Assigning variables

At the end of the previous chapter, we ran a simple Python function for the sake of testing:

print('Hello world!')

Here, "Hello world" is an argument, that is, a data point used as an input for the function. In this particular case, we used a raw data value. However, this approach won't get us far—what if we need to change this value, or use it in some other code? This can be done by using variables!

Variables are one of the most basic and powerful concepts in programming. You can think of them as aliases, similar to variables in math equations. Variables are representations of the actual underlying data in the code, which allow us to write operations and describe relations without knowing the exact values the code will operate on. This allows us to write generalized code, which can be used multiple times and in different situations.

In order to assign any value to the variable, we use the equals sign (=), as shown in the following line of code:

<variable_name> = <value>

Take a look at these examples showing the equals sign used to assign any type of value to variables:

pi = 3.14159265359    # Decimal
name = ‘Philipp'      # Text
age = 31              # Integer
sky_is_blue = True    # Boolean

Note the text after the # sign on the same lines as code. Those are comments. They are ignored by the code (because of the symbol before them) and are very useful for explaining the code, jotting an idea, or commenting on specific solutions. Most editors, including VS Code and Jupyter, comment and uncomment whole lines with the command/Ctrl + / shortcut key.

Once a variable is assigned, it is stored in the machine's memory for the entire session, until script execution is over or the notebook's kernel is shut down. Now we can use them in our code:

>>> print(pi)
3.14159265359

Sometimes, it is convenient to assign a few variables on the same line by using packing, which makes the code more readable:

>>> x, y = 10, 5
>>> print(x)
10

>>> print(y)
5

We can always reassign the variable by using the same process:

>>> pi = 'Philipp'
>>> print(pi)
Philipp

Reassignment can be also done with packing, like this: x, y = y, x.

If a variable is not defined, then Python will raise an error. This is a classic Jupyter rookie mistake: people tend to skip cells with variable assignment in the notebooks. The following is an example code block for this:

>>> print(non_existent_variable)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-1-4e13ec8d6c49> in <module>()
----> 1 print(non_existent_variable)
NameError: name 'non_existent_variable' is not defined

In addition to the notation used in the preceding snippet, integers and floats have additional ways to be defined. Integers can use an underscore (_) to mark the number of thousands for readability purposes, for example, ten_million = 10_000_000. Floats do not need this first zero if the value is lower than one, for example, small = .25. On top of this, floats support scientific notations, for example, sci_thousand = 10e3.

Table of Contents for Assigning variables

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Table of Contents for
Assigning variables