$$ \newcommand{\floor}[1]{\left\lfloor{#1}\right\rfloor} \newcommand{\ceil}[1]{\left\lceil{#1}\right\rceil} \renewcommand{\mod}{\,\mathrm{mod}\,} \renewcommand{\div}{\,\mathrm{div}\,} \newcommand{\metar}{\,\mathrm{m}} \newcommand{\cm}{\,\mathrm{cm}} \newcommand{\dm}{\,\mathrm{dm}} \newcommand{\litar}{\,\mathrm{l}} \newcommand{\km}{\,\mathrm{km}} \newcommand{\s}{\,\mathrm{s}} \newcommand{\h}{\,\mathrm{h}} \newcommand{\minut}{\,\mathrm{min}} \newcommand{\kmh}{\,\mathrm{\frac{km}{h}}} \newcommand{\ms}{\,\mathrm{\frac{m}{s}}} \newcommand{\mss}{\,\mathrm{\frac{m}{s^2}}} \newcommand{\mmin}{\,\mathrm{\frac{m}{min}}} \newcommand{\smin}{\,\mathrm{\frac{s}{min}}} $$

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Python programs

As we saw in the chapter on Karel, programs consist of stetments. Let’s look at some of the basic Python stetments we will use to write the first programs.

Assigning a value to a variable

Variable is a named space in the memory of a computer, in which we can store values of any kind (number, text, logical value, or something else). Intermediate results are often placed in variables when calculating. When we run a Python shell, we can assign a value to a variable with one command, and then use the value of that variable in the following commands. For example:

>>> base = 6*8
>>> base
>>> base * 1.5
>>> base * 1.6

Value assignment statement

The value assignment statement is written by writing the name of a variable, followed by the equals sign =, and then the expression whose value we assign to the variable. We also consider integers and real numbers to be expressions (these are the simplest possible expressions).

Names of variables

Variable names (as well as other names in programs we write) can consist of uppercase and lowercase letters, digits and underscore, but they cannot begin with a digit.

Python distinguishes between uppercase and lowercase letters. N and n are different names and if we use them both, they would represent two different variables.

The variable name can be as long as we need it.

When writing programs (or individual statements), we try to give the variables meaningful names so that the commands and programs are as clear as possible.

In Python, it is a common style that capital letters are not used (though allowed), and when a name is made up of more than one word, those words are separated by an uderscore, for example, price_of_one_piece. Numbers are used in names when it makes sense (which is not often).

Variable names - check your understanding:

    Match the proposed variable names with the answers. Try again!
  • 2_date
  • incorrect, starts with an illegal character
  • pet_no_2
  • correct name
  • state_at_23:59
  • incorrect, contains an illegal character

    Q-8: Which of these can be the name of a variable?

  • vArIaBlE
  • ?1
  • 2D_graphics
  • _3D_graphics
  • pet-no-2

Starting a program

Running programs in browser

To help you get started, we used the ActiveCode component of the Runestone Interactive project and enabled you to run Python programs in the web pages of this course. For example, below are the statements we previously entered interactively, but this time written as a program. You can start the program by clicking the “Save & Run” button.

Running programs from the IDLE environment

It is recommended that you, in addition to writing programs on these web pages, run programs at least occasionally in the IDLE environment. Getting used to the IDLE environment is important for you to become more independent in programming.

When you run IDLE on your computer, open the integrated text editor (File / New File menu) and type in the previous (or any other) program.

When you finish the program, save it (menu File / Save) and then run it (menu Run / Run Module).


You will see the result in the interactive shell window.

Running programs from an online environment

Another way to run your Python program is to use one of the online programming environments. One such environment is https://repl.it/.


Click on the + new repl button, select the Python language and click Create repl. Your web browser will open a page where you can type a program and run it.


Program errors

Sometimes, you may not type a statement in the program exactly as required by the Python rules. In such case, the Python interpreter cannot understand the statement and you receive an error message. Each runtime environment reports an error in a slightly different way, but each of them tells in which line of the program the error occurred and what type of error it is.

The occurrence of errors (also known as bugs) should not worry you as it is a common thing and happens to experienced developers as well. Look at the message carefully, make sure you understand what is wrong, then correct it and run the program again. Understanding error messages is an integral part of programming and can be practiced like many other skills.

To help you understand the error messages you will be getting (to become less anxious about errors), we recommend that you now try to deliberately make some small mistakes that might anyway happen to you when writing a program.

When you know exactly what you did wrong and look at the message, it will be easier to understand the same message latter when you see it after an unintentional error.

We have also prepared a few programs with intentionally made mistakes, which we then explain. Programs are short to make errors more noticeable, but in longer programs, debugging is almost the same. Since the error message contains the program line number in which the error occured, in longer programs you just need to first find the program line mentioned and look at that (and possibly the previous) line.

Run each of the following programs, then see the error message and explanation.

The message says that it is an error of type NameError. This means that some of the names in the specified line are unknown to the Python interpreter (name is not defined). Note that the name of the print function is not spelled correctly (and the function prit does not exist). By inserting the letter n the error is corrected and the program works.

The error is of the same type, only this time it refers to the name resultt. By removing the superfluous letter t, the program becomes correct.

The error is of type SyntaxError, which means that Python statement construction rules are not followed. In this case, a comma between a and b is missing.

Another syntax error. Again, the rules of statement construction were not followed, and this time the brackets are missing.

The error is of type ZeroDivisionError. This error is different from the previous ones, because the statement was written correctly and was successfully interpreted. However, the execution of this command resulted in zero division, which is not a permitted operation. The program should be written in such a way that it does not attempt a zero division. The correction in this case depends on what we want our program to do in this situation. One possibility is to check that the divisor is not zero before dividing.

Make sure you understand these three types of error by answering the question.

    Match the error type with the program. Try again!
  • SyntaxError
  • print(3*(2+2)
  • NameError
  • ?=3
    print(a / b)
  • ZeroDivisionError
  • b=3//6
    print(3 // b)