Variables #

An important concept in programming are variables. A variable is nothing but a naming of something that we will want to use later. Programmers use variables to store data so that their code is more readable and they don't have to remember specific values.

Let's say you want to name a string with your name as name. It would be written like this:

>>> name = 'Ola'

The variable name will now have the value 'Ola'.

As you may have noticed, this command did not return anything - Python did not output any result. How do you then find out if the variable really exists?

Enter the variable name alone (i.e. name, without quotation marks) and press Enter:

>>> jmeno
'Ola'

Try to set a different variable - for example, your favorite color.

>>> color = 'blue'
>>> color
'blue'

You can assign a value to a variable anytime again and thus change what is hidden under the given name:

>>> name
'Ola'
>>> name = "Soňa"
>>> name
'Soňa'

You can also pass it to a function or use it in an expression. Python will substitute the current value for the variable name.

>>> len(name)
4
>>> name * 4
'SoňaSoňaSoňaSoňa'

Great, isn't it? The variable can contain anything, for example numbers. Try this:

>>> width = 4
>>> length = 6
>>> width * length
24

But what if you use the wrong name? Can you guess what will happen?

>>> city = "Tokyo"
>>> ccity
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'ccity' is not defined

Error!

Python has various types of errors. This one is called NameError. Python will return this error if you try to use a variable that has not been set yet. It is often a typo. So when you see NameError, check if you made a typo when you were setting or using the variable.

Variable names #

Professional programmers name variables in English so that as many colleagues around the world as possible can understand them.

In any case, it is good not to use diacritics and avoid capital letters: use name instead of Name.

Try it out: Which of these names will Python allow you to use as a variable?

• button5
• 5button
• button
• favorite color
• favorite-color
• favoriteColor

In more complex variable names, an underscore is used. For example, favorite_color will be considered as one word, the name of one variable, by Python, but a person sees two words.

>>> favourite_color = 'blue'
>>> favourite_color.upper()
'BLUE'

Summary #

• Variables are names for values.
• By using the assignment operator (=), you can set a variable to any value.
• Variables are named using lowercase letters without diacritics.
• We can use an underscore to separate words within a variable name.