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Variables

Info

The structure of following sections is based on the official Python tutorial.

Many thanks to @jhumci for providing the initial resource materials!

Getting started

Important

We encourage you to execute all upcoming code snippets on your machine. You can easily copy each code snippet to your clipboard, by clicking the icon in the top right corner. By doing so, you will be prepared for all upcoming tasks within the sections. Tasks are indicated by a -icon.

We recommend to create a new notebook for each chapter, e.g. create variables.ipynb for this chapter. Doing so, your notebooks will follow the structure of this crash course.

You will encounter multiple info boxes throughout the course. They provide additional information, tips, and tricks. Be sure to read them thoroughly.

Let's start with the first task.

Notebook & first code cell

Create a new Jupyter notebook and name it variables.ipynb. Paste the following code snippet into a new cell and execute it.

print("Hello World!")

The output should be:

>>> Output
Hello World!
Info 
print(1+1)

The upcoming content contains a lot of code snippets. They are easily recognizable due to their colourful syntax highlighting, such as:

Code snippets are an integral part, to illustrate concepts, which are introduced and explained along the way. Commonly, these code snippets are accompanied by an output block to display the result, for instance:

>>> Output
2

Nevertheless note, output blocks can be missing as there is not always an explicit result.

Again, execute and experiment with all code snippets on your machine to verify the results and get familiar with Python !

Variable

Computers can store lots of information. To do so, in Python we use variables. A variable is a name that refers to a value. The following code snippet, assigns the value 4 to the variable number. In general, you pick the variable name on the left hand side, assign a value with = and the value itself is on the right hand side.

number = 4

You can change the value of a variable in your program at any time, and Python will always keep track of its current value.

number = 4
number = 4000

You will notice that none of the cells had any output. To display the value of a variable we use the print() function.

print()

number = 4
print(number)
>>> Output
4

Now, we can also verify that in the above snippet the value of number was actually changed.

number = 4
print(number)
number = 4000
print(number)
>>> Output
4
4000
Info

Within a notebook, the variables are stored in the background and can be overwritten at any time. Therefore, it is good practice to execute all cells from top to bottom of the notebook in the right order so that nothing unexpected is stored in a variable.

Comments

Comments exist within your code but are not executed. They are used to describe your code and are ignored by the Python interpreter. Comments are prefaced by a #.

# this is a comment
print("Hello World!")
>>> Output
Hello World!
Info

Comments help you and others understand what your code is doing. It is good practice to use comments as a tool for documentation.

Variable naming

When you’re using variables in Python, you need to adhere to a few rules and guidelines. Breaking some of these rules will cause errors; other guidelines just help you write code that’s easier to read and understand. Be sure to keep the following rules in mind:

  • Variable names are lower case and can contain only letters, numbers, and underscores. They can start with a letter or an underscore, but not with a number. For instance, you can call a variable message_1 but not 1_message.
  • Whitespace is not allowed in variable names, but an underscores _ can be used to separate words in variable names. For example, greeting_message works, but greeting message won't.
  • Avoid using Python keywords and function names as variable names; that is, do not use words that Python has reserved for a particular programmatic purpose, such as the word print.
  • Variable names should be short but descriptive. For example, name is better than n, student_name is better than s_n, and name_length is better than length_of_persons_name.

Errors (NameError)

Every programmer makes mistakes and even after years of experience, mistakes are part of the process. With time, you get more efficient in debugging (=process of finding and fixing errors).

Debugging tactics
byu/0ajs0jas inProgrammerHumor

Let’s look at an error you’re likely to make early on and learn how to fix it. We’ll write some code that throws an error message on purpose. Copy the code and run your cell.

message = "Hello Python Crash Course reader!"
print(mesage)

Which should result in:

NameError: name 'mesage' is not defined

When an error occurs in your program, the Python interpreter does its best to help you figure out where the problem is. The interpreter provides a traceback which is a record of where the interpreter ran into trouble when trying to execute your code. Here’s an example of the traceback that Python provides, after you’ve accidentally misspelled a variable’s name:

Traceback (most recent call last):
  File "C:\\IPython\core\interactiveshell.py", line 3577, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-35-c8f2adeaed02>", line 2, in <module>
    print(mesage)
          ^^^^^^
NameError: name 'mesage' is not defined. Did you mean: 'message'?

The output reports that an error occurs in line 2. The interpreter shows this line to help us spot the error quickly and tells us what kind of error it found. In this case, it found a NameError and reports that the variable mesage has not been defined. A name error usually means we made a spelling mistake when entering the variable’s name or that the variable simply does not exist.

Your first fix

Fix the NameError in your code cell.

Recap

In this section, we have covered variables in Python .

You have learned (about):

  • To create and assign a value to a variable
  • print() to display the value of a variable
  • Comments
  • Naming conventions for variables
  • How to fix a NameError