Data Visualization is a big part of a data scientist’s jobs. In the early stages of a project, you’ll often be doing an Exploratory Data Analysis (EDA) to gain some insights into your data.

Data Visualization is a big part of a data scientist’s jobs. In the early stages of a project, you’ll often be doing an Exploratory Data Analysis (EDA) to gain some insights into your data.

Creating visualizations really helps make things clearer and easier to understand, especially with larger, high dimensional datasets. Towards the end of your project, it’s important to be able to present your final results in a clear, concise, and compelling manner that your audience, whom are often non-technical clients, can understand.

Matplotlib is a popular Python library that can be used to create your Data Visualizations quite easily. However, setting up the data, parameters, figures, and plotting can get quite messy and tedious to do every time you do a new project. In this blog post, we’re going to look at 5 data visualizations and write some quick and easy functions for them with Python’s Matplotlib. In the meantime, here’s a great chart for selecting the right visualization for the job!

Scatter plots are great for showing the relationship between two variables since you can directly see the raw distribution of the data. You can also view this relationship for different groups of data simple by colour coding the groups as seen in the first figure below. Want to visualize the relationship between three variables? No problemo! Just use another parameters, like point size, to encode that third variable as we can see in the second figure below. All of these points we just discussed also line right up with the first chart.

Now for the code. We first import Matplotlib’s pyplot with the alias “plt”. To create a new plot figure we call `plt.subplots()`

. We pass the x-axis and y-axis data to the function and then pass those to `ax.scatter()`

to plot the scatter plot. We can also set the point size, point color, and alpha transparency. You can even set the y-axis to have a logarithmic scale. The title and axis labels are then set specifically for the figure. That’s an easy to use function that creates a scatter plot end to end!

```
import matplotlib.pyplot as plt
import numpy as np
def scatterplot(x_data, y_data, x_label="", y_label="", title="", color = "r", yscale_log=False):
# Create the plot object
_, ax = plt.subplots()
# Plot the data, set the size (s), color and transparency (alpha)
# of the points
ax.scatter(x_data, y_data, s = 10, color = color, alpha = 0.75)
if yscale_log == True:
ax.set_yscale('log')
# Label the axes and provide a title
ax.set_title(title)
ax.set_xlabel(x_label)
ax.set_ylabel(y_label)
```

Line plots are best used when you can clearly see that one variable varies greatly with another i.e they have a high covariance. Lets take a look at the figure below to illustrate. We can clearly see that there is a large amount of variation in the percentages over time for all majors. Plotting these with a scatter plot would be extremely cluttered and quite messy, making it hard to really understand and see what’s going on. Line plots are perfect for this situation because they basically give us a quick summary of the covariance of the two variables (percentage and time). Again, we can also use grouping by colour encoding. Line charts fall into the “over-time” category from our first chart.

Here’s the code for the line plot. It’s quite similar to the scatter above. with just some minor variations in variables.

```
def lineplot(x_data, y_data, x_label="", y_label="", title=""):
# Create the plot object
_, ax = plt.subplots()
# Plot the best fit line, set the linewidth (lw), color and
# transparency (alpha) of the line
ax.plot(x_data, y_data, lw = 2, color = '#539caf', alpha = 1)
# Label the axes and provide a title
ax.set_title(title)
ax.set_xlabel(x_label)
ax.set_ylabel(y_label)
```

Histograms are useful for viewing (or really discovering)the distribution of data points. Check out the histogram below where we plot the frequency vs IQ histogram. We can clearly see the concentration towards the center and what the median is. We can also see that it follows a Gaussian distribution. Using the bars (rather than scatter points, for example) really gives us a clearly visualization of the relative difference between the frequency of each bin. The use of bins (discretization) really helps us see the “bigger picture” where as if we use all of the data points without discrete bins, there would probably be a lot of noise in the visualization, making it hard to see what is really going on.

The code for the histogram in Matplotlib is shown below. There are two parameters to take note of. Firstly, the `n_bins`

parameters controls how many discrete bins we want for our histogram. More bins will give us finer information but may also introduce noise and take us away from the bigger picture; on the other hand, less bins gives us a more “birds eye view” and a bigger picture of what’s going on without the finer details. Secondly, the `cumulative`

parameter is a boolean which allows us to select whether our histogram is cumulative or not. This is basically selecting either the Probability Density Function (PDF) or the Cumulative Density Function (CDF).

```
def histogram(data, n_bins, cumulative=False, x_label = "", y_label = "", title = ""):
_, ax = plt.subplots()
ax.hist(data, n_bins = n_bins, cumulative = cumulative, color = '#539caf')
ax.set_ylabel(y_label)
ax.set_xlabel(x_label)
ax.set_title(title)
```

Imagine we want to compare the distribution of two variables in our data. One might think that you’d have to make two separate histograms and put them side-by-side to compare them. But, there’s actually a better way: we can overlay the histograms with varying transparency. Check out the figure below. The Uniform distribution is set to have a transparency of 0.5 so that we can see what’s behind it. This allows use to directly view the two distributions on the same figure.

There are a few things to set up in code for the overlaid histograms. First, we set the horizontal range to accommodate both variable distributions. According to this range and the desired number of bins we can actually computer the width of each bin. Finally, we plot the two histograms on the same plot, with one of them being slightly more transparent.

```
# Overlay 2 histograms to compare them
def overlaid_histogram(data1, data2, n_bins = 0, data1_name="", data1_color="#539caf", data2_name="", data2_color="#7663b0", x_label="", y_label="", title=""):
# Set the bounds for the bins so that the two distributions are fairly compared
max_nbins = 10
data_range = [min(min(data1), min(data2)), max(max(data1), max(data2))]
binwidth = (data_range[1] - data_range[0]) / max_nbins
if n_bins == 0
bins = np.arange(data_range[0], data_range[1] + binwidth, binwidth)
else:
bins = n_bins
# Create the plot
_, ax = plt.subplots()
ax.hist(data1, bins = bins, color = data1_color, alpha = 1, label = data1_name)
ax.hist(data2, bins = bins, color = data2_color, alpha = 0.75, label = data2_name)
ax.set_ylabel(y_label)
ax.set_xlabel(x_label)
ax.set_title(title)
ax.legend(loc = 'best')
```

Bar plots are most effective when you are trying to visualize categorical data that has few (probably < 10) categories. If we have too many categories then the bars will be very cluttered in the figure and hard to understand. They’re nice for categorical data because you can easily see the difference between the categories based on the size of the bar (i.e magnitude); categories are also easily divided and colour coded too. There are 3 different types of bar plots we’re going to look at: regular, grouped, and stacked. Check out the code below the figures as we go along.

The regular barplot is in the first figure below. In the `barplot()`

function, `x_data`

represents the tickers on the x-axis and `y_data`

represents the bar height on the y-axis. The error bar is an extra line centered on each bar that can be drawn to show the standard deviation.

Grouped bar plots allow us to compare multiple categorical variables. Check out the second bar plot below. The first variable we are comparing is how the scores vary by group (groups G1, G2, … etc). We are also comparing the genders themselves with the colour codes. Taking a look at the code, the `y_data_list`

variable is now actually a list of lists, where each sublist represents a different group. We then loop through each group, and for each group we draw the bar for each tick on the x-axis; each group is also colour coded.

Stacked bar plots are great for visualizing the categorical make-up of different variables. In the stacked bar plot figure below we are comparing the server load from day-to-day. With the colour coded stacks, we can easily see and understand which servers are worked the most on each day and how the loads compare to the other servers on all days. The code for this follows the same style as the grouped bar plot. We loop through each group, except this time we draw the new bars on top of the old ones rather than beside them.

```
def barplot(x_data, y_data, error_data, x_label="", y_label="", title=""):
_, ax = plt.subplots()
# Draw bars, position them in the center of the tick mark on the x-axis
ax.bar(x_data, y_data, color = '#539caf', align = 'center')
# Draw error bars to show standard deviation, set ls to 'none'
# to remove line between points
ax.errorbar(x_data, y_data, yerr = error_data, color = '#297083', ls = 'none', lw = 2, capthick = 2)
ax.set_ylabel(y_label)
ax.set_xlabel(x_label)
ax.set_title(title)
def stackedbarplot(x_data, y_data_list, colors, y_data_names="", x_label="", y_label="", title=""):
_, ax = plt.subplots()
# Draw bars, one category at a time
for i in range(0, len(y_data_list)):
if i == 0:
ax.bar(x_data, y_data_list[i], color = colors[i], align = 'center', label = y_data_names[i])
else:
# For each category after the first, the bottom of the
# bar will be the top of the last category
ax.bar(x_data, y_data_list[i], color = colors[i], bottom = y_data_list[i - 1], align = 'center', label = y_data_names[i])
ax.set_ylabel(y_label)
ax.set_xlabel(x_label)
ax.set_title(title)
ax.legend(loc = 'upper right')
def groupedbarplot(x_data, y_data_list, colors, y_data_names="", x_label="", y_label="", title=""):
_, ax = plt.subplots()
# Total width for all bars at one x location
total_width = 0.8
# Width of each individual bar
ind_width = total_width / len(y_data_list)
# This centers each cluster of bars about the x tick mark
alteration = np.arange(-(total_width/2), total_width/2, ind_width)
# Draw bars, one category at a time
for i in range(0, len(y_data_list)):
# Move the bar to the right on the x-axis so it doesn't
# overlap with previously drawn ones
ax.bar(x_data + alteration[i], y_data_list[i], color = colors[i], label = y_data_names[i], width = ind_width)
ax.set_ylabel(y_label)
ax.set_xlabel(x_label)
ax.set_title(title)
ax.legend(loc = 'upper right')
```

We previously looked at histograms which were great for visualizing the distribution of variables. But what if we need more information than that? Perhaps we want a clearer view of the standard deviation? Perhaps the median is quite different from the mean and thus we have many outliers? What if there is so skew and many of the values are concentrated to one side?

That’s where boxplots come in. Box plots give us all of the information above. The bottom and top of the solid-lined box are always the first and third quartiles (i.e 25% and 75% of the data), and the band inside the box is always the second quartile (the median). The whiskers (i.e the dashed lines with the bars on the end) extend from the box to show the range of the data.

Since the box plot is drawn for each group/variable it’s quite easy to set up. The `x_data`

is a list of the groups/variables. The Matplotlib function `boxplot()`

makes a box plot for each column of the `y_data`

or each vector in sequence `y_data`

; thus each value in `x_data`

corresponds to a column/vector in `y_data`

. All we have to set then are the aesthetics of the plot.

```
def boxplot(x_data, y_data, base_color="#539caf", median_color="#297083", x_label="", y_label="", title=""):
_, ax = plt.subplots()
# Draw boxplots, specifying desired style
ax.boxplot(y_data
# patch_artist must be True to control box fill
, patch_artist = True
# Properties of median line
, medianprops = {'color': median_color}
# Properties of box
, boxprops = {'color': base_color, 'facecolor': base_color}
# Properties of whiskers
, whiskerprops = {'color': base_color}
# Properties of whisker caps
, capprops = {'color': base_color})
# By default, the tick label starts at 1 and increments by 1 for
# each box drawn. This sets the labels to the ones we want
ax.set_xticklabels(x_data)
ax.set_ylabel(y_label)
ax.set_xlabel(x_label)
ax.set_title(title)
```

There are your 5 quick and easy data visualizations using Matplotlib. Abstracting things into functions always makes your code easier to read and use! I hope you enjoyed this post and learned something new and useful.

An overview of using Python for data science including Numpy, Scipy, pandas, Scikit-Learn, XGBoost, TensorFlow and Keras.

An overview of using Python for data science including Numpy, Scipy, pandas, Scikit-Learn, XGBoost, TensorFlow and Keras.

So you’ve heard of data science and you’ve heard of Python.

You want to explore both but have no idea where to start — data science is pretty complicated, after all.

Don’t worry — Python is one of the easiest programming languages to learn. And thanks to the hard work of thousands of open source contributors, **you** can do data science, too.

If you look at the contents of this article, you may think there’s a lot to master, but this article has been designed to gently increase the difficulty as we go along.

One article obviously can’t teach you everything you need to know about data science with python, but once you’ve followed along you’ll know exactly where to look to take the next steps in your data science journey.

Table contents:

- Why Python?
- Installing Python
- Using Python for Data Science
- Numeric computation in Python
- Statistical analysis in Python
- Data manipulation in Python
- Working with databases in Python
- Data engineering in Python
- Big data engineering in Python
- Further statistics in Python
- Machine learning in Python
- Deep learning in Python
- Data science APIs in Python
- Applications in Python
- Summary

Python, as a language, has a lot of features that make it an excellent choice for data science projects.

It’s easy to learn, simple to install (in fact, if you use a Mac you probably already have it installed), and it has a lot of extensions that make it great for doing data science.

Just because Python is easy to learn doesn’t mean its a toy programming language — huge companies like Google use Python for their data science projects, too. They even contribute packages back to the community, so you can use the same tools in your projects!

You can use Python to do way more than just data science — you can write helpful scripts, build APIs, build websites, and much much more. Learning it for data science means you can easily pick up all these other things as well.

There are a few important things to note about Python.

Right now, there are two versions of Python that are in common use. They are versions 2 and 3.

Most tutorials, and the rest of this article, will assume that you’re using the latest version of Python 3. It’s just good to be aware that sometimes you can come across books or articles that use Python 2.

The difference between the versions isn’t huge, but sometimes copying and pasting version 2 code when you’re running version 3 won’t work — you’ll have to do some light editing.

The second important thing to note is that Python really cares about whitespace (that’s spaces and return characters). If you put whitespace in the wrong place, your programme will very likely throw an error.

There are tools out there to help you avoid doing this, but with practice you’ll get the hang of it.

If you’ve come from programming in other languages, Python might feel like a bit of a relief: there’s no need to manage memory and the community is very supportive.

If Python is your first programming language you’ve made an excellent choice. I really hope you enjoy your time using it to build awesome things.

The best way to install Python for data science is to use the Anaconda distribution (you’ll notice a fair amount of snake-related words in the community).

It has everything you need to get started using Python for data science including a lot of the packages that we’ll be covering in the article.

If you click on Products -> Distribution and scroll down, you’ll see installers available for Mac, Windows and Linux.

Even if you have Python available on your Mac already, you should consider installing the Anaconda distribution as it makes installing other packages easier.

If you prefer to do things yourself, you can go to the official Python website and download an installer there.

Packages are pieces of Python code that aren’t a part of the language but are really helpful for doing certain tasks. We’ll be talking a lot about packages throughout this article so it’s important that we’re set up to use them.

Because the packages are just pieces of Python code, we could copy and paste the code and put it somewhere the Python interpreter (the thing that runs your code) can find it.

But that’s a hassle — it means that you’ll have to copy and paste stuff every time you start a new project or if the package gets updated.

To sidestep all of that, we’ll instead use a package manager.

If you chose to use the Anaconda distribution, congratulations — you already have a package manager installed. If you didn’t, I’d recommend installing pip.

No matter which one you choose, you’ll be able to use commands at the terminal (or command prompt) to install and update packages easily.

Now that you’ve got Python installed, you’re ready to start doing data science.

But how do you start?

Because Python caters to so many different requirements (web developers, data analysts, data scientists) there are lots of different ways to work with the language.

Python is an interpreted language which means that you don’t have to compile your code into an executable file, you can just pass text documents containing code to the interpreter!

Let’s take a quick look at the different ways you can interact with the Python interpreter.

If you open up the terminal (or command prompt) and type the word ‘python’, you’ll start a shell session. You can type any valid Python commands in there and they’d work just like you’d expect.

This can be a good way to quickly debug something but working in a terminal is difficult over the course of even a small project.

If you write a series of Python commands in a text file and save it with a .py extension, you can navigate to the file using the terminal and, by typing python YOUR_FILE_NAME.py, can run the programme.

This is essentially the same as typing the commands one-by-one into the terminal, it’s just much easier to fix mistakes and change what your program does.

An IDE is a professional-grade piece of software that helps you manage software projects.

One of the benefits of an IDE is that you can use debugging features which tell you where you’ve made a mistake before you try to run your programme.

Some IDEs come with project templates (for specific tasks) that you can use to set your project out according to best practices.

None of these ways are the best for doing data science with python — that particular honour belongs to Jupyter notebooks.

Jupyter notebooks give you the capability to run your code one ‘block’ at a time, meaning that you can see the output before you decide what to do next — that’s really crucial in data science projects where we often need to see charts before taking the next step.

If you’re using Anaconda, you’ll already have Jupyter lab installed. To start it you’ll just need to type ‘jupyter lab’ into the terminal.

If you’re using pip, you’ll have to install Jupyter lab with the command ‘python pip install jupyter’.

It probably won’t surprise you to learn that data science is mostly about numbers.

The NumPy package includes lots of helpful functions for performing the kind of mathematical operations you’ll need to do data science work.

It comes installed as part of the Anaconda distribution, and installing it with pip is just as easy as installing Jupyter notebooks (‘pip install numpy’).

The most common mathematical operations we’ll need to do in data science are things like matrix multiplication, computing the dot product of vectors, changing the data types of arrays and creating the arrays in the first place!

Here’s how you can make a list into a NumPy array:

Here’s how you can do array multiplication and calculate dot products in NumPy:

And here’s how you can do matrix multiplication in NumPy:

With mathematics out of the way, we must move forward to statistics.

The Scipy package contains a module (a subsection of a package’s code) specifically for statistics.

You can import it (make its functions available in your programme) into your notebook using the command ‘from scipy import stats’.

This package contains everything you’ll need to calculate statistical measurements on your data, perform statistical tests, calculate correlations, summarise your data and investigate various probability distributions.

Here’s how to quickly access summary statistics (minimum, maximum, mean, variance, skew, and kurtosis) of an array using Scipy:

Data scientists have to spend an unfortunate amount of time cleaning and wrangling data. Luckily, the Pandas package helps us do this with code rather than by hand.

The most common tasks that I use Pandas for are reading data from CSV files and databases.

It also has a powerful syntax for combining different datasets together (datasets are called DataFrames in Pandas) and performing data manipulation.

You can see the first few rows of a DataFrame using the .head method:

You can select just one column using square brackets:

And you can create new columns by combining others:

In order to use the pandas read_sql method, you’ll have to establish a connection to a database.

The most bulletproof method of connecting to a database is by using the SQLAlchemy package for Python.

Because SQL is a language of its own and connecting to a database depends on which database you’re using, I’ll leave you to read the documentation if you’re interested in learning more.

Sometimes we’d prefer to do some calculations on our data before they arrive in our projects as a Pandas DataFrame.

If you’re working with databases or scraping data from the web (and storing it somewhere), this process of moving data and transforming it is called ETL (Extract, transform, load).

You extract the data from one place, do some transformations to it (summarise the data by adding it up, finding the mean, changing data types, and so on) and then load it to a place where you can access it.

There’s a really cool tool called Airflow which is very good at helping you manage ETL workflows. Even better, it’s written in Python.

It was developed by Airbnb when they had to move incredible amounts of data around, you can find out more about it here.

Sometimes ETL processes can be really slow. If you have billions of rows of data (or if they’re a strange data type like text), you can recruit lots of different computers to work on the transformation separately and pull everything back together at the last second.

This architecture pattern is called MapReduce and it was made popular by Hadoop.

Nowadays, lots of people use Spark to do this kind of data transformation / retrieval work and there’s a Python interface to Spark called (surprise, surprise) PySpark.

Both the MapReduce architecture and Spark are very complex tools, so I’m not going to go into detail here. Just know that they exist and that if you find yourself dealing with a very slow ETL process, PySpark might help. Here’s a link to the official site.

We already know that we can run statistical tests, calculate descriptive statistics, p-values, and things like skew and kurtosis using the stats module from Scipy, but what else can Python do with statistics?

One particular package that I think you should know about is the lifelines package.

Using the lifelines package, you can calculate a variety of functions from a subfield of statistics called survival analysis.

Survival analysis has a lot of applications. I’ve used it to predict churn (when a customer will cancel a subscription) and when a retail store might be burglarised.

These are totally different to the applications the creators of the package imagined it would be used for (survival analysis is traditionally a medical statistics tool). But that just shows how many different ways there are to frame data science problems!

The documentation for the package is really good, check it out here.

Machine Learning in PythonNow this is a major topic — machine learning is taking the world by storm and is a crucial part of a data scientist’s work.

Simply put, machine learning is a set of techniques that allows a computer to map input data to output data. There are a few instances where this isn’t the case but they’re in the minority and it’s generally helpful to think of ML this way.

There are two really good machine learning packages for Python, let’s talk about them both.

Most of the time you spend doing machine learning in Python will be spent using the Scikit-Learn package (sometimes abbreviated sklearn).

This package implements a whole heap of machine learning algorithms and exposes them all through a consistent syntax. This makes it really easy for data scientists to take full advantage of every algorithm.

The general framework for using Scikit-Learn goes something like this –

You split your dataset into train and test datasets:

Then you instantiate and train a model:

And then you use the metrics module to test how well your model works:

The second package that is commonly used for machine learning in Python is XGBoost.

Where Scikit-Learn implements a whole range of algorithms XGBoost only implements a single one — gradient boosted decision trees.

This package (and algorithm) has become very popular recently due to its success at Kaggle competitions (online data science competitions that anyone can participate in).

Training the model works in much the same way as a Scikit-Learn algorithm.

Deep Learning in PythonThe machine learning algorithms available in Scikit-Learn are sufficient for nearly any problem. That being said, sometimes you need to use the most advanced thing available.

Deep neural networks have skyrocketed in popularity due to the fact that systems using them have outperformed nearly every other class of algorithm.

There’s a problem though — it’s very hard to say what a neural net is doing and why it’s making the decisions that it is. Because of this, their use in finance, medicine, the law and related professions isn’t widely endorsed.

The two major classes of neural network are convolutional neural networks (which are used to classify images and complete a host of other tasks in computer vision) and recurrent neural nets (which are used to understand and generate text).

Exploring how neural nets work is outside the scope of this article, but just know that the packages you’ll need to look for if you want to do this kind of work are TensorFlow (a Google contibution!) and Keras.

Keras is essentially a wrapper for TensorFlow that makes it easier to work with.

Once you’ve trained a model, you’d like to be able to access predictions from it in other software. The way you do this is by creating an API.

An API allows your model to receive data one row at a time from an external source and return a prediction.

Because Python is a general purpose programming language that can also be used to create web services, it’s easy to use Python to serve your model via API.

If you need to build an API you should look into the pickle and Flask. Pickle allows you to save trained models on your hard-drive so that you can use them later. And Flask is the simplest way to create web services.

Finally, if you’d like to build a full-featured web application around your data science project, you should use the Django framework.

Django is immensely popular in the web development community and was used to build the first version of Instagram and Pinterest (among many others).

And with that we’ve concluded our whirlwind tour of data science with Python.

We’ve covered everything you’d need to learn to become a full-fledged data scientist. If it still seems intimidating, you should know that nobody knows all of this stuff and that even the best of us still Google the basics from time to time.

Learn Data Science | How to Learn Data Science for Free. In this post, I have described a learning path and free online courses and tutorials that will enable you to learn data science for free.

The average cost of obtaining a masters degree at traditional bricks and mortar institutions will set you back anywhere between $30,000 and $120,000. Even online data science degree programs don’t come cheap costing a minimum of $9,000. So what do you do if you want to learn data science but can’t afford to pay this?

I trained into a career as a data scientist without taking any formal education in the subject. In this article, I am going to share with you my own personal curriculum for learning data science if you can’t or don’t want to pay thousands of dollars for more formal study.

The curriculum will consist of 3 main parts, technical skills, theory and practical experience. I will include links to free resources for every element of the learning path and will also be including some links to additional ‘low cost’ options. So if you want to spend a little money to accelerate your learning you can add these resources to the curriculum. I will include the estimated costs for each of these.

The first part of the curriculum will focus on technical skills. I recommend learning these first so that you can take a practical first approach rather than say learning the mathematical theory first. Python is by far the most widely used programming language used for data science. In the Kaggle Machine Learning and Data Science survey carried out in 2018 83% of respondents said that they used Python on a daily basis. I would, therefore, recommend focusing on this language but also spending a little time on other languages such as R.

Before you can start to use Python for data science you need a basic grasp of the fundamentals behind the language. So you will want to take a Python introductory course. There are lots of free ones out there but I like the Codeacademy ones best as they include hands-on in-browser coding throughout.

I would suggest taking the introductory course to learn Python. This covers basic syntax, functions, control flow, loops, modules and classes.

Next, you will want to get a good understanding of using Python for data analysis. There are a number of good resources for this.

To start with I suggest taking at least the free parts of the data analyst learning path on dataquest.io. Dataquest offers complete learning paths for data analyst, data scientist and data engineer. Quite a lot of the content, particularly on the data analyst path is available for free. If you do have some money to put towards learning then I strongly suggest putting it towards paying for a few months of the premium subscription. I took this course and it provided a fantastic grounding in the fundamentals of data science. It took me 6 months to complete the data scientist path. The price varies from $24.50 to $49 per month depending on whether you pay annually or not. It is better value to purchase the annual subscription if you can afford it.

If you have chosen to pay for the full data science course on Dataquest then you will have a good grasp of the fundamentals of machine learning with Python. If not then there are plenty of other free resources. I would focus to start with on scikit-learn which is by far the most commonly used Python library for machine learning.

When I was learning I was lucky enough to attend a two-day workshop run by Andreas Mueller one of the core developers of scikit-learn. He has however published all the material from this course, and others, on this Github repo. These consist of slides, course notes and notebooks that you can work through. I would definitely recommend working through this material.

Then I would suggest taking some of the tutorials in the scikit-learn documentation. After that, I would suggest building some practical machine learning applications and learning the theory behind how the models work — which I will cover a bit later on.

**SQL**

SQL is a vital skill to learn if you want to become a data scientist as one of the fundamental processes in data modelling is extracting data in the first place. This will more often than not involve running SQL queries against a database. Again if you haven’t opted to take the full Dataquest course then here are a few free resources to learn this skill.

Codeacamdemy has a free introduction to SQL course. Again this is very practical with in-browser coding all the way through. If you also want to learn about cloud-based database querying then Google Cloud BigQuery is very accessible. There is a free tier so you can try queries for free, an extensive range of public datasets to try and very good documentation.

To be a well-rounded data scientist it is a good idea to diversify a little from just Python. I would, therefore, suggest also taking an introductory course in R. Codeacademy have an introductory course on their free plan. It is probably worth noting here that similar to Dataquest Codeacademy also offers a complete data science learning plan as part of their pro account (this costs from $31.99 to $15.99 per month depending on how many months you pay for up front). I personally found the Dataquest course to be much more comprehensive but this may work out a little cheaper if you are looking to follow a learning path on a single platform.

It is a good idea to get a grasp of software engineering skills and best practices. This will help your code to be more readable and extensible both for yourself and others. Additionally, when you start to put models into production you will need to be able to write good quality well-tested code and work with tools like version control.

There are two great free resources for this. Python like you mean it covers things like the PEP8 style guide, documentation and also covers object-oriented programming really well.

The scikit-learn contribution guidelines, although written to facilitate contributions to the library, actually cover the best practices really well. This covers topics such as Github, unit testing and debugging and is all written in the context of a data science application.

For a comprehensive introduction to deep learning, I don’t think that you can get any better than the totally free and totally ad-free fast.ai. This course includes an introduction to machine learning, practical deep learning, computational linear algebra and a code-first introduction to natural language processing. All their courses have a practical first approach and I highly recommend them.

Whilst you are learning the technical elements of the curriculum you will encounter some of the theory behind the code you are implementing. I recommend that you learn the theoretical elements alongside the practical. The way that I do this is that I learn the code to be able to implement a technique, let’s take KMeans as an example, once I have something working I will then look deeper into concepts such as inertia. Again the scikit-learn documentation contains all the mathematical concepts behind the algorithms.

In this section, I will introduce the key foundational elements of theory that you should learn alongside the more practical elements.

The khan academy covers almost all the concepts I have listed below for free. You can tailor the subjects you would like to study when you sign up and you then have a nice tailored curriculum for this part of the learning path. Checking all of the boxes below will give you an overview of most elements I have listed below.

*Calculus*

Calculus is defined by Wikipedia as “the mathematical study of continuous change.” In other words calculus can find patterns between functions, for example, in the case of derivatives, it can help you to understand how a function changes over time.

Many machine learning algorithms utilise calculus to optimise the performance of models. If you have studied even a little machine learning you will probably have heard of Gradient descent. This functions by iteratively adjusting the parameter values of a model to find the optimum values to minimise the cost function. Gradient descent is a good example of how calculus is used in machine learning.

What you need to know:

*Derivatives*

- Geometric definition
- Calculating the derivative of a function
- Nonlinear functions

*Chain rule*

- Composite functions
- Composite function derivatives
- Multiple functions

*Gradients*

- Partial derivatives
- Directional derivatives
- Integrals

*Linear Algebra*

Many popular machine learning methods, including XGBOOST, use matrices to store inputs and process data. Matrices alongside vector spaces and linear equations form the mathematical branch known as Linear Algebra. In order to understand how many machine learning methods work it is essential to get a good understanding of this field.

What you need to learn:

*Vectors and spaces*

- Vectors
- Linear combinations
- Linear dependence and independence
- Vector dot and cross products

*Matrix transformations*

- Functions and linear transformations
- Matrix multiplication
- Inverse functions
- Transpose of a matrix

Here is a list of the key concepts you need to know:

*Descriptive/Summary statistics*

- How to summarise a sample of data
- Different types of distributions
- Skewness, kurtosis, central tendency (e.g. mean, median, mode)
- Measures of dependence, and relationships between variables such as correlation and covariance

*Experiment design*

- Hypothesis testing
- Sampling
- Significance tests
- Randomness
- Probability
- Confidence intervals and two-sample inference

*Machine learning*

- Inference about slope
- Linear and non-linear regression
- Classification

The third section of the curriculum is all about practice. In order to truly master the concepts above you will need to use the skills in some projects that ideally closely resemble a real-world application. By doing this you will encounter problems to work through such as missing and erroneous data and develop a deep level of expertise in the subject. In this last section, I will list some good places you can get this practical experience from for free.

“With deliberate practice, however, the goal is not just to reach your potential but to build it, to make things possible that were not possible before. This requires challenging homeostasis — getting out of your comfort zone — and forcing your brain or your body to adapt.”,

Anders Ericsson,Peak: Secrets from the New Science of Expertise

Machine learning competitions are a good place to get practice with building machine learning models. They give access to a wide range of data sets, each with a specific problem to solve and have a leaderboard. The leaderboard is a good way to benchmark how good your knowledge at developing a good model actually is and where you may need to improve further.

In addition to Kaggle, there are other platforms for machine learning competitions including Analytics Vidhya and DrivenData.

The UCI machine learning repository is a large source of publically available data sets. You can use these data sets to put together your own data projects this could include data analysis and machine learning models, you could even try building a deployed model with a web front end. It is a good idea to store your projects somewhere publically such as Github as this can create a portfolio showcasing your skills to use for future job applications.

One other option to consider is contributing to open source projects. There are many Python libraries that rely on the community to maintain them and there are often hackathons held at meetups and conferences where even beginners can join in. Attending one of these events would certainly give you some practical experience and an environment where you can learn from others whilst giving something back at the same time. Numfocus is a good example of a project like this.

In this post, I have described a learning path and free online courses and tutorials that will enable you to learn data science for free. Showcasing what you are able to do in the form of a portfolio is a great tool for future job applications in lieu of formal qualifications and certificates. I really believe that education should be accessible to everyone and, certainly, for data science at least, the internet provides that opportunity. In addition to the resources listed here, I have previously published a recommended reading list for learning data science available here. These are also all freely available online and are a great way to complement the more practical resources covered above.

Thanks for reading!

Python For Data Analysis - Build a Data Analysis Library from Scratch - Learn Python in 2019

**

**

Immerse yourself in a long, comprehensive project that teaches advanced Python concepts to build an entire library

You’ll learn

- How to build a Python library similar pandas
- How to complete a large, comprehensive project
- Test-driven development with pytest
- Environment creation
- Advanced Python topics such as special methods and property decorators
- A fully-functioning library that you can use to data analysis