Learn Data Science using Python Step by Step

Here is how you can learn Python step by step. Please feel free to reach out to me on my personal email id rpdatascience@gmail.com if you have any question or comments related to any topics.

  1. Setup Python environment
  2. How to start jupyter notebook
  3. Install and check Packages
  4. Arithmetic operations
  5. Comparison or logical operations
  6. Assignment and augmented assignment in Python
  7. Variables naming conventions
  8. Types of variables in Python and typecasting
  9. Python Functions
  10. Exception handling in Python
  11. String manipulation and indexing
  12. Conditional and loops in Python
  13. Python data structure and containers
  14. Introduction to Python Numpy
  15. Introduction to Python SciPy
  16. Introduction to Python Pandas
  17. Python pivot tables
  18. Pandas join tables
  19. Missing value treatment
  20. Dummy coding of categorical variables 
  21. Basic statistics and visualization
  22. Data standardization or normalization
  23. Linear Regression with scikit- learn (Machine Learning library)
  24. Lasso, Ridge and Elasticnet Regularization in GLM
  25. Logistic Regression with scikit- learn (Machine Learning library)
  26. Hierarchical clustering with Python
  27. K-means clustering with Scikit Python
  28. Decision trees using Scikit Python
  29. Principal Component Analysis (PCA) using Scikit Python- Dimension Reduction
  30. Linear Discriminant Analysis (LDA) using Scikit Python- Dimension Reduction and Classification
  31. Market Basket Analysis or Association Rules or Affinity Analysis or Apriori Algorithm
  32. Recommendation Engines using Scikit-Surprise
  33. Price Elasticity of Demand using Log-Log Ordinary Least Square (OLS) Model
  34. Timeseries Forecasting using Facebook Prophet Package
  35. Model Persistence and Productionalization Using Python Pickle
  36. Deep Learning- Introduction to deep learning and environment setup
  37. Deep Learning- Multilayer perceptron (MLP) in Python
  38. Deep Learning- Convolution Neural Network (CNN) in Python
  39. Other topics (coming soon)

Cheers!

Analytical Problem Solving- Types of Reasoning

To solve any problem we need to have some way of breaking down the problem. There are two main ways of reasoning to that effect-

  • Deductive Reasoning–  This is also called as “Top Down” approach or “Formal Logic” approach. The key here is to form hypotheses to explain a certain phenomenon and then go to reject or accept related hypotheses. The conclusions and recommendation coming out from this sort of reasoning are more certain and factual in nature.
    • For example, let’s say you are trying to explain why a certain car gives lower miles per gallon. Because you know the business and have more context on this problem, you can start with potential hypotheses-
      • Weight of the car is high
      • Car has higher number or cylinders
      • Car has higher horse power
      •  and so on…

You will check each of the above hypotheses and reach to a definite conclusion.

  • Inductive Reasoning– On the other hand, this is a “Bottom Up” approach or “Informal Logic” approach. This sort of reasoning is more exploratory in nature. The end goal is to form some hypotheses to give possible reasons to explain certain phenomenon.
    • For example, let’s say you are trying to explain why sales of an eCommerce company has gone down in a particular quarter. You may begin by an exploratory analysis where you can begin with potential driver factors such as-
      • Marketing spend of the company
      • Pricing
      • Competitive landscape
      •  Macro economic factors

You will do data analysis to correlate each of the above factors to the sales and find potential reasons or build potential hypotheses to be tested further.

Cheers!

 

Lasso, Ridge and Elastic Net Regularization

Regularization techniques in Generalized Linear Models (GLM) are used during a modeling process for many reasons. A regularization technique helps in the following main ways-

  1. Doesn’t assume any particular distribution of the dependent variable ( DV). The DV can follow any distribution such as normal, binomial, possison etc. Hence the name Generalized Linear Models (GLMs)
  2. Address Variance-Bias Tradeoffs. Generally will lower the variance from the model
  3. More robust to handle multicollinearity
  4. Better sparse data (observations < features) handling
  5. Natural feature selection
  6. More accurate prediction on new data as it minimizes overfitting on the train data
  7. Easier interpretation of the output

And so on…

What is a regularization technique you may ask? A regularization technique is in simple terms a penalty mechanism which applies shrinkage (driving them closer to zero) of coefficient to build a more robust and parsimonious model. Although there are many ways to regularize a model, few of the common ones are-

  1. L1 Regularization aka Lasso Regularization– This add regularization terms in the model which are function of absolute value of the coefficients of parameters. The coefficient of the paratmeters can be driven to zero as well during the regularization process. Hence this technique can be used for feature selection and generating more parsimonious model
  2. L2 Regularization aka Ridge Regularization – This add regularization terms in the model which are function of square of coefficients of parameters. Coefficient of parameters can approach to zero but never become zero.
  3. Combination of the above two such as Elastic Nets– This add regularization terms in the model which are combination of both L1 and L2 regularization.

For more on the regularization techniques you can visit this paper.

Scikit help on Lasso Regression

Here is a working example code on the Boston Housing data. Please note, generally before doing regularized GLM regression it is advised to scale variables. However, in the below example we are working with the variables on the original scale to demonstrate each algorithms working.

lasso1lasso2lasso3lasso4lasso5lasso6lasso7lasso8lasso9lasso10lasso11lasso12lasso13

Cheers!

Model Persistence Using Python Pickle

After you have built a machine learning model which is doing a great job in prediction, you don’t have to retrain your model again and again for future usage. Instead, you can use Python pickle serialization for reusing this model in future and transferring it into a production environment where non modelers can also use this model to make predictions.

 

512px-Pickle

By Renee Comet (photographer) [Public domain], via Wikimedia Commons

First let’s look at how Wikipedia defines a pickle

Pickling is the process of preserving or expanding the lifespan of food by either anaerobic fermentation in brine or immersion in vinegar. The resulting food is called a pickle.

Python pickling is the same process without brine or vinegar, whereas you will pickle your model for longer usage without the need for you to recook your models. In a “Pickling” process a Python object is converted into a byte stream. On the other hand, in an “Unpickling” process a byte stream is converted back into Python object.

I strongly recommend that you read Python Official Documentation on this topic before moving forward.

Now let’s see this live in action. We will first look at a simple example and then look at a model example.

Example 1- In this we will pickle and un-pickle a simple Python list

pickle1

Example 2- In this we will pickle and un-pickle a Decision Tree classifier and use it later for making predictions on a new data

pickle2pickle3

For more details, do check out this excellent presentation.

Cheers!

 

Recurrent Neural Network (RNN) in Python

Recurrent Neural Network (RNN) are a special type of feed-forward network used for sequential data analysis where inputs are not independent and are not of fixed length as is assumed in some of the other neural networks such as MLP.  Rather in this case, inputs are dependent on each other along the time dimension. In other words, what happens in time ‘t’ may depend on what happened in time ‘t-1’, ‘t-2’ and so on.

These are also called ‘memory’ networks as previous inputs and states persist in the model for doing a more optimal sequential analysis. They can have both short term and long term time dependence. Due to their capabilities of handling sequential data very well, these networks are typically very suitable for speech recognition, sentiment analysis, forecasting, language translation and other such applications.

Let’s now spend sometime looking at how a RNN work-

Recurrent Neural Network (RNN)

Recurrent Neural Network (RNN)

As you may recall, in a typical feed-forward neural network input is fed at beginning and then hidden layers do the processing and finally output layer spits out the output. On the other hand, in a RNN generally speaking we will have different input, output and cost function for each time stamp. However, the same weight matrix is fed to all layers in the network.

One point to note is that RNNs are also trained using backward propagation of errors and gradient descent to minimize cost function. However, backward propagation in RNN happen over different time stamps and hence it’s called Backward Propagation Through Time (BPTT). In a typical RNN, we may have several time stamp layers which sometimes may range in hundreds or thousands and therein lies the problem of vanishing gradient or exploding gradient that these pure vanilla RNNs are particularly susceptible for.

There are various techniques such as gradient clipping and architecture such as Long Short Term Memory (LSTM) or Gated Recurrent Unit (GRU) which help in fixing the vanishing gradient and exploding gradient issues. We will delve deeper into how an LSTM work.

A LSTM network consist of hidden layers that have many LSTM blocks or units. In turn each LSTM unit will have the following components-

  • Memory Cell- The component that remembers the values over a period of time. This has an activation function
  • Input gate- Enables addition of info to the memory cell. Generally has as an tanh activation to squash the values between -1 and +1
  • Forget gate- Enables removing or retaining from the memory cell. This will generally have a sigmoid activation function and hence the output values will range between 0 and 1. If the gate is on, then all memories are retained. If the gate is turned-off, all values will be removed.
  • Output gate- Retrieve information from the memory cell passed through the tanh activation
Long Short Term Memory Cell or Block

Long Short Term Memory Cell or Block (Source- Wiki)

Let’s work through an example which we used in a previous article.

lstm1lstm2lstm3lstm4lstm5lstm6lstm7lstm8

lstm9

Here is an excellent article in case you want to explore more.

Cheers!

 

Ensemble Modeling using Python

Ensemble models are a great tool to fix the variance-bias trade-off which a typical machine learning model faces, i.e. when you try to lower bias, variance will go higher and vice-versa. This generally results in higher error rates.

Total Error in Model = Bias + Variance + Random Noise

Variance and Bias Trade-off

Variance and Bias Trade-off

Ensemble models typically combine several weak learners to build a stronger model, which will reduce variance and bias at the same time. Since ensemble models follow a community learning or divide and conquer approach, output from ensemble models will be wrong only when the majority of underlying learners are wrong.

One of the biggest flip side of ensemble models is that they may become “Black Box” and not very explainable as opposed a simple machine learning model. However, the gains in model performances generally outweigh any loss in transparency. That is the reason why you will see top performing models in many high ranking competitions will be generally an ensemble model.

Ensemble models can be broken down into the following three main categories-

  1. Bagging
  2. Boosting
  3. Stacking

Let’s look at each one of them-

Bagging-

  • One good example of such model is Random Forest
  • These types of ensemble models work on reducing the variance by removing instability in the underlying complex models
  • Each learner is asked to do the classification or regression independently and in parallel and then either a voting or averaging of the output of all the learners is done to create the final output
  • Since these ensemble models are predominantly focuses on reducing the variance, the underlying models are fairly complex ( such as Decision Tree or Neural Network) to begin with low bias
  • An underlying decision tree will have higher depth and many branches. In other words, the tree will be deep and dense and with lower bias

Boosting-

  • Some good examples of these types of models are Gradient Boosting Tree, Adaboost, XGboost among others.
  • These ensemble models work with weak learners and try to improve the bias and variance simultaneously by working sequentially.
  •  These are also called adaptive learners, as learning of one learner is dependent on how other learners are performing. For example, if a certain set of the data has higher mis-classification rate, this sample’s weight in the overall learning will be increased so that the other learners focus more on correctly classifying the tougher samples.
  • An underlying decision tree will be shallow and a weak learner with higher bias

There are various approaches for building a bagging model such as- pasting, bagging, random subspaces, random patches etc. You can find all details over here.

Stacking-

  • These meta learning models are what the name suggest. They are stacked models. Or in other words, a particular learner’s output will become an input to another model and so on.

Working examples-

First install xgboost via conda install-

Step1 – search packages  using “anaconda search -t conda xgboost”

Step 2- install a particular package such as ” conda install py-xgboost”

Coming soon….

Cheers!

 

Deep Learning- Convolution Neural Network (CNN) in Python

Convolution Neural Network (CNN) are particularly useful for spatial data analysis, image recognition, computer vision, natural language processing, signal processing and variety of other different purposes. They are biologically motivated by functioning of neurons in visual cortex to a visual stimuli.

What makes CNN much more powerful compared to the other feedback forward networks for image recognition is the fact that they do not require as much human intervention and parameters as some of the other networks such as MLP do. This is primarily driven by the fact that CNNs have neurons arranged in three dimensions.

CNNs make all of this magic happen by taking a set of input and passing it on to one or more of following main hidden layers in a network to generate an output.

  • Convolution Layers
  • Pooling Layers
  • Fully Connected Layers

Click here to see a live demo of a CNN

Let’s dig deeper into utility of each of the above layers.

Convolution Layers– Before we move this discussion any further, let’s remember that any image or similar object can be represented as a matrix of numbers ranging between 0-255. Size of this matrix will be determined by the size the image in the following fashion-

Height X Width X Channels

Channels =1 for grey-scale images

Channels =3 for colored images

For example, if we feed an image which is 28 by 28 square in pixels and on the grey scale. This image will be a matrix of numbers in the below fashion-

28*28*1. Each of the 784 pixels can any values between 0-255 depending on the intensity of grey-scale.

Now let’s talk about what happens in a convolution layer. The main objective of this layer is to derive features of an image by sliding smaller matrix called kernel or filter over the entire image through convolution.

What is convolution? Convolution is taking a dot product between the filter and the local regions

Kernels can be many types such as edge detection, blob of color, sharpening, blurring etc. You can find some main kernels over here.  Please note that we can specify the number of filters during the network training process, however network will learn the filters on its own.

As a result of this convolution layers, the network creates numbers of features maps. The size of feature maps depends on the # of filters (kernels), size of filters, padding (zero padding to preserve size), and strides (steps by which a filter scans the original image). Please note that a non linear activation function such Relu or Tanh is applied at each convolution layer to generate modified feature maps.

giphy.gif

Source: https://cs.nyu.edu/~fergus/tutorials/deep_learning_cvpr12/

Pooling Layer– The arrays generated from the convolution layers are generally very big and hence pooling layer is used predominantly to reduce the feature maps and retain the most important aspect.  In other words this facilitate “Downsampling” using algorithms such as max pooling or average pooling etc. Moreover, as the numbers of parameters in the network are truncated, this layer also helps in avoiding over fitting.  It is common to have pooling layers in between different convolution layers.

Fully Connected Layer– This enables every neuron in the layers to be interconnected to the neurons from the previous and next layer to take the matrix inputs from the previous layers and flatten it to pass on to the output layer. Which in turn will make prediction such as classification probability.

Here is an excellent write-up which provides further details on all of the above steps.

Since we know enough about how a CNN works, let’s code now-

In this example, we will be working with MNIST dataset and build a CNN to recognize handwritten digits from 0-9. We will be using classification accuracy as a metric to evaluate the model’s performance. Please see link for MNIST CNN working

Please note that CNN need very high amount of computational power and memory and hence it’s recommended that you run this in GPUs or Cloud. CPUs may not be able to fit the model.  Furthermore, you may need to reduce batch size to a lower level to ensure algorithm runs successfully.

cnn1cnn2cnn3cnn4cnn5cnn6cnn7cnn8

cnn11cnn12cnn13

As you can see, the above model gives 99%+ accuracy in the classification.

Cheers!

Time-series Forecasting Using Facebook Prophet Package

Forecasting is a technique that is used for a variety of different purposes and situations such as sales forecasting, operational and budget planning etc. Similarly there are a variety of different techniques such as moving averages, smoothening, ARIMA etc to statistically make a forecast.

In this article we will talk about an open source package called “Prophet” from Facebook which takes away the complexity of other techniques without compromising on accuracy of the forecast. The guiding principle of this approach is General Additive Models (GAMs). More on which can be found over here.

Let’s look at an example of how to deploy Prophet in Python.

 

ts1ts2ts3ts4ts5ts6ts7ts8
Cheers!

Fundamentals of Deep Learning and Artificial Intelligence

Here are some good links that ought to give you a broader context of what is machine learning, deep learning, artificial intelligence etc.

Is machine learning learning same as deep learning?

HBR article explaining what is machine learning and deep learning

Opportunities and challenges in AI

Introduction to neural network and deep learning

Setup deep learning environment in Python

Deep learning with Keras

Free book

Tensorflow Playground

Cheers!

How to Start Jupyter Notebook From Anaconda Prompt

Jupyter Notebook can be started using many ways, most common ones are-

  1. From the Windows or Mac search interface. Type “Jupyter Notebook” and it should show you to application to start
  2. From Anaconda prompt by typing “jupyter notebook” at the anaconda prompt
  3. For high graphics display such as with plotly package, you are advised to start the jupyter notebook using the following command- “jupyter notebook –NotebookApp.iopub_data_rate_limit=1e10”
Jupyter Notebook Start from Anaconda

Jupyter Notebook Start from Anaconda for High Resolution Graphics

otherwise you can get an error message similar to the one shown below-

IOPub data rate exceeded. The notebook server will temporarily stop sending output to the client in order to avoid crashing it. To change this limit, set the config variable `--NotebookApp.iopub_data_rate_limit`.

Python Error Message for High Graphics Images

Cheers!