3.1.4. Variables

Variables are fundamental in programming. A variable is a space in memory to store a value. For example, we could create two variables, a and b, that both store a different number:

a = 4
b = 3

From there, we can refer to the value of a variable by referring to its name:

a + 1

5

a + b

7

We can even assign the result of an operation to a new variable:

c = a + b

c

7

Note

Contrarily to other programming languages such as C/C++, we do not need to declare a variable before assigning a value to it. The variable is created as we assign its value.

Keep in mind that in the last example, we did not instruct Python that c must always be equal to a + b. This is not how a sequential programming language such as Python works. Instead, we instructed Python, at this very instant, to calculate the result of a + b and to store it in a new variable named c. This sequential nature allows is illustrated in this example:

c = c + 1

c

8

where we instructed Python to calculate the result of c + 1 and to store it in the variable c.

Tip

The exemple above can be written in a shorter form using the increment += operator. This notation is very common and may help avoid typographic errors in more complex statements.

These shorthands exist for every arithmetical operation:

a += b      # equivalent to a = a + b
a -= b      # equivalent to a = a - b
a *= b      # equivalent to a = a * b
a /= b      # equivalent to a = a / b

Good practice: Variable names

It is generally a good idea to use words rather than letters for variable names. For example, this code:

velocity = distance / duration

is clearer than:

v = d / t

In addition, the standard Python coding style recommends to use all lower case for variables, and to generally separate multiple words by underscores (_), e.g., power, mean_power, peak_power.