Python IO and Import

Python provides some built-in functions that are readily available at the Python prompt

Some of the functions like input()and print() are widely used for standard input and output operations

Python Output Using print() function

The print() function is used to output data to the standard output device (screen)

>>>print("A string")
A string
var = 15
>>>print("A variable:", var)
A variable: 15

In the second print() statement, a space was added between the string and the value of variable var

This is by default, but can be configured

Data can also be written to a file

The 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

The sepseparator is used between the values, which defaults into a space character

After all values are printed, endis printed, which defaults into a new line

The file is the object where the values are printed and its default value is sys.stdout (screen)

Output formatting

Output can also be formatted by using str.format() method of a string str

This method is accessible by string objects

>>> x = 15; y = 20
>>>print('Value of y is {} and x is {}'.format(y, x))
Value of y is 20 and x is 15

Here the curly braces {} are used as placeholders

Values are assigned to the placeholders with the arguments of format function, and in the order provided

Keyword arguments can also be used to format the string

>>>print('{name} can be {age} years old'.format(age=16, name='Johnson'))
Johnson can be 16 years old

Strings can also be formatted like the old sprintf()style used in C programming language

The % operator is used here with variable type

>>> y = 15.3156789
>>>print('The value of y is %3.2f'%y)
The value of y is 15.32
>>>print('The value of y is %3.4f'%y)
The value of y is 15.3157 

Python Input

A program might require input to be taken from the user

Python provides the input() function to facilitate this

The syntax for input() is :

input([prompt])

where promptis the string to be displayed on the screen, which is optional

>>> num = input('Enter a number: ')
Enter a number: 20
>>> num
'20' 

Here, the entered value 20 is taken as a string, not a number

To convert this into a number, int()or float()functions can be used

>>>int('20')
20
>>>float('20')
20.0 

This same operation can be performed using the eval()function

It can evaluate even expressions, provided the input is a string

>>>int('4+5')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '4+5'
>>>eval('4+5')
9

Python Import

When a program grows bigger, it is often broken into different modules

A module is a file containing Python definitions and statements

Python modules have a filename and end with the extension .py

Definitions inside a module can be imported to another module or the interactive interpreter in Python

The import keyword is used to do this

For example, math module can be imported by import math

The definitions inside mathmodule are then available in current file or scope

Some specific attributes and functions can also be imported from a module, by using the from keyword

>>>from math import pi
>>> pi
3.141592653589793 

While importing a module, Python looks at several places defined in sys.path, which is a list of directory locations

>>>import sys
>>> sys.path
['', '/usr/lib/python36.zip', '/usr/lib/python3.6', '/usr/lib/python3.6/lib-dynload', '/usr/local/lib/python3.6/dist-packages', '/usr/lib/python3/dist-packages'] 

More locations can also be added to this list