Python Input Methods for Competitive Programming

2.3

Python is an amazingly user friendly language with the only flaw of being slow. In comparison to C, C++ and Java, it is quite slower. On Online coding platforms, if C/C++ limit provided is X. Usually, in Java time provided is 2X and Python, it’s 5X.

To improve the speed of code execution for input/output intensive problems, languages have various input and output procedures.

An Example Problem :

Consider a question of finding the sum of N numbers inputted from the user.
Input a number N.
Input N numbers separated by a single space in a line.
Examples:

Input : 
5
1 2 3 4 5
Output :
10

Different Python solutions for above Problem :

Normal Method Python: (Python 2.7)
1. raw_input() takes an optional prompt argument. It also strips the trailing newline character from the string it returns.
2. print is just a thin wrapper that formats the inputs (space between args and newline at the end) and calls the write function of a given object.

# basic method of input output
# input N
n = int(raw_input())

# input the array
arr = [int(x) for x in raw_input().split()]

# initialize variable
summation = 0

# calculate sum
for x in arr:
    summation += x
    
# print answer
print(summation)

A bit faster method using inbuilt stdin, stdout: (Python 2.7)
1. sys.stdin on the other hand is a File Object. It is like creating any other file object one could create to read input from the file. In this case, the file will be standard input buffer.
2. stdout.write(‘D\n’) is faster than print ‘D’.
3. Even faster is to write all once by stdout.write(“”.join(list-comprehension)) but this makes memory usage dependent on size of input.

# import inbuilt standard input output 
from sys import stdin, stdout 

# suppose a function called main() and
# all the operations are performed
def main():

    # input via readline method
    n = stdin.readline()

    # array input similar method
    arr = [int(x) for x in stdin.readline().split()]

    #initialize variable
    summation = 0
    
    # calculate sum
    for x in arr:
        summation += x

    # could use inbuilt summation = sum(arr)

    # print answer via write
    # write method writes only
    # string operations
    # so we need to convert any
    # data into string for input
    stdout.write(str(summation))

# call the main method
if __name__ == "__main__":
    main()    

Difference in time:
photo
Adding a buffered pipe io: (Python 2.7)
1. Simply, adding the buffered IO code before your submission code to make the output faster.
2. The benefit of io.BytesIO objects is that they implement a common-ish interface (commonly known as a ‘file-like’ object). BytesIO objects have an internal pointer and for every call to read(n) the pointer advances.
3. The atexit module provides a simple interface to register functions to be called when a program closes down normally. The sys module also provides a hook, sys.exitfunc, but only one function can be registered there. The atexit registry can be used by multiple modules and libraries simultaneously.

# template begins
#####################################

# import libraries for input/ output handling
# on generic level
import atexit, io, sys

# A stream implementation using an in-memory bytes 
# buffer. It inherits BufferedIOBase.
buffer = io.BytesIO()
sys.stdout = buffer

# print via here
@atexit.register
def write():
    sys.__stdout__.write(buffer.getvalue())

#####################################
# template ends

# normal method followed
# input N
n = int(raw_input())

# input the array
arr = [int(x) for x in raw_input().split()]

# initialize variable
summation = 0

# calculate sum
for x in arr:
    summation += x

# print answer
print(summation)

While handling large amount of data usually, the normal method fails to execute within the time limit. The method 2 helps in maintaining the large amount of I/O data. The method 3 is fastest. Usually, handling of input data file greater than 2 or 3 MBs is helped via method 2 and 3.

Note : above mention codes are in Python 2.7, to use in Python 3.X versions. Simply replace the raw_input() with Python 3.X’s input() syntax. Rest should work fine.

References:
1.More About Input in Python 2.7
2.Output via sys library and other commands.
3.Input via sys library and other commands.
4. Python atexit Module docs.

This article is contributed by Shubham Saxena. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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