Module 1, Practical 1¶

Setting up the environment¶

We will need to install several pieces of software to get a working programming environment suitable for this practical. In this section we will install everything that we are going to need in the next few weeks.

Python3 is available for Windows, Linux and Mac, therefore you can run it on your preferred platform.

The following section explains how to install Linux on a windows machine. This is for your reference, you can read the following instructions before the next practical and try to instally Linux if you want to test it out.

Our toolbox¶

If you decide to work on Windows or Mac, you can safely skip the following information and go straight to the section “Installing Python3 in Windows/Mac”. Note that, regardless your operating system, a useful source of information on how to install python can be found here.

Installing Python3 in Linux¶

1. The Python interpreter. In this course we will use python version 3.x. A lot of information on python can be found on the python web page. Open a terminal and try typing in:

   python3

if you get an error like “python3 command not found” you need to install it, while if you get something like this (note that the version might be different):

you are already sorted, just press Ctrl-D to exit.

Installation on a debian-like linux distribution (e.g. Ubuntu) can be done by typing the following commands on a terminal:

sudo apt-get update

sudo apt-get install python3

While if you are using Fedora you can use:

sudo dnf install python3

2. Install now the package manager pip, which is a very convenient tool to install python packages, with the following command (on Fedora, the command above should have already installed it):

   sudo apt-get install python3-pip

   Note:

   If pip is already installed in your system you will get a message like: python3-pip is already the newest version (3.x.y)

3. Finally, install the Integrated Development Environment (IDE) that we will be using. This is called Visual Studio Code and is available for all platforms. You can read about it here. Downloads for all platforms can be found here. On a debian-like distribution go to the folder where you downloaded the .deb package and type:

   sudo dpkg -i code*.deb

   While if you are using Fedora you can use:

   sudo dnf install code*.rpm

Installing Python3 in Windows/Mac¶

Two options are available, please read them both CAREFULLY and then pick the one you are more comfortable with.

OPTION 1:¶

1. The python interpreter. In this course we will use python version 3.x. A lot of information on python can be found on the python web page. Installers for Windows and Mac can be downloaded from this page. Click on Download Python 3.8.x. PLEASE REFRAIN FROM DOUBLE-CLICKING ON THE INSTALLER LIKE THERE IS NO TOMORROW AND READ BELOW FIRST.

   Attention! Important note

   When executing the installer, please remember to tick the flag “Add Python 3.8.x to PATH” and then click on Install now (see picture below noting that the current version might differ from the picture).

2. Install now the Integrated Development Environment (IDE) that we will be using. This is called Visual Studio Code and is available for all platforms. You can read about it here. Downloads for all platforms can be found here.

OPTION 2 (easier):¶

Additional Information:

It is also possible to install python through the Anaconda package manager. You can install Visual Studio Code together with Anaconda(the Anaconda installer will ask if you want it, just say yes!).

Anaconda is available here

Upon launching the installer you should be prompted something like:

at the next step flag the correct items as in the figure below (i.e. Flag Register Anaconda as my Default Python 3.x):

When installation is complete, start anaconda through the Anaconda Navigator in the windows menu. When the navigator starts, you should see a screen similar to:

from which you can install Visual Studio Code as IDE (by clicking on Install).

For more information please have a look here.

The console¶

To access the console on Linux just open a terminal and type:

python3

while in Windows you have to look for “Python” and run “Python 3.x”. The console should look like this:

Now we are all set to start interacting with the Python interpreter. In the console, type the following instructions (i.e. the first line and then press ENTER):

[1]:

5 + 3

[1]:

8

All as expected. The “In [1]” line is the input, while the “Out [1]” reports the output of the interpreter. Let’s challenge python with some other operations:

[2]:

12 / 5

[2]:

2.4

[3]:

1/133

[3]:

0.007518796992481203

[4]:

2**1000

[4]:

10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376

And some assignments:

[5]:

a = 10
b = 7
s = a + b
d = a / b

print("sum is:",s, " division is:",d)

sum is: 17  division is: 1.4285714285714286

In the first four lines, values have been assigned to variables through the = operator. In the last line, the print function is used to display the output. For the time being, we will skip all the details and just notice that the print function somehow managed to get text and variables in input and coherently merged them in an output text. Although quite useful in some occasions, the console is quite limited therefore you can close it for now. To exit press Ctrl-D or type exit() and press ENTER.

Visual Studio Code¶

Once you open the IDE Visual Studio Code you will see the welcome screen:

You can find useful information on this tool here. Please spend some time having a look at that page. Once you are done with it you can close this window pressing on the “x”.

The first thing to do is to set the python interpreter to use. Click on View –> Command Palette and type “Python” in the text search space. Select Python: Select Workspace Interpreter as shown in the picture below.

Finally, select the python version you want to use (e.g. Python3.x).

Now you can click on Open Folder to create a new folder to place all the scripts you are going to create. You can call it something like “exercises”. Next you can create a new file, example1.py (as you might have guessed the .py extension stands for python).

Visual Studio Code will understand that you are writing Python code and will help you writing valid syntax in your scripts.

Warning:

If you get the following error message:

click on Install Pylint which is a useful tool to help your coding experience.

Add the following text to your example1.py file.

[6]:

"""
This is the first example of Python script.
"""
a = 10 # variable a
b = 33 # variable b
c = a / b # variable c holds the ratio

# Let's print the result to screen.
print("a:", a, " b:", b, " a/b=", c)

a: 10  b: 33  a/b= 0.30303030303030304

A couple of things worth nothing: the first three lines opened and closed by “”” are some text describing the content of the script. Moreover, comments are proceeded by the hash key (#) and they are just ignored by the python interpreter.

Please notice that Visual Studio Code will help you writing your Python scripts. For example, when you start writing the print line it will complete the code for you (if the Pylint extension mentioned above is installed), suggesting the functions that match the letters typed in. This useful feature is called code completion and, alongside suggesting possible matches, it also visualizes a description of the function and parameters it needs. Here is an example:

Save the file (Ctrl+S as shortcut). It is convenient to ask the IDE to highlight potential syntactic problems found in the code. You can toggle this function on/off by clicking on View –> Problems. The Problems panel should look like this

Visual Studio Code is warning us that the variable names a,b,c at lines 4,5,6 do not follow Python naming conventions for constants (do you understand why? Check here to find the answer). This warning is because they have been defined at the top level (there is no structure to our script yet) and therefore are interpreted as constants. The naming convention for constants states that they should be in capital letters. To amend the code, you can just replace all the names with the corresponding capitalized name (i.e. A,B,C). If you do that, and you save the file again (Ctrl+S), you will see all these problems disappearing as well as the green underlining of the variable names. If your code does not have an empty line before the end, you might get another warning “Final new line missing”.

Had we by mistake mispelled the print function name (something that should not happen with the code completion tool that suggests functions names!) writing printt (note the double t), upon saving the file, the IDE would have underlined in red the function name and flagged it up as a problem.

This is because the builtin function printt does not exist and the python interpreter does not know what to do when it reads it. Note that printt is actually underlined in red, meaning that there is an error which will cause the interpreter to stop the execution with a failure. Please remember ALL ERRORS MUST BE FIXED before running any piece of code.

Now it is time to execute the code. By right-clicking in the code panel and selecting Run Python File in Terminal (see picture below) you can execute the code you have just written.

Upon clicking on Run Python File in Terminal a terminal panel should pop up in the lower section of the coding panel and the result shown above should be reported.

Saving script files like the example1.py above is also handy because they can be invoked several times (later on we will learn how to get inputs from the command line to make them more useful…). To do so, you just need to call the python intepreter passing the script file as parameter. From the folder containing the example1.py script:

python3 example1.py

will in fact return:

a: 10 b: 33 a/b= 0.30303030303030304

Before getting into that, consider the following lines of code (do not focus on the import line, this is only to load the mathematics module and use its method sqrt to compute the square root of its parameter):

[7]:

"""
Runtime error example, compute square root of numbers
"""
import math

A = 16
B = math.sqrt(A)
C = 5*B
print("A:", A, " B:", B, " C:", C)

D = math.sqrt(A-C) # whoops, A-C is now -4!!!
print(D)

A: 16  B: 4.0  C: 20.0

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Input In [7], in <cell line: 11>()
      8 C = 5*B
      9 print("A:", A, " B:", B, " C:", C)
---> 11 D = math.sqrt(A-C) # whoops, A-C is now -4!!!
     12 print(D)

ValueError: math domain error

If you add that code to a python file (e.g. sqrt_example.py), you save it and you try to execute it, you should get an error message as reported above. You can see that the interpreter has happily printed off the vaule of A,B and C but then stumbled into an error at line 9 (math domain error) when trying to compute \(\sqrt{A-C} = \sqrt{-4}\), because the sqrt method of the math module cannot be applied to negative values (i.e. it works in the domain of real numbers).

Please take some time to familiarize with Visual Studio Code (creating files, saving files etc.) as in the next practicals we will take this ability for granted.

The debugger¶

Another important feature of advanced Integrated Development Environments (IDEs) is their debugging capabilities. Visual Studio Code comes with a debugging tool that can help you trace the execution of your code and understand where possible errors hide.

Write the following code on a new file (let’s call it integer_sum.py) and execute it to get the result.

[8]:

""" integer_sum.py is a script to
 compute the sum of the first 1200 integers. """

S = 0
for i in range(0, 1201):
    S = S + i

print("The sum of the first 1200 integers is: ", S)

The sum of the first 1200 integers is:  720600

Without getting into too many details, the code you just wrote starts initializing a variable S to zero, and then loops from 0 to 1200 assigning each time the value to a variable i, accumulating the sum of S + i in the variable S.

A final thing to notice is indentation.

How does this code work? How does the value of S and i change as the code is executed? These are questions that can be answered by the debugger.

To start the debugger, click on Debug –> Start Debugging (shortcut F5). The following small panel should pop up:

We will use it shortly, but before that, let’s focus on what we want to track. On the left hand side of the main panel, a Watch panel appeared. This is where we need to add the things we want to monitor as the execution of the program goes. With respect to the code written above, we are interested in keeping an eye on the variables S, i and also of the expression S+i (that will give us the value of S of the next iteration). Add these three expressions in the watch panel (click on + to add new expressions). The watch panel should look like this:

do not worry about the message “name X is not defined”, this is normal as no execution has taken place yet and the interpreter still does not know the value of these expressions.

The final thing before starting to debug is to set some breakpoints, places where the execution will stop so that we can check the value of the watched expressions. This can be done by hovering with the mouse on the left of the line number. A small reddish dot should appear, place the mouse over the correct line (e.g. the line corresponding to S = S + 1 and click to add the breakpoint (a red dot should appear once you click).

Now we are ready to start debugging the code. Click on the green triangle on the small debug panel and you will see that the yellow arrow moved to the breakpoint and that the watch panel updated the value of all our expressions.

The value of all expressions is zero because the debugger stopped before executing the code specified at the breakpoint line (recall that S is initialized to 0 and that i will range from 0 to 1200). If you click again on the green arrow, execution will continue until the next breakpoint (we are in a for loop, so this will be again the same line - trust me for the time being).

Now i has been increased to 1, S is still 0 (remember that the execution stopped before executing the code at the breakpoint) and therefore S + i is now 1. Click one more time on the green arrow and values should update accordingly (i.e. S to 1, i to 2 and S + i to 3), another round of execution should update S to 3, i to 3 and S + i to 6. Got how this works? Variable i is increased by one each time, while S increases by i. You can go on for a few more iterations and see if this makes any sense to you, once you are done with debugging you can stop the execution by pressing the red square on the small debug panel.

Other editors are available, if you already have your favourite one you can stick to it. Some examples are:

• Spyder

• PyCharm Community Edition

• Jupyter Notebook. Note: we might use it later on in the course.

A quick Jupyter primer (just for your information, skip if not interested)¶

Jupyter allows to write notebooks organized in cells (these can be saved in files with .ipynb extension). Notebooks contain both the code, some text describing the code and the output of the code execution, they are quite useful to produce some quick reports on data analysis. where there is both code, output of running that code and text. The code by default is Python, but can also support other languages like R). The text is formatted using the Markdown language - see cheatsheet for its details. Jupyter is becoming the de-facto standard for writing technical documentation.

Installation¶

To install it (if you have not installed python with Anaconda otherwise you should have it already):

python3 -m pip install jupyter

you can find more information here

Upon successful installation, you can run it with:

jupyter-notebook

This should fire up a browser on a page where you can start creating your notebooks or modifying existing ones. To create a new notebook you simply click on New:

and then you can start adding cells (i.e. containers of code and text). The type of each cell is specified by selecting the cell and selecting the right type in the dropdown list:

Cells can be executed by clicking on the Run button. This will get the code to execute (and output to be written) and text to be processed to provide the final page layout. To go back to the edit mode, just double click on an executed cell.

Please take some more time to familiarize with Visual Studio Code (creating files, saving files, interacting with the debugger etc.) as in the next practicals we will take this ability for granted. Once you are done you can move on and do the following exercises.

Let’s try together¶

1. Compute the area of a triangle having base 120 units (B) and height 33 (H). Assign the result to a variable named area and print it.

[9]:

B = 120
H = 33
Area = B*H/2
print("Triangle area is:", Area)

Triangle area is: 1980.0

2. Aquire the side S (base of a square) from the user at runtime and compute the area of a square. Then, assign the result to a variable named area and print it. Hint: use the input function (details here) and remember to convert the acquired value into an int.

[10]:

S_str = input("Insert size: ")
print(type(S_str))
print(S_str)
S = int(S_str)
print(type(S))
print(S)
Area = S**2
print("Square area is:",Area)

<class 'str'>
15
<class 'int'>
15
Square area is: 225

Exercises¶