Python snippets

Python is full of cool shortcuts that I can't seem to remember when I need to, so I'm gonna document them here.

Default dictionary

This can be used to avoid having to check for existence of keys by providing default values.          
from collections import defaultdict

    from collections import defaultdict
    counter = defaultdict(int) # Default to integer set to 0
    for akey in alist:
        counter[akey] += 1

Or one to many lists.

    from collections import defaultdict

    onetomany = defaultdict(list)
    for akey, anitem in acollection:
        onetomany[akey].append(anitem)

Output Latex Formats

print r"\begin{tabular}{|l|l|}"
print r"\hline"
print r"MAC & IP\\"
print r"\hline"
for snd,rcv in ans:
    print rcv.sprintf(r"%Ether.src% & %ARP.psrc%\\")
print r"\hline"
print r"\end{tabular}"

List Comprehensions

Convert a string to an array of ASCII characters

[ord(ch) for ch in "Hallo"]

Nested(?) access to list
in: r = [1,2]
in: [ (x, y) for x in r for y in r ]
out: [(1, 1), (1, 2), (2, 1), (2, 2)]

Create various shapes and sizes of sets of points

from random import random as rnd
_2x1 = [rnd() for _ in range(2)]
_3x2 = [[rnd() for _ in range(2)] for _ in range(3)]
_4x3x2 = [[[rnd() for _ in range(2)] for _ in range(3)] for _ in range(4)] 
ThreePoints = [[rnd(),rnd()] for _ in range(3)]

Strange Objects

Evaluate completely dynamic code

a=5
b="a*a"
eval(b)
25

Language Features

Built in list iteration

# x is a list of integers.

In : x = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

In : x[::1]  # Every item

Out: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]In : x[::2]  # Get every 2nd item

Out: [0, 2, 4, 6, 8]In : x[::3]  # Every 3rd item

Out: [0, 3, 6, 9]

# Backwards

In : x[::-1]Out: [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]In : x[::-2]Out: [9, 7, 5, 3, 1]

Decorators to dynamically add functionality to class instance from StackOverflow

class Pizza(object):
    def __init__(self):
        self.toppings = []
    def __call__(self, topping):
        # Use '@instance_of_pizza' before a function def and the function gets passed onto 'topping'
        self.toppings.append(topping())
    def __repr__(self):
        return str(self.toppings)
myPizza = Pizza()
@myPizza
def cheese():
    return 'cheese'
@myPizza
def sauce():
    return 'sauce'
print myPizza
# ['cheese', 'sauce']

Iterators allow objects to work with for statements, require 'next' method

class PizzaFactory(object):
    def __init__(self):
        self.orders = {1:'ted", 2:"joe"}
    def __iter__(self):
        return OrderIterator(self)

class OrderIterator(object):

    def __init__(self, container):

        self.items = container

        self.index = 0

    def next(self):

        if self.index >= len(self.items): raise StopIteration

        ret = self.items[self.index]

        self.index += 1
        return ret

# pf = PizzaFactory()
for order in pizza: print order
1:ted
2:joe

Generators allow function to return intermediate values

def fib(n):

  a = 0

  yield a

  b = 1

  yield b

  while b < n:

    a, b = b, a + b

    yield b

# for e in fib(5): print e

0

1

1

2

3

5

Ternary operator

def factorial(n):
  return 1 if n == 0 else n * factorial(n - 1)
print factorial(5)