Python input/output

Python language provides numerous built-in functions that are readily available to us at the Python prompt. Some of the functions like input() and print() are widely used for standard input and output operations respectively. Let us see the output section first.

Output

We use the print() function to output data to the standard output device (screen). We can also output data to a file, but this will be discussed later. An example use is given below.

>>> print('This sentence is output to the screen')
This sentence is output to the screen
>>> a = 5
>>> print('The value of a is',a)
The value of a is 5

In the second example, we can notice that a space was added between the string and the value of variable a. This is by default, but we can change it. The actual syntax of the print() function is

print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)

Here, objects is the value(s) to be printed. sep is the separator used between the values. It defaults into a space character. end is printed after printing all the values. It defaults into a new line. file is the object where the values are printed and its default value is sys.stdout (screen). Here are an example to illustrate this.

print(1,2,3,4)
print(1,2,3,4,sep='*')
print(1,2,3,4,sep='#',end='&')

Output

1 2 3 4
1*2*3*4
1#2#3#4&

Output formatting

Sometimes we would like to format our output to make it look attractive. This can be done by using the str.format() method. This method is visible to any string object.

>>> x = 5; y = 10
>>> print('The value of x is {} and y is {}'.format(x,y))
The value of x is 5 and y is 10

Here the curly braces {} are used as placeholders. We can specify the order in which it is printed by using numbers (tuple index).

>>> print('I love {0} and {1}'.format('bread','butter'))
I love bread and butter
>>> print('I love {1} and {0}'.format('bread','butter'))
I love butter and bread

We can even use keyword arguments to format the string.

>>> print('Hello {name}, {greeting}'.format(greeting='Goodmorning',name='John'))
Hello John, Goodmorning

We can even format strings like the old sprintf() style used in C programming language. We use the % operator to accomplish this.

>>> x = 12.3456789
>>> print('The value of x is %3.2f' %x)
The value of x is 12.35
>>> print('The value of x is %3.4f' %x)
The value of x is 12.3457

Input

Up till now, our programs were static. The value of variables were defined or hard coded into the source code. To allow flexibility we might want to take the input from the user. In Python, we have the input() function to allow this. The syntax for input() is

input([prompt])

where prompt is the string we wish to display on the screen. It is optional.

>>> num = input('Enter a number: ')
Enter a number: 10
>>> num
'10'

Here, we can see that the entered value 10 is a string, not a number. To convert this into a number we can use int() or float() functions.

>>> int('10')
10
>>> float('10')
10.0

This same operation can be performed using the eval() function. But it takes it further. It can evaluate even expressions, provided the input is a string

>>> int('2+3')
Traceback (most recent call last):
  File "<string>", line 301, in runcode
  File "<interactive input>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '2+3'
>>> eval('2+3')
5