python-tricks.md

Cool Python Tricks

1. List/Map Comprehensions

This is a shortcut for writing an iterator to create a list. (Lists are like Ruby arrays.)

Details in the docs.

2. Python Generators for Large (or Infinte) Data Sets

For a large data set, a generator allows for lazy evaluation of each item one at a time, rather than loading the entire data set into memory for iteration.

For creating a generator for a large range of numbers, Python has the [built-in xrange method][xrange-python].

Example:

# Large data set
# Example: sum of a large set of numbers

total = sum(n for n in xrange(1, 1000000000))
=> 499999999500000000

# Infinite data set
# Example: Finding the nth next leap year

import datetime

def leap_year_generator(n):
  current_year = datetime.datetime.now().year
  i = 0
  year = current_year
  while i < n:
    if (year % 4 == 0):
	    if (year % 100 != 0 or year % 400 == 0):
				# it's a leap year
				i += 1
				yield year
    year += 1
      
# using the generator
for yr in leap_year_generator(10):
  print "It's leap year:", yr

For more, try the Advanced Generators Tutorial at Dream in Code.