Python Introduction

Python as a programming language

Programming is the execution of instructions by computer, which are given by a person. But, in order to succeed, the computer must understand what we want from it to do for us. This is possible with programming languages, which resemble a way of a communication between a person and a computer. Python is one of the most popular and easiest-to-learn programming languages.

Python is a high-level, general-purpose programming language, which supports numerous programming paradigms, such as procedural, imperative, object-oriented and functional programming. Python is a very fun-to-learn and expandable language, whose syntax allows us to write highly readable programs. Programs written in Python are most often being interpreted, which means the commands are being executed without having to be compiled into a form readable for machines (machine language). It was first released in 1991 as Python 0.9.0. by Guido van Rossum and was meant to be a successor to the ABC programming language.

Python environment

A lot of Python environments have an interactive mode, which is good for people who are new in Python. That means we can test a line of code by writing it and the output(s) is(are) immediately displayed. Here’s a few examples:

>>> 3 + 4 
7

Every new line begins with >>>, which means we can start writing our code. Then we write the command 3 + 4 and after tapping ENTER on the keyboard we immediately get the output, which is 7. As we can see, output line doesn’t begin with >>> and we cannot change it.

>>> print("Wipher is good")
Wipher is good

Now we want to write a program that will show us a message on the output. We can do that using the function print and inside its brackets ( () ) writing a message we want under quotation marks ( ” ” ). As a result, we get the message on the output which says Wipher is good. Is it not?

>>> x = 12
>>> print(x)
12

There is one more example of how we can use the print function, but this time it prints the value of previously defined variable x, because we didn’t use quotation marks ( ” “ ). The output is 12, which is the value of the variable x. More about variables and data types will be explained in the next chapter of this lesson.

Python scripts

Programs in Python can be written in files with the .py extension. Those files are called scripts and they can be ran from the command line of your computer. In this explanation, the name of our script will be script_name.py.

If you have Windows OS, in your script you just write your program and then from the Windows command line run this command:

python script_name.py

If you have Linux OS, your program in script must begin with this line of code:

#!/usr/bin/python3

After that, you just write your code. When you want to run your script, in the Linux terminal run this command (make sure the script file has the executable flag):

./script_name.py

Keywords

Keywords are words that cannot be used for naming variables. They are reserved for a special use. Some keywords are used in loops (for, while, etc.), others are used in logical operations (True, False, and, etc.) and the list goes on. Here is a table with all keywords in Python:

Here’s an example of correct use of a keyword:

>>> x = 8 and 15
>>> print(x)
15

We used keyword and for logical and operation between 8 and 15 to set a value for variable x, which is 15.

Here’s an example of incorrect use of a keyword:

>>> import = 'Jordan'
SyntaxError: Can't assign to keyword

We tried to use the keyword import as a name for the variable and set its value to ‘Jordan’ (string). As we can see, the program reports an error.

Comments

Comments are part of program code visible only to programmer, which means they can not be executed. They are used for some additional explanation of how some lines of code work.

There are two types of comments in Python: comments and docstrings.

Comments (in the narrow sense) are comments that spread in a single line. They are written behind the hash symbol (#) and usually are used for short explanations.

>>> x = 7 #definition of variable x 
>>> y = 3.14 #definition of variable y

Docstrings are comments that can spread in multiple lines. They are written inside triple quotation marks (“”” “”” or ”’ ”’) and usually are used for documentation.

>>> y = 9 
"""
variable 
y
with 
value 
9
"""

Now that we’ve learnt about comments, we will use them for the most of explanations in future examples.