Python

Introduction to Python

First Python Program

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Prashant Lakhera

This is the introduction to Python Series in which the below Program shows how to take input from Keyboard and ask for Name and Age

#First Python Program to ask for age and name
print("What is your name:")
name = input()
print("Hello: " + name)
print("Length of your name is: ")
print(len(name))
print("What is your age: ")
age = input()
print("You are growing old " + name + " " + age)

Output

python3 firstprogram.py
What is your name:
Prashant
Hello: Prashant
Length of your name is: 
8
What is your age: 
33
You are growing old Prashant 33

Now let see what going on

• Python reads the program from the top and read line by line(which refer as execution)
• # is used as comment to make your program more readable and completely ignored by Python
• print is a function(mini-programs in our program)which display the values passed to it
• When input() function is called python is waiting for user to enter some text via keyboard,it like a variable which holds some value(input always holds the string value)
• Then we are performing string concatenation
• What len does it takes a string value and returns an integer value of the string length

>>len("Prashant")
8

As mentioned above input return a string so we must cast it, if we are working with numbers

>>> age = input(“What is your age: “)
What is your age: 5
>>> type(age)
<class ‘str’>
# Which is not what we are expecting
>>> age * 5
'55555'

And the way to do it

>>> age = int(input(“What is your age: “))
What is your age: 5
>>> age * 5
25

Now let look at some data types,first and second are self explanatory but in the last case, as we see it return as string

>>> a = 4
>>> type(a)
<class ‘int’>
>>> b = “hello”
>>> type(b)
<class ‘str’>
>>> c = “4”
>>> type(c)
<class ‘str’>

As we can’t perform mathematical operation on string, we must need to convert them to integer

>>> “4” + 2
Traceback (most recent call last):
File “<stdin>”, line 1, in <module>
TypeError: must be str, not int
>>> int(“4”) + 2
6

Same way, if we need to concatenate string with integer we need to convert integer to string

>>> “how old are you” + 2
Traceback (most recent call last):
File “<stdin>”, line 1, in <module>
TypeError: must be str, not int
>>> “how old are you” + “ 2”
‘how old are you 2’

Some interesting thing to note while we are doing variable assignment/swapping variable in case of Python

>>> x = 1
>>> y = 2
>>> y = x
>>> x = y
>>> x
1
>>> y
1

We are not expecting this output, we want x=2 and y=1 so we need to be really careful so the better way to do that is using extra variable called temp.

>>> x = 1
>>> y = 2
>>> temp = y
>>> y = x
>>> x = temp
>>> x
2
>>> y
1