Variables and Datatypes

Every programming language has its own grammar rules just like the languages we speak.

Keywords and Identifiers

The following identifiers are used as reserved words, or keywords of the language, and cannot be used as ordinary identifiers. They must be typed exactly as written here:

False      class      finally    is         return
None       continue   for        lambda     try
True       def        from       nonlocal   while
and        del        global     not        with
as         elif       if         or         yield
assert     else       import     pass
break      except     in         raise

In Python we don’t specify what kind of data we are going to put in a variable. So you can directly write abc = 1 and abc will become an integer datatype. If you write abc = 1.0 abc will become of floating type. Here is a small program to add two given numbers

>>> a = 13
>>> b = 23
>>> a + b
36

From the above example you can understand that to declare a variable in Python , what you need is just to type the name and the value. Python can also manipulate strings They can be enclosed in single quotes or double quotes like

>>> 'India'
'India'
>>> 'India\'s best'
"India's best"
>>> "Hello World!"
'Hello World!'

Reading input from the Keyboard

Generally the real life Python codes do not need to read input from the keyboard. In Python we use input function to do input. input(“String to show”) , this will return a string as output. Let us write a program to read a number from the keyboard and check if it is less than 100 or not. Name of the program is testhundred.py

The output

$ ./testhundred.py
Enter an integer: 13
Your number is smaller than 100
$ ./testhundred.py
Enter an integer: 123
Your number is greater than 100

In the next program we are going to calculate investments.

The output

$ ./investment.py
Enter amount: 10000
Enter Interest rate: 0.14
Enter period: 5
Year 1 Rs. 11400.00
Year 2 Rs. 12996.00
Year 3 Rs. 14815.44
Year 4 Rs. 16889.60
Year 5 Rs. 19254.15

Some Examples

Some examples of variables and datatypes:

Average of N numbers

In the next program we will do an average of N numbers.

The output

$ ./averagen.py
1
2.3
4.67
1.42
7
3.67
4.08
2.2
4.25
8.21
N = 10 , Sum = 38.800000
Average = 3.880000

Temperature conversion

In this program we will convert the given temperature to Celsius from Fahrenheit by using the formula C=(F-32)/1.8

The output

$ ./temperature.py
Fahrenheit Celsius
0.0  -17.78
25.0   -3.89
50.0   10.00
75.0   23.89
100.0   37.78
125.0   51.67
150.0   65.56
175.0   79.44
200.0   93.33
225.0  107.22
250.0  121.11

Multiple assignments in a single line

You can even assign values to multiple variables in a single line, like

>>> a , b = 45, 54
>>> a
45
>>> b
54

Using this swapping two numbers becomes very easy

>>> a, b = b , a
>>> a
54
>>> b
45

To understand how this works, you will have to learn about a data type called tuple. We use comma to create tuple. In the right hand side we create the tuple (we call this as tuple packing) and in the left hand side we do tuple unpacking into a new tuple.

Below we have another example of tuple unpacking.

>>> data = ("Kushal Das", "India", "Python")
>>> name, country, language = data
>>> name
'Kushal Das'
>>> country
'India'
>>> language
'Python'

Tuples can not be modified. You will have to create another new tuple if you want have any changes. Many times, we create variables written in CAPS to mark values which will not be changed when the program is running (constants). For example, if we think about colours as tuples of RGB values, then we can define them as:

>>> RED = (255, 0, 0)
>>> GREEN = (0, 255, 0)
>>> BLUE = (0, 0, 255)
>>> print(RED)
(255, 0, 0)

Formatting strings

In Python 3, there are a few different ways to format a string. We use these methods to format a text dynamically. I will go though a few examples below.

In Python 3.6, we have a new way to do string formatting. PEP 498 introduces the concept called f-strings.

Here is the same example using f-strings:

>>> name = "Kushal"
>>> language = "Python"
>>> msg = f"{name} loves {language}."
>>> print(msg)
Kushal loves Python.

F-strings provide a simple and readable way to embed Python expressions in a string. Here are a few more examples.

>>> answer = 42
>>> print(f"The answer is {answer}")
The answer is 42
>>> import datetime
>>> d = datetime.date(2004, 9, 8)
>>> f"{d} was a {d:%A}, we started the mailing list back then."
'2004-09-08 was a Wednesday, we started the mailing list back then.'

If you want to know more about how this feature came into Python, watch this talk from Mariatta Wijaya.

From Python3.8 we can use the following style to help printing values along with the variable name. This is very useful while debugging the code.

>>> a = 1
>>> b = 2
>>> print(f"{a=} and {b=}")
a=1 and b=2

.format method

We can also use format method in a string inside.

>>> name = "Kushal"
>>> language = "Python"
>>> msg = "{0} loves {1}.".format(name, language)
>>> print(msg)
Kushal loves Python.

Left of right aligned

We can use :< or :> in .format call to print left or right aligned. In the following example we are printing first left aligned (10 characters) and right aligned.

>>> print("{:<10}".format("Sweden"))
Sweden
>>> print("{:>10}".format("Sweden"))
    Sweden