Writing a simple script

Learning objectives

At the end of this chapter you will be able to:

  • write a simple script,
  • handle basic data types such as int, float, string and list,
  • execute basic operations with these data types,
  • use the control statement if ,
  • Iterate using for or while,
  • test whether a variable is of a certain datatype,
  • convert data to another data type,
  • ask input from the user,
  • return output in a formatted way,
  • use arguments in a script,
  • use string operations (slicing, ...)

Extra documentation

From the official documentation

Variabels en data types

As we described earlier Python is dynamically types. We do not have to declare the data type of a variable beforehand. The data type is inferred at the first assignment.

Numbers

We know two numeric data types:

  • int: integers or whole numbers
  • float: numbers with a fractional part, with decimals.

Decimal numbers use a decimal point.

age = 23
weight = 78.5

The variable age will be of type int and the variable weight will be of type float.

Python has the following numeric operators:

  • Add: +
  • Subtract: -
  • Multiply: *
  • Power: **
  • Division: /
  • Integer division: //
  • Modula (remainder of integer division): %

The following numeric functions are also interesting:

  • abs(x): absolute value of x.
  • max(x,y): the largest of x and y.
  • min(x,y): the smallers number of x and y.

If we import the built-in module math - and we do this by putting the statement import math at the beginning of our script -, we add a few usefull functions to our repertoire:

  • math.ceil(x): returns the smallest integer larger than or equal to x.
  • math.floor(x): returns the largest integer smaller than or equal to x.

Examples:

import math
print(math.ceil(2.3)) # 3
print(math.floor(2.3)) # 2
print(abs(-5)) # 5
print(max(5,7)) # 7

Boolean values

As you know, a boolean value is either true (True) of not true (False).

We know the traditional operations on booelan values. In Python they look like this: or, and and not.

The comparison operators in Python look like this:

Operator Meaning
< smaller than
<= smaller than or equal to
> larger than
>= groter than or equal to
== equal to
!= not equal to
is object equality
is not object inequality

Text

Text of str should be put between single or double quotes. The following statements are equivalent:

print("Hello world")
print('Hello world')

If you want to use literal quotes in a piece of text, you can do that by switching between single and double quotes:

print('And father said: "Son, you must start studying in time."')

To concatenate strings, we use the plus operator (+). You can use the star operator (*) to create a string that is a repitition of the original string.

The command:

print("VIVES" + " University of Applied Sciences")

will give the following output: VIVES University of Applied Sciences.

The command:

print(3 * "ai")

will give the output: aiaiai.

A few usefull string functions :

  • str.lower(): str in lower case.
  • str.upper(): str in capitals.
  • str.title(): every word will start with a capital letter.
  • len(str): returns the number of characters in a string.
  • str.count(sub [,start [,end]]): count the number of times a non-overlapping piece of text occurs in the string.
  • str.find(sub, [, start [,end]]): return -1 if sub does not occur in str and if it does, it return the index of the first character of sub (indexes start at 0).
  • sub in str: return True if sub if found in str , else False.
  • str.replace(str1, str2): replaces str1 by str2.

Keep in mind that string are immutable. A changed string is printed or assigned to a new variable.

Examples:

print("VIVES University").lower() # vives university
print("VIVES University").upper() # VIVES UNIVERSITY
print("vives university").title() # Vives University
print(len("This is text")) # 12
message = "This is text"
print(message.count('i')) # 2
print(message.count('t')) # 3
print(message.count('t', 4)) # 2 (I start counting from position 4)
print(message.find('t', 4)) # 8
print("text" in "This is text.") # True
print("word" in "This is text.") # False
new_message = message.replace("text", "a message") # This is a message.

In a string \t is a tab and \n a new line. Beware, in Windows a new line is a combination of a new line and a carriage return: \n\r'.

When you want to escape a character, to indicate that it must be taken literally, you put a backslash in front of it, the so called escape sign.

print("We use the character \", or double quotes in a citation.")
print("The directory is C:\\Windows\\System32.")

When you want to select a character from a string, you can use the index. The index starts at 0.

message = "Hello, World!"
print(message[1]) # resultaat: e

With the slice operator, you can select a substring. Keep in mind that the start index is included, the end index is not.

message = "Hello, World!"
print(message[2:5]) # result: llo

The start i1 of the slice i1:i2 can be omitted if i1 is the first index, and i2 can be omitted if i2 is the last index:

print(message[:5]) # result: Hello
print(message[7:]) # result: World!

Conversion van data types

We do not always get a value in the correct data type. In that case we must do type casting, or conversion to another data type.

  • int(x): conversion to integer.

  • float(x): conversion to float.

  • str(x): conversion to string.

Test whether a value is of a certain data type

The most general function to test whether a value is of a certain data type is: isinstance(object, type).

You can use one type or a list of types:

x = isinstance("Hello", str) # x is True
x = isinstance(2, str) # x is False
x = isinstance(2.5, int) # is is False
x = isinstance(2.5, (int, float)) # x is True

Another way is using the function type().

x = 5
y = type(x) == int # y is True

The function str.isnumeric() will check whether all characters are numeric.

x = "122343".isnumeric() # x is True
x = "2.5".isnumeric() # x is False

You can use the module number to really test whether a value is numeric:

import numbers

x = 5
print(isintance(x, numbers.Number)) #resultaat True

Mutliple assignment

You can assign multiple values in one command (although this makes your code less readable):

x, y, z = 10.0, 20.0, 30.0
print(x + y + z) # 60.0

Read input

A script can do a meaningful action is it can ask input to the user. To do that we use the function input(). This function can contain a promt, a piece of text that is presented to the user as a message. Keep in mind that the input is always of type str. You may have to convert your input to another data types;

first_name = input("Give your first name: ")
print("Welcome " + first_name)

my_number = input("Give a whole number: ")
print("The square is this number is " + int(my_number)**2)

Make desisions (if)

Python uses the traditional if.

speed = 31.54
if speed <= 30:
    print("Flying allowed.")
else:
    print("Flying not allowed, due to strong winds.")

age = 14
if (age <= 12):
    print("Free entrance.")
elif (age < 18):
    print("Ticket price: €5.")
else:
    print("Ticket price: €8.")

voltage = 3.9
temperature = 36
if voltage > 3.7 and temperature < 49:
    print("All ok.")
else:
    print("Warning, check parameters!")

This notation is also possible for short statements:

cost = 24.54
revenue = 93.01
profit = revenue - cost
print("Nice, keep going!") if profit > 0 else print("Watch out for bankruptcy.")

If you need an empty statement that you will define later, you can use pass . This way you avoid an error.

height = 150
if height < 300:
    pass
else:
    print("Too high.")

You can also use three dots instead of pass, the so called Ellipsis object:

naam = 'KULeuven'
if naam.lower() == 'vives':
  print('Hogeschool')
else:
  ...

Comments and indentation

As you noticed in the previous examples, Python does not use curly braces or semicolons to structure the code. It uses indentation instead. You indent four spaces to indicate a code block. In most text editors you can use the tab key. This is automatically converted to four spaces.

To give comments, pieces of text that are not compiles, you use #. All code after this sign (on the same line) is ignored.

A comment that spans multiple lines begins and ends with ''' or """.

Multiple lines

Sometimes you want to spread a long statement over multiple lines. You can use a backslash at the end of the line as a continuation symbol.

if school1[1] == school2[1]
        and school1[2] == school2[2] \
        and school1[3] == school2[3] \
        and school1[4] == school[4]:
        return True
else:
        return False

An alternative method is to include your code in rounded brackets, square brackets or curly braces.

 if (school1[1] == school2[1]
        and school1[2] == school2[2]
        and school1[3] == school2[3]
        and school1[4] == school2[4]):
        return True
    else:
        return False

If you want a string to run over multiple lines, you can use the same method, of work with triple quotes. In the latter case the line break is included in the string.

my_text = """Python is
a very readable
programming language
"""
print(my_text)

This code will produce the following output:

Python is
a very readable
programming language

Lists

In a later chapter we will discuss lists in detail, but we will already show how basic lists or arrays work. You can declare a list in two ways.

myList = ["car", "bike", "airplane", "step"]
myUnimportantList = list(("a", "b", "c"))

To indicate a specific member of the list, you use indexes, as you do with strings. The index is zero based. Use square brackets with indexes.

my_list = ["car", "bike", "airplane", "step"]
print(my_ist[1]) # bike

In contrast to strings, list are mutable. You can change an individual member of a list:

my_list[1] = "e-bike"
print(my_list[1]) # e-bike

As with strings you can use slicing to select part of a list. You create a new shorter list. Don't forget that the element at the end index is not included (exclusive boundary).

my_list = ["car", "bike", "airplane", "step"]
print(my_list[1:3]) # ['bike', 'airplane']

To add items at the end of an ordered list, use append().

my_list = ['bike', 'car']
my_list.append("boat") # ['bike', 'car', 'boat']

We will learn more about lists later.

Iteration

When an element is "iterable" , that means that you can go through is as a list, use the for-loop. Strings as well as lists are iterable.

my_list = ["car", "bike", "airplane", "step"]
for item in my_list:
    print(item)

You have no counter in the for-loop. If you need a fixed number of iteration, use the range() function.

for i in range(10):
    print(i*2)

Python also knows the while-loop to iterate until a condition is met.

i = 0;
while i < 5:
    i += 1
    print("i has the value of: " + str(i))

You can interrupt an iteration with the statement break. You can go on with an iteration using the statement continue.

Formatted String output

You have noticed that the function print() prints output to the terminal. This function can have multiple arguments. These are concatenated.

my_number = int(input("Give a number: "))
print("The square of this number is: ", my_number**2)

In most cases we want to present our output in a nice, structure way. We usually want to mix literal text with the value of a variable.

The easiest way to do this if to put the letter 'f' before the quotes. When you want to use the value of a variable, you put the variable name between curly braces.

name = "Saïd"
age = 23
print(f"Hello {name}, I heard that you are {age} years old.")

In fact this a brief notation of the functionformat(). You could rewrite this as:

name = "Saïd"
age = 23
salutation = "Hello {}, I heard that your {} years old."
print(salutation.format(name, age))

The empty curly braces are placeholders. You fill in the placeholders using the format function.

You can add extra formatting using the function `format()'. A few examples may illustrate this.

Center: you can center a piece of text using the ^ sign in a placeholder, following by the width of the field.

Centreren: Je kunt tekst centreren in een bepaald aantal tekens met behulp van het ^-teken in de plaatsaanduiding, gevolgd door een getal dat de breedte van de velden aangeeft. Hier is een voorbeeld:

text = "Python"
centered_text = "{:^10}".format(text) # Alternative notation: centered_text = f"{text:^10}"
print(centered_text)

This Python code with center the text 'Python' in a field of 10 characters.

Left align: you can left align the text with the character < in the placeholder, followed by the field width.

text = "Python"
left_aligned_text = f"{text:<10}"
print(left_aligned_text)

Right align: you can right align the text with the > symbol in the placeholder, followed by the width of the field.

text = "Python"
right_aligned_text = f"{text:>10}"
print(right_aligned_text)

With a floating comma number, you can define the number of decimals in the output with .2f (or another number of decimals).

my_number = 3.14159265
rounded_output = "{:.2f}".format(getal)
print(rounded_output)

The output will show the number with two decimals: 3.14.

You can use the f-notation to get the same effect.

my_number = 3.14159265
print(f"Het number pi is {my_number:.2f}.")"

Arguments

Script do no only ask input from a user, but you can also pass values when calling the script. Suppose you want to calculate the circumference of a rectangle. You will call the script as follows:

python3 perimeter.py 5 10

5 is the width of the rectangle and 10 the height.

How do you process these arguments width and height. In the module sys there is a list sys.argv. This list contains all the arguments that were passed to the script, including the name of the script itself as the first element (index 0). An example will illustrate this.

import sys

# The first element is the name of the script
script_name = sys.argv[0]

# De rest of the list are the arguments
arguments = sys.argv[1:]

# Je count the arguments as follows
number_of_arguments = len(sys.argv) - 1

# Now you can use the arguments
print("Name of the script:", script_name)
print("Arguments:", arguments)

In the example of the rectangle the script would look like thios:

import sys

script_name, width, height = sys.argv

perimeter = int(width) * int(height)
print(f"The perimeter of the rectangle is {perimeter}.")

We will make this script more robust by checking the number of arguments.

import sys

if len(sys.argv) -1 != 2:
    print("You have to pass two arguments, width and height")
    sys.exit(1)

script_name, width, height = sys.argv

perimeter = int(width) * int(height)
print(f"The perimeter of the rectangle is {perimeter}.")

If the number of arguments is not equal to two, a error message is given and the script is stopped with error code 1, which is equal to a non-successful termination (0 is successful)