=======================
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*
.. image:: img/testhundred.png
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.
.. image:: img/investment.png
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.
.. image:: img/averagen.png
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
.. image:: img/temperature.png
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