# Basics¶

“The canonical, ‘Python is a great first language’, elicited, ‘Python is a great last language!’”

—Noah Spurrier

Now we are going to introduce the basics of the Python programming language. We start with the infamous Hello, World! program and the basic syntax of a Python script.

Attention

Make sure you have your virtual environment activated! If you do not have (python101) in front of your command line prompt you need to activate it using

$source ~/.virtualenvs/python101/bin/activate  Execute the following command to start the interactive Python interpreter: $ python


You should see a couple of lines printed in the terminal, with the first line stating, among the current date and time, the version of Python you use. The very last line should start with >>> which indicates the Python prompt. You can write Python commands there and execute them.

Attention

In the previous chapter The Unix shell the basic commands for the Unix shell were introduced. Notice how every command was preceeded by a $ character. In this tutorial code blocks that start with a $ sign are meant to be executed in the Unix shell. If the code block is preceeded by >>> it means that it should either be executed in the Python interpreter or be used in a Python script.

## Python as an interactive calculator¶

To get your feet wet with Python you can use the Python interpreter as a calculator. You have the usual mathematical operators at your disposal, like

+: addition, subtraction, multiplication, division, exponent, integer division, and modulus.

If you are not familiar with one of them just give it a try in the Python interpreter—do not only use integers, but also floating point numbers. You can also use brackets as you would use them in mathematical expressions. Try whether Python uses the proper mathematical rules with regards to the order of execution of the operators.

Note

At one point you will enter an “invalid” expression like 1 + * 2. Python will then raise a SyntaxError to tell you that whatever you typed is not valid Python syntax. In many cases Python will also give you additional information about the error. There are many more errors you can encounter, and it is perfectly normal to have errors. The only difference between a seasoned programmer and a beginner is the time it takes to fix those errors. The more errors and mistakes you made the better you know how to solve them.

To leave the Python interpreter you either execute

>>> quit()


or you press Ctrl + D. Besides the interactive Python interpreter you can also write scripts with Python. Scripts are files that can be executed from the command line interface. They contain Python expressions that get executed once you call the scripts. A script can be simple and merely rename files or it can be complex and run a complete simulation of a car crash. You decide how simple they are.

## Your first Python script¶

We will start with the infamous Hello, World! Program. Open a new terminal, activate your virtual environment, and create a new file named hello_world.py via

$touch hello_world.py  Open it with your favorite text editor, e.g., Atom or SublimeText. In the former case you would open the file via $ atom hello_world.py


Now type (not copy!) the following into the file

hello_world.py
print('Hello, World!')


Save the file, switch to your command line interface, and execute

\$ python hello_world.py


If you did everything correctly you should see the phrase Hello, World! popping up in your command line interface. If you see something like

  File "hello_world.py", line 1
print('Hello, World!)
^
SyntaxError: EOL while scanning string literal


or

  File "hello_world.py", line 2

^
SyntaxError: unexpected EOF while parsing


it means that you have either forgotten the closing ' or ), respectively. As you can see Python tries its best to describe the error to you so that it can be fixed quickly.

If everything went fine: Congratulations! You wrote your first Python script!

## The print function¶

The function you used in your first Python script, the print() function, has a rather simple goal: Take whatever you have in there and display it in the command line interface. In the Python interpreter (the command line starting with >>>) the result of an expression was displayed automatically. Try creating a new file math_expressions.py and enter several mathematical expressions like you did earlier. Save the file, switch to your terminal and execute the file via

python math_expressions.py


You should not see a single thing happening. That is because you never told Python what to actually do with those expressions. So what it does is evaluate them and nothing more. Now wrap the mathematical expressions in the print() function, for example like this:

print((3 + 4)*6)


If you execute the script again you should see the expected output.

## Integers, floats and strings¶

In the previous examples you worked with integers, floating-point numbers, and with strings. -4, 0, and 2 are all integers. 1.2, 1.0 and -2e2 (which is the scientific notation for -200.0) are floating-point numbers. Finally, 'Hello, World!' is a string. These categories are called data types. Every value in Python is of a certain data type.

The meaning of operators may depend on the data types of the values surrounding it. Take, e.g., the addition operator +:

>>> 1 + 2
3
>>> 1.2 + 3.4
4.6
>>> 'My first sentence.' + 'My second sentence.'
'My first sentence.My second sentence.'
>>> 'My ' + 3 + 'rd sentence.'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Can't convert 'int' object to str implicitly


In the last case the addition operator has no idea how to combine the integer 1 with the strings. What you can do to solve this is to convert the integer to a string using str():

>>> 'My ' + str(3) + 'rd sentence.'
'My 3rd sentence.'


If you want to convert something to a string you use str(), to convert to an integer you use int(), for floating-point numbers you use float().

>>> '1.2' + '3.4'
'1.23.4'
>>> float('1.2') + float('3.4')
4.6
>>> int('1.4')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '1.4'
>>> str(1e2)
'100.0'


Play around with those three functions to see what can be converted and what can not. Try the different operators, e.g., try to multiply a string with an integer, etc.

## Variables¶

Like in mathematics you can also use variables to store values. A variable has a name by which it is called and a value. There are three rules that a variable name must comply:

1. It must be exactly one word.
2. It must comprise only letters, numbers, and the underscore character.
3. It must not begin with a number.

Other than that anything goes.

Warning

Although anything else is a viable variable name, you should take special care not to use names of built-in objects like, e.g., int. If you name a variable after some function or class it is not usable anymore in the subsequent code.

To assign a value to a variable you use the equal sign = with the variable name on the left and the value on the right:

>>> my_first_variable = 21
>>> 2*my_first_variable
42
>>> my_second_variable = 3
>>> my_first_variable/my_second_variable
7.0
>>> my_third_variable = my_first_variable
>>> print(my_third_variable)
21


Here is a slightly more complex example:

students = 35
tutors = 2
classrooms = 1
pizza_orders = 20

students_per_tutor = students / tutors
persons = students + tutors
persons_per_classroom = persons / classrooms
hungry_persons = persons - pizza_orders

print('There are', students, 'students and', tutors, 'tutors.')
print('That makes', persons, 'persons in', classrooms, 'class room(s).')
print(hungry_persons, 'have to stay hungry...')


The advantages of using variables are two-fold:

• If the amount of students, tutors, classrooms or pizza_orders changes you only have to update one line instead of many. This is less error-prone and faster.
• You give the values some meaning which should be represented in the variable name. You could in principle read “the students per tutor is the amount of students divided by the amount of tutors.” This makes your code easily comprehensible and you need fewer comments. But you still should write them when they make sense!

And here is what the output should look like:

There are 35 students and 2 tutors.
That makes 37 persons in 1 class room(s).
17 have to stay hungry...


Notice how we used , to separate strings and variable names in print(), but everything was composed in a nice way? The reason for this is that print() can take an arbitrary amount of arguments. Just chain them using , and you are good to go. How this works is part of the section Functions.

## User input¶

In some cases you may want to ask the user of your script to provide some additional information, like the path to a file or parameters for a simulation. For this the input() can be used.

print('What is your name?')
name = input()
print('Nice to meet you,', name)


Note

The value returned by input() is always a string. So when you are asking for numbers you have to convert them.

print('What is your age in years?')
age = int(input())
print('In 5 years you will be', age+5, 'years old.')


## Imports¶

Sometimes the features that Python offers by default are not enough. What if you want to use the $$\sin(x)$$ function? For more specialized topics Python offers modules or packages, either ones that already ship with every Python installation or packages from external parties. The packages that Python ships with are called the standard library. External packages may be, e.g., NumPy and SciPy for scientific computing with Python, or Matplotlib for plotting.

You activate this additional functionality by importing these packages:

>>> import math


Now we have access to all functions available in the math module.

>>> math.pi
3.141592653589793
>>> math.sin(0.5*math.pi)
1.0


Take your time and browse the documentation of the math module, try some of the provided functions like math.ceil(), math.exp(), etc.

## Summary¶

• You can use the interactive Python interpreter to execute small commands.
• You can execute scripts that hold several commands using Python.
• You can display results of computations or strings using the print() function
• You can use str(), int(), float() to convert from one data type to another—if it is somehow possible.
• You can store values in variables to access them at a later point in your script.
• You can import modules or packages to extend Pythons builtin functionality using the import statement.

## Exercises¶

1. Write a script that asks the user for the radius of a circle and subsequently shows the circumference and the area of the circle in the terminal.