• AI & Data Science
October 13, 2021

OpenAI Codex: The automated programmer?

Codex is the latest extension of OpenAI’s GPT-3 large transformer-based language model, pushing the limits of what artificial intelligence can do in the world of natural language generation. One of our engineers was given access to a limited release beta preview of the Codex API and this article explores some of the interesting things they discovered whilst playing around.

Training corpus

The model was trained on 10’s of millions of public code repositories on GitHub. The primary goal of the model is that it be used for research purposes and produce original output, rather than simply regurgitating the code it has been trained on. In fact, OpenAI has found that “the vast majority (>99%) of output does not match the training data”. This means that the model was able to capture the structure, syntax, and semantics of programming languages in the same way it has previously done with human languages.


Figure 1: Transformer architecture

The GPT-3 model is built on the atomic unit of a Transformer, first proposed in 2017 in the research paper “Attention is all you need” by a team at Google. The key finding of this research was the idea of self-attention, which allows generative models to be trained on unstructured and unlabeled text – leading to an exponential increase in training data available to such models.

The main difference between the different iterations of GPT is the number of layers, training corpus size, vocabulary sizes and the number of parameters in the model – all increasing successively with every iteration. GPT-2 increased the number of parameters to 1.5 billion from 117 million. The latest model uses a mind-bending 175 billion parameters. As the models expand, so do their capabilities, as we see in Figure 2 of model accuracy in “next word prediction”, i.e., predicting the next word from a given context. What is important to note, however, is that model performance seems to be approaching a plateau, at least in the case of GPT-3, which may point to the idea that future model iterations will focus on other aspects than training data size to improve generalization ability.

Figure 2: GPT model accuracy comparison (next token prediction)


The Codex API currently supports a limited range of tasks that demonstrates some of the more developed modelling capabilities. These include familiar benchmarked tasks such as chat, Q&A, and text summarization. But this growing list also includes tasks that extend to more nuanced assignments, such as programming or code explanation. It currently offers full support for Python, and supports JavaScript, Go, Perl, PHP, Ruby, Swift, TypeScript, SQL, and Shell to a lesser extent.

If Codex can automate code generation from natural language, it also raises the question of whether it can replace an actual programmer. We decided to start with a simple problem and move to a more involved prompt and finally something more in-line with what a seasoned programmer would need to perform.


  1. Simple prompt – Sort a list of numbers

Figure 3: Sample screenshot from OpenAI Codex Beta


  1. More specific prompt along the same lines – Sort a list of numbers with o(n2) time complexity

  1. Application-specific prompt – Create a LSTM-based time series prediction model with a training window of 3 days and plot the errors of predictions

Overall, the results seem quite impressive. The first two prompts are handled with relative ease, and the model is even able to identify suitable algorithms based on complexity and use built-in Python functions when they are more appropriate. The final prompt also provides some interesting insights into the generation process. The general outline is clearly there, as well as the common pre-processing steps when working with time series data.

It is, however, clear to any Data Scientist / ML Engineer that the third output is not a generalized solution and would require significant reworking. There is no function to plots the errors, and it reads like a copy-paste implementation of a toy problem, including irrelevant comments and unused lines of code. To many, this is akin to a “Googled” solution in that it is not tailor-made to specifications but rather a vaguely relevant code snippet that needs to be largely altered.

Potential and limitations

The Codex model has gone a long way in illustrating what we can expect from future language models, which will be better able to understand us than ever before. With a simple instruction, Codex can find relevant information and structure it in a way that programmers understand, or even create unique content that helps to bridge the translation gap between programmers and non-programmers. This exciting development will continue to fuel interest and research in this area for years to come.

Although the Codex model can code with surprising coherence and speed, it is evidently limited by its exposure to code repositories and their inherit imperfections. The model is also prone to offering up the same solution to slightly varying prompts, and in some cases, presents a solution that is identical to a code excerpt from its training corpus. This means that the model does not always deliver an original and relevant solution to an unseen prompt, something that most trained programmers with exposure to only 1% of the Codex corpora would be able to do.


Codex demonstrates that by combining enormous unstructured corpora and sufficiently complex architectures, models can produce realistic and veritable programming output from simple natural language prompts. However, when digging a little deeper, it is not the panacea it first appears to be. It is still a work in progress, and though exciting, there is still much to be done. For this reason, automated programmers may still be a little way off, and instead the immediate relevance of Codex will be to augment the ability of current programmers, and hopefully clean up some of our dusty, syntactically littered repositories on GitHub.

Other interesting resources related to Codex:

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Jurie Germishuys

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