machine code dynamically. Numba is known for its ability to accelerate numerical computations, making it an excellent choice for tasks such as array processing, simulations, and data analysis.

To use Numba, you’ll need to install it using a package manager like pip:

pip install numba

Once installed, you can decorate your Python functions with the @jit decorator provided by Numba to trigger just-in-time compilation. Here’s an example:

from numba import jit

def calculate_sum(n):
    cdef int i
    cdef int total = 0
    for i in range(n):
        total += i
    return total

In the above code snippet, the calculate_sum function is decorated with @jit, indicating that Numba should apply just-in-time compilation to optimize its execution.

You can now run this code normally, like you would any other Python script.

Numba also supports parallel execution using the @jit(parallel=True) decorator for suitable code patterns. You can also disable the GIL using the @jit(nogil=True).

Note that not all Python code will benefit equally from Numba optimization. Code that heavily relies on numerical computations or tight loops tends to show the most significant performance improvements. This rings true for other JIT Compilers as well. For example, attempting to optimize a I/O bound function with a JIT will have no effect.

Cython Compiler:

Cython is a programming language that blends Python and C, providing a seamless way to write Python extensions with C-like performance. It translates Python-like code into C, which is then compiled into efficient machine code. Cython allows you to annotate variables and function signatures with type information, enabling static typing and more efficient memory access.

Cython is not a “JIT” compiler exactly, but is often compared to JIT compilers like Numba. At the end of the day, it’s a compiler too, which applies many optimizations like JIT compilers. So it’s worth talking about Cython too.

To use Cython, you’ll need to install it using a package manager like pip:

pip install cython

Once installed, you can create a .pyx file containing your Cython code and compile it into a Python extension module. Here’s an example:

# mymodule.pyx
def calculate_sum(n):
    cdef int i
    cdef int total = 0
    for i in range(n):
        total += i
    return total

To compile the Cython module, you can use the cythonize command:

cythonize -i mymodule.pyx

The -i flag tells Cython to generate the C source code and compile it into a Python extension module. Once compiled, you can import and use the module in your Python code.

import mymodule

result = mymodule.calculate_sum(100)

If you are interested in seeing a performance comparison between Cython and CPython, follow the link.

Best Practices and Tips for Using JIT Compilers in Python:

When working with Python JIT compilers, keep the following best practices in mind:

  1. Identify performance bottlenecks: Profile your code to identify sections that consume the most execution time and would benefit from JIT compilation.
  2. Leverage compiler-specific features: Each JIT compiler offers unique features and optimizations. Explore the documentation and learn how to take full advantage of them.
  3. Optimize data structures: Use appropriate data structures and algorithms to maximize the benefits of JIT compilation.
  4. Measure and validate: Regularly benchmark your code to ensure that the JIT compiler is providing the desired performance improvements.

Cython vs PyPy vs Numba

Let’s provide a more detailed comparison between Cython, PyPy, and Numba, highlighting their unique features, strengths, limitations, and areas where they outperform each other:


Cython is an excellent choice when you need to optimize Python code that interacts with C libraries or requires low-level programming. It allows you to write Python-like code with added static typing and explicit memory management, resulting in significant performance improvements.


  • Seamless integration with existing C code or libraries, making it suitable for wrapping C/C++ libraries or creating Python extensions.
  • Fine-grained control over memory management, type annotations, and direct access to C-level operations.
  • Ability to optimize specific code sections by annotating variables and functions with static types.
  • Generating efficient C extension modules for usage across different Python implementations.


  • Requires adding type annotations and making code modifications for optimization, which may increase complexity and development time.
  • Limited performance improvements for code that doesn’t heavily rely on interactions with C code or low-level operations.
  • Limited performance improvements when working with libraries that are already optimized with C/C++ code (like numpy). Although this is true (in varying degrees) for other compilers too.


PyPy is a suitable option when you want a drop-in replacement for CPython with overall improved performance. It excels in scenarios involving tight loops, numerical computations, and computationally intensive tasks. PyPy’s just-in-time compilation and specialized optimizations can provide significant speedups compared to CPython.


  • Generally delivers improved performance across the entire codebase without major modifications.
  • Outperforms CPython in scenarios involving tight loops, numerical computations, and computationally intensive tasks.
  • Provides compatibility with most Python libraries and frameworks.
  • Automatic memory management and garbage collection improvements.


  • Limited support for C extensions, which may impact compatibility with certain libraries or modules that rely heavily on C code.
  • Memory usage can be higher compared to CPython in some cases.
  • Warm-up time for JIT compilation may impact performance for short-lived programs.


Numba is an ideal choice when working with scientific computing, numerical analysis, and data processing tasks. It specializes in accelerating numerical computations using just-in-time compilation. Numba seamlessly integrates with the NumPy library, making it convenient for optimizing array operations and mathematical algorithms.


  • Provides excellent performance optimization for numerical computations and array operations.
  • Deep integration with NumPy, enabling efficient execution of operations on arrays and mathematical functions.
  • Easy to use with decorators and minimal code modifications required for optimization.
  • Supports parallel execution with the @jit(parallel=True) decorator for suitable code patterns.


  • Limited compatibility with certain Python language features, such as complex control flow or dynamic data structures.
  • Optimizations are specific to numerical computations and may not provide significant benefits for non-numeric code.
  • Relatively slower performance compared to Cython or PyPy for non-numeric code.

In conclusion, consider the following guidelines when choosing a compiler:

  • Use Cython when you require integration with C code, low-level control, and performance optimization for specific code sections.
  • Opt for PyPy when you seek overall performance improvements across the entire codebase, especially for tight loops and numerical computations, without extensive modifications.
  • Select Numba when you focus on accelerating numerical computations, array operations, and leveraging the NumPy ecosystem.

Remember that the performance gains and suitability of each compiler heavily depend on the specific codebase and use case. It’s recommended to profile and benchmark your code with different compilers to make an informed decision.

This marks the end of the Python JIT Compilers article. Any suggestions or contributions for CodersLegacy are more than welcome. Questions regarding the tutorial content can be asked in the comments section below.

Notify of
Newest Most Voted
Inline Feedbacks
View all comments