Python
Submissions are either programs or functions. They should print or return the output. These examples compute the factorial of a positive integer.
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)
Your may not expect input in a pre-assigned variable.
# Invalid, expects input in 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, saves result to p
n=input();p=1
for i in range(1,n+1):p*=i
Nor can the result just be the value of an expression like in the interactive shell.
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.
Libaries
You may import standard libraries. Just include the import statement in the code. You may also import non-standard ones like scipy, but then call the language "Python with scipy" in the header.
Truthy/Falsey
Some challenges ask for a Truthy or Falsey output. In Python, Truthy values 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 may use extra optional arguments. For example, this factorial function expects a single number, but uses the optional input i to help recurse:
f=lambda n,i=1:1 if i>n else i*f(n,i+1)
You may not however expect the user to call the function with a specific additional argument, i.e. like
# You may not say "call this with 2nd argument 1"
f=lambda n,i:1 if i>n else i*f(n,i+1)