Building Your Own BERT Model: Practical Step-By-Step Implementation Guide

BERT Model

The Bidirectional Encoder Representations from Transformers (BERT) concept has had a significant impact on Natural Language Processing (NLP). The BERT Model was introduced by Google in 2018 through its paper called “Pre-training of deep bidirectional transformers for language understanding”. This was where they introduced the BERT model, which was extremely efficient with problems related to GLUE (General Language Understanding Evaluation. GLUE is a benchmark that measures the performance of natural language understanding systems on a variety of tasks. BERT was able to achieve state-of-the-art results on all of the GLUE tasks, which showed that it was a very effective model for natural language understanding. BERT has since been used in a wide variety of NLP applications, including text classification, question answering, and natural language inference. It has also been used to improve the performance of other NLP models, such as machine translation and text summarization.

Building Your Own BERT Model: Practical Step-By-Step Implementation Guide

The BERT model is a significant advancement in NLP, and it has had a major impact on the field. It has made it possible for NLP systems to better understand and utilize textual data, which has led to advancements in a wide range of NLP tasks. However, it is a complex topic in the vast field of Machine Learning and Transformers. It is understandable for one to not know where to start and how to proceed with the different models and how to implement, operate and finetune a BERT Model. However, you should not worry, as we will walk you through every step of setting up your own BERT model in this highly in-depth guide. Regardless of your level of NLP experience, we will break down each step straightforwardly and understandably, integrating code samples to encourage hands-on learning.

Laying Foundation

Before we begin our journey of implementing a BERT Model on our own with step-by-step explanation and code snippet examples, let us first realize the extent of this guide, the contents therein, and all the things that we cover in it. In this guide, we will undergo these steps to unravel, understand and, implement the BERT concept and create our own BERT Model. These are the core stages of our guide.

  • Understanding BERT: First of all, we will understand the underlying principles powering the BERT model, how it works and how it is so exceptionally efficient at what it does.
  • Data Preprocessing: Then comes the exploring of essential data preparation steps, through which we can pave the way to the feeding of contextual data to the BERT model.
  • Constructing the BERT Model: After that, we will finally delve into the architecture of a BERT Model and the meticulous process of creating a model from scratch.
  • Fine-Tuning: After the BERT Model’s construction, we will go over the important step of fine-tuning which will adapt the pre-trained model to address more specific tasks.
  • Inference: Finally, we will demonstrate the utilization potential of the trained model to make accurate text predictions on the fresh text data provided.

We will go through all these steps one by one, including Python’s code snippets with each line explained to further help you understand the concept so you can use this in real life and create your BERT Model with ease.

Step 1: Understanding BERT Model

As we have stated, BERT (Bidirectional Encoder Representations from Transformers) is an example of a transformer-based model. Transformer-based models are a type of deep learning architecture that has gained significant popularity and achieved state-of-the-art performance in various natural language processing (NLP) and machine translation tasks. The model can evaluate the relative weights of various words in a sentence while taking into account their contextual relationships thanks to a technique termed “self-attention,” which is the foundation of the transformer design. Even in transformer-based models, BERT is groundbreaking. By considering both the preceding and following context, BERT transcends the limitations of unidirectional models, leading to a more profound understanding of language nuances.

BERT is a bidirectional model, which means that it can consider the context of a word by looking at the words that come before and after it. This is in contrast to unidirectional models, which can only look at the words that come before a word. By considering both the preceding and following context, BERT can better understand the meaning of a word and its role in a sentence. This can lead to a more profound understanding of language nuances. For example, BERT can understand that the word “bank” can refer to a financial institution or the edge of a river, depending on the context. This goes beyond the traditional transformer-based model, making BERT exceptional in the category as we have discussed.

Step 2: Preparing Our Data for BERT

Data preparation is the process of cleaning, organizing, and transforming data into a format that can be used by machine learning models. It is a critical step in any machine learning project, as the quality of the data will have a direct impact on the performance of the model. Data preparation can be a time-consuming and challenging task, but it is essential for the success of any machine learning project. By taking the time to prepare the data properly, you can ensure that your model will be accurate and reliable. The same is true when preparing the data for your BERT model. The steps involved in the preparation of the BERT model are very crucial, as we have shown below:

  • Tokenization: The process of disintegrating text into smaller units (tokens) is called as tokenization. Tokens can be words, sub-words, or even individual characters. The purpose of tokenization is to make text easier to process by computers. For example, if you want to search for a particular word in a text, it is much easier to do so if the text has been tokenized into individual words.
  • Special Tokens: Introducing special tokens like CLS (classification) at the beginning and SEP (separator) between sentences can help BERT understand the text’s structure in a few ways. First, the CLS token can be used to represent the entire text, which can help BERT learn the overall meaning of the text. Second, the SEP tokens can be used to represent the boundaries between sentences, which can help BERT learn the relationships between sentences. Finally, the special tokens can also be used to represent other parts of speech, such as nouns, verbs, and adjectives, which can help BERT learn the meaning of individual words.
  • Padding and Truncation: Ensuring uniformity in sequence length by either adding padding or truncating text is a common practice in natural language processing. This is done to make sure that all sequences are of the same length, which can help to improve the accuracy of models. Padding is when extra characters are added to the end of a sequence to make it the desired length. Truncating is when characters are removed from the beginning or end of a sequence to make it the desired length.
  • Word Embedding: Mapping tokens to numerical vectors using pre-trained word embeddings. Mapping tokens to numerical vectors using pre-trained word embeddings is a process of converting words into vectors of numbers. This is done by using a pre-trained model that has been trained on a large corpus of text. The model learns to associate each word with a vector of numbers that represents the meaning of the word. This process can be used to represent words in a way that is easier for computers to understand. It can also be used to find similarities between words. For example, the vectors for the words “cat” and “dog” would be similar, because they both represent animals. This information can be used to find related words or to cluster words into groups.

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Data Preprocessing Code Snippet With Explanation:

# Import the necessary libraries
from transformers import BertTokenizer# Load the BERT tokenizer
tokenizer = BertTokenizer.from_pretrained(‘bert-base-uncased’)# Tokenize and convert the text to input IDs
text = “Implementing BERT model is exciting!”
input_ids = tokenizer.encode(text, add_special_tokens=True)# Display tokenized input IDs
print(“Tokenized Input IDs:”, input_ids)


  • First, we import the BertTokenizer from the trans formers library.
  • Then we will load the pre-trained BERT tokenizer for the ‘bert-base-uncased’ variant.
  • After which, we will tokenize the input text “Implementing the BERT model is exciting!” and encode it as input IDs.

Step 3: Constructing Your BERT Model

Creating a BERT model involves understanding its architecture, which comprises embedding layers, transformer layers, and an output layer. Embedding layers convert words into vectors of numbers. This allows the model to understand the meaning of words, even if they are not present in the training data. Transformer layers are responsible for learning the relationships between words. They do this by attending to each other, which means that they pay attention to the words that are close to them in the sentence. The output layer is responsible for predicting the label of the sentence. It does this by taking the vectors from the embedding layers and the transformer layers and combining them to produce a single vector. This vector is then used to predict the label of the sentence. In addition to these three main layers, BERT also has several other layers, such as a dropout layer and a regularization layer. These layers help to prevent the model from overfitting the training data. Understanding the concepts of the BERT model’s architecture will help in constructing the BERT model of your own making. We show the PyTorch Python code snippet below.

BERT Model Architecture Code Snippet With Explanation:

import torch
import torch.nn as nn
from transformers import BertModelclass CustomBERT(nn.Module):
def __init__(self):
super(CustomBERT, self).__init__()
self.bert = BertModel.from_pretrained(‘bert-base-uncased’)def forward(self, input_ids):
outputs = self.bert(input_ids)
return outputs# Instantiate the custom BERT model
model = CustomBERT()

# Display the model architecture


  • First, we import the necessary libraries, including ‘torch’ for PyTorch, ‘nn’ for neural network modules, and BertModel from transformers.
  • Then, we define a custom class CustomBERT that inherits from ‘nn.Module’.
  • Inside the class constructor, we then have to initialize the BERT model using the pre-trained weights of ‘bert-base-uncased’.
  • After this, we have to define the ‘forward’ method, which will take the ‘input_ids’ as the input and return the BERT output.
  • Now, we instantiate the custom BERT output.
  • Finally, we print the architecture of the model.

Step 4: Fine-Tuning Your Model

Fine-tuning is a process of adjusting a pre-trained model to a specific task. This is done by feeding the model data that is specific to the task and then adjusting the model’s parameters so that it can perform the task better. In the case of BERT, the pre-trained model is trained on a massive dataset of text and code. This dataset is used to teach the model the relationships between words and concepts. When fine-tuning BERT, the model is trained on a dataset that is specific to the task at hand. For example, if the task is to classify text as spam or not spam, the model would be trained on a dataset of text that has been labeled as spam or not spam. The model would then be adjusted so that it can classify new text as spam or not spam.  It’s an important step in making BERT more accurate for a specific task. By training the model on data that is specific to the task, the model can learn the nuances of the task and perform better. The code snippet on fine-tuning the BERT model is given below.

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Fine-Tuning the Model Code Snippet With Explanation:

# Assuming the availability of a labelled dataset for sentiment analysis
from transformers import BertForSequenceClassification, AdamW
from import DataLoader, RandomSampler, SequentialSampler# Load the pre-trained BERT model for sequence classificationmodel = BertForSequenceClassification.from_pretrained(‘bert-base-uncased’, num_labels=2)# Define the optimizer and data loaders
optimizer = AdamW(model.parameters(), lr=1e-5)
train_dataloader = DataLoader(train_dataset, sampler=RandomSampler(train_dataset), batch_size=32)

# Initiate the fine-tuning loop
for epoch in range(epochs):
for batch in train_dataloader:
input_ids, labels = batch
outputs = model(input_ids, labels=labels)
loss = outputs.loss


  • First, we will import ‘BertForSequenceClassification’, ‘AdamW optimizer’, and any other tools required for data management, assuming that a labeled dataset already exists.
  • Then we will place a predetermined number of labels into the pre-trained BERT model for sequence categorization.
  • Utilising AdamW and a 1e-5 learning rate, we will then define the optimizer.
  • We will then set up data loaders for training data, taking into account batch size and sampling.
  • After that, we will create a loop that iterates through epochs and batches to be used for fine-tuning.
  • We have to make sure that the model should be in training mode.
  • Then, we can calculate the loss, backpropagate, and change the model parameters for each batch using the optimizer.

Step 5: Using The BERT Model For Inference

After fine-tuning, your BERT model is now ready to make predictions on new text data. This means that it can be used to answer questions, generate text, and translate languages. For example, you could ask your BERT model to summarize a text or to write a poem. You could also use it to translate a text from one language to another. We give an example of inference below.

Inference Code Snippet With Explanation:

# Assuming the availability of new text data for prediction
text = “BERT is astonishing!”
input_ids = tokenizer.encode(text, add_special_tokens=True)
input_tensor = torch.tensor(input_ids).unsqueeze(0)# Transition the model to evaluation mode
model.eval()# Generate predictions
with torch.no_grad():
outputs = model(input_tensor)
logits = outputs.logits# Transform logits into probabilities and extract the predicted label
probs = torch.nn.functional.softmax(logits, dim=-1)
predicted_label = torch.argmax(probs).item()

# Display the predicted label and associated probabilities
print(“Predicted Label:”, predicted_label)
print(“Probabilities:”, probs)


  • Assuming new text data is available, we first tokenize the text with the BERT tokenizer and then encode it into input IDs.
  • Now we adjust the tensor’s dimensions after converting the input IDs to it.
  • After that, we can enter the evaluation mode on the model.
  • The predictions are then produced by running the input tensor through the model without computing the gradient.
  • After which, The predicted label with the highest probability is extracted after computing softmax probabilities from the logits.
  • Then, the probability and the expected label are printed.

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We set out on a mission to implement our very own BERT model in this extensive guide. BERT is a fantastic model pioneered by the Google (parent company Alphabet) which we utilized in this guide. We started by comprehending the basic ideas behind BERT, then we dove into the complex world of data preprocessing, painstakingly built the BERT model from scratch, and last we understood the essence of fine-tuning for task-specific greatness. Finally, we used our trained model’s power to make precise predictions. With a solid understanding of each stage and a thorough investigation of the code samples, you are now prepared to start your own NLP projects and use BERT’s transforming powers to decode the nuanced intricacies of human language. So, now you can easily go forward with ease and let the power of BERT help your language research thrive.

Frequently Asked Questions (FAQs)

Q) Are there different versions of BERT available? Which is the right one for me?

Ans: Yes, there are other BERT model variations, including “bert-base,” “bert-large,” and others. The decision is based on the task’s complexity and computational capabilities. Smaller variations are easier to perfect but may have trouble comprehending complex linguistic systems.

Q) Can I integrate BERT into my existing machine-learning pipeline?

Ans: Yes, just like any other machine learning model, BERT can easily be incorporated into your current pipeline by refining it on your task-specific data and then using the trained model for inference.

About Om Thakur

Om Thakur is a proficient content writer at Selectyourdeals. An aspiring author, he has been writing content for as long as he can remember. Apart from being keen about writing and literature, he is also passionate about technological developments, and is currently pursuing BTech in Computer Science.

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