June 1, 2019 6 min to read

Writing your first Python programme

By the end of this article, you will have written a Python programme that outputs a sentence based on a number of variables.

If you’ve made it to this article you should have read the Python Introduction, and be familiar with variables and data-types.

As we stated in the Python Introduction, Python is a weakly-typed language so its types are inferred.

There is only one way to declare a variable in Python.

In the following example:

• x is assigned the value "Hello World" and has an inferred type of String.
• y is assigned the value True and has an inferred type of Bool.
• a is assigned the value 1 and has an inferred type of Int.
• b is assigned the value 3.14159 and has an inferred type of Float.

x = "Hello World"
y = True
a = 1
b = 3.14159

As Python is a weakly-typed language, there is no way to declare a variable as a specific data-type.

It is, however, possible to perform logic based on the type, using the type() function.

The following if statement checks to see if the type of y is bool and, if True, will run the <do something> action. Otherwise, it will skip the indented section of code.

y = True

if type(y) is bool:
    <do something>

It is also possible to perform “type casting” in Python. In the following example:

• The variable x is assigned the value 1 and has an inferred type of Int.
• The value of x (1) is “casted” to a float and assigned to the variable y. The type of y is therefore float.

x = 1
y = float(x)

print(type(x)) // => outputs int 
print(type(y)) // => outputs float

##Your first Python programme

It’s finally time to write your first Python programme. You can write this anywhere on your machine, however, it is best practice to keep your development code in a specific place and well organized.

Within your home directory ($HOME on Linux and Mac, %userprofile% on Windows) create a new directory called python (e.g. the full Linux path would be /home/codetips/python ). Navigate to this directory and create another directory called firstApplication (e.g. the full Linux path would be /home/codetips/python/firstApplication) and open it in your code editor of choice.

We’re going to leave out most of the specifics of Python best practices, so you’re not overwhelmed with too much information, but everything will be explained in future articles.

In your new firstApplication directory, create a new file called main.py, add the below code into it, and save the file.

#!/bin/env python

# Start of our code

print("Welcome to my first Python programme!")

# End of our code

Now, open a terminal, navigate to your firstApplication folder, and run the following command:

python main.py

Your output should look very similar to the following:

$ [user@linux]: python main.py
Welcome to my first Python programme!

As you can see, the print function outputs text to the terminal. Unfortunately, as a first programme goes, this is pretty boring. Let’s make this more personal with Variables!

Replace the contents of main.py with the following code:

#!/bin/env python

# Start of our code

name = "Jordan"
age = 31
profession = "Developer"

print("""Welcome to my first Python programme!\n\
My name is {0}\n\
I am {1} years old\n\
I work as a {2}""".format(name, age, profession))

# End of our code

This time we’re outputting four different sentences. One from the previous example and the other three are using the same print function, but notice the .format() function at the end of the string definition.

The """ statement in Python means that we are going to create a multi-line string.

The format function allows the programmer to pass variables through to the string. In the above example:

• the name variable (in position 0, referenced in the string as {0})
• the age variable (in position 1, referenced in the string as {1})
• the profession variable (in position 2, referenced in the string as {2}).

Knowing the position of the variables, also known as the index, allows the programmer to repeat the same thing any number of times by simply referencing the position within the string. We will cover this in a future article.

We can define strings in the following ways:

string1 = "This is a string using double-qoutes"
string2 = 'This is a string using single-quotes"
string3 = `This is a string using back-ticks`
string4 = """This is a multi-line string \
using double-quotes"""
string5 = '''This is a multi-line string \
using single-quotes'''

It is best practice to use double-quotes by default, but all of the above are valid python syntax.

Now see if you can complete the following challenges:

• Change the variables so the programme prints out your name, age and profession.
• Change the programme to output everything on one line, instead of four lines.
• Add a new variable, favoriteAnimal , and incorporate that into your outputted sentence.

Tweet us at @RealCodeTips and show us your solutions. If you get stuck along the way, reach out and we’ll do our best to help.