Python
Submissions are either programs or functions. They should print or return the output. These examples compute the factorial.
General I/O
# Program that prints
n=input();p=1
for i in range(1,n+1):p*=i
print p
# Defined function that outputs or prints
def f(n):
p=1
for i in range(1,n+1):p*=i
print p
# Lambda function, no name needed
lambda n:reduce(int.__mul__,range(1,n+1))
# Lambda function, named to use recursive call
f=lambda n:1 if n==0 else n*f(n-1)
Snippets are not valid. You may not expect input in a pre-assigned variable.
# Invalid, expects input in variable n
p=1
for i in range(1,n+1):p*=i
print p
Nor may you output just by saving the result to a variable.
# Invalid, only saves output to variable
n=input();p=1
for i in range(1,n+1):p*=i
Inputs
We're pretty free about input formats. For example, if a challenge says to take a list of numbers, you can assume it will be in a Python list like l=[1,2,3], not like "1 2 3". So, a Python 2 program can do
l=input()
rather than
l=map(int,raw_input().split())
Likewise, when a program takes string input, you can assume it's given in quotes and use input() for Python 2 rather than raw_input().
Version
Use Python 2 or Python 3 as you prefer. If the version matters, say it in the header.
Truthy/Falsey
Some challenges mention giving a Truthy or Falsey output. In Python, Truthy values x are those where bool(x) == True. That is, everything but False, 0, None, and empty collections, which are all Falsey.
More on functions
A function submission may have additional helper code outside the function definition, for example
import re;r=range
lambda l: ...
Functions can have extra optional arguments. For example, the factorial function is called with a single number, but can use the helper input i to help recurse:
f=lambda n,i=1:1 if i>n else i*f(n,i+1)