Variables
As in other programming languages, it's often essential in Python to save values for later using variables. Python assigns values to variables using the equal sign ("="):
length = 15
width = 3 * 5
length * width
The output is:
225
If you have a programming background in another language, such as Java, you might have noticed that we didn't specify the variable type when we declared our length and width variables. Python doesn't require you to specify type, and you can change variable types freely. For example:
length = 15
length
The output is:
15
You can change length to floating point:
length = 15.0
length
The output is:
15.0
Or, you can change length to a string:
length = 'fifteen'
length
The output is:
'fifteen'
For all the flexibility of variables in Python, you do still have to define them. If you try to use an undefined variable, you'll produce an error:
n
The error output is:
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
<ipython-input-34-ab0680a89434> in <module>
----> 1 n
NameError: name 'n' is not defined
In Python's interactive mode and in Jupyter Notebook, you can use the built-in variable _ (underscore). This variable automatically takes the value of the last printed expression. For example, start with this expression:
tax = 11.3 / 100
price = 19.95
price * tax
The output is:
2.25435
Then, use _ to reference the result of the most recent calculation in the next expression:
price + _
The output is:
22.204349999999998
Always treat the _ variable as read-only. Explicitly assigning a value to it will create an independent local variable with the same name, and will mask the built-in variable and its behavior.
Our last output was kind of a mess, however. We typically use only two decimal points or fewer when working with prices. To clean up the output, we can use a built-in function called round().
round(_, 2)
The output is:
22.2
We'll cover some of the other functions that are built into Python later in this module, and we'll cover user-defined functions in a later module.
You don't have to define variables one at a time. You can define multiple variables on a single line:
a, b, c, = 3.2, 1, 6
a, b, c
The output is:
(3.2, 1, 6)
You can also augment variable assignments. This ability will be useful when we tackle loops in a later module.
x = 5
x = x + 1 # Un-pythonic variable augmentation
x += 1 # Pythonic variable augmentation
x
The output is:
7
Augmented assignment isn't limited to increments of 1, or to addition operations. Beyond +=, augmented assignment, statements in Python include -=, *=, /=, %=, and **=. Try playing around with different augmentation assignments until this concept makes sense.
Python supports other types of numbers beyond int and float, such as Decimal and Fraction. Python also has built-in support for complex numbers, which are beyond the scope of this learning path.