There is no magic bullet that leads to the fastest input and output in a programming contest. Having said this, there are typically multiple ways of performing and processing input and output in Python3. By understanding the options that are available and how to optimize them (e.g., conducting tests) you can see a real world performance increase, in particular when faced with high volumes of reads and writes.

Input in Python3 is handled via the input() function, and reads a newline-terminated string from standard input.¹⁾ Additional processing is done to the resulting string.

>>> x = input()
Hello World! # x="Hello World!"

>>> x = int(input())
42 # x=42

>>> x = input().split()
Hello World! # x=["Hello", "World!"]

>>> (x, y) = map(int, input().split())
1 2 # x=1, y=2

map() applies a function across a sequence or iterable structure. The comma , unpacks the map object into x and y.

>>> x = list(map(float, input().split()))
1.2 2.3 3.4 4.5 # x=[1.2, 2.3, 3.4, 4.5]

Output is handled with the print() function.

>>> print() # '\n'

>>> print("Hello World!") # "Hello World!\n"

>>> print("Hello", "World!") # "Hello World!\n"

>>> print(1, 2, 3, sep='') # "123\n"

>>> print(1, 2, 3, end='-')
>>> print("a b c") # "1 2 3-a b c\n"

input() will scale well for many easy and mid-level contest problems. For mid to upper problems with significant bounds on reads and writes input() will carry an increased risk of time limit exceeded judgements.²⁾

>>> from sys import *
>>> x = stdin.readline()
Hello World! # x="Hello World!\n"

>>> from sys import *
>>> x = stdin.readline().split()
Hello World! # x=["Hello", "World!"]

readlines() is stopped by an End Of File (EOF) character. You can send this from the terminal with <ctrl-D>

>>> from sys import *
>>> x = stdin.readlines()
Hello
To Everyone! # x=["Hello\n", "To Everyone!\n"]

The following benchmarks demonstrate the increased likelihood of failure of input() as input sizes increase. All files used for testing can be found here.

10 characters per line (n= number of lines):

1000 characters per line (n= number of lines):

The readlines() version is actually slower than readline() on the largest dataset. It is attempting to store about 1GB in memory here, causing a slowdown, but still faster than input().