Python CookBook 3 Reading Note Part 1

Data Structures

  • Python “start expressions” can be used to unpack elements from iterable elements.
def drop_first_last(grades):
    first, *middle, last = grades            
    return middle
  • Defaultdict will automatically create a dictionary entry for keys accessed late on. But it can be changed by using setdefault()
d = {}
d.setdefault('a',[]).append(1)

  • An Ordered Dict internally maintains a double linked list that orders the keys according to the insertion order

  • it is often useful to invert the keys and values of the dictionary using zip().

 prices = { 'A' : 23, 'B' : 24, 'C' : 25 }
 min_price = min(zip(prices.values(),prices.keys())
  • Just simply performing the common set operations using the keys() and items() method could find commonalities between two dictionaries.
a.keys() & b.keys()
a.items() - b.items()

  • A useful induction expression: val = item if key is None else key(item)

  • Slice obj can be used to extract a sepcific part from List

record = '...123......'        
SLICE = slice(3,5)
record[SLICE]
  • Determining the Most Frequently reoccuriring Items in the Sequence.
#words is a word sequences
from collections import Counter
word_counts = Counter(words)
top_three = word_count.most_common(3)
print(top_three)
  • Some sorting techniques:
#dict
sorted(rows,key=itemgetter('fname')
#obj
sorted(users, key=attrgetter('user_id')
  • extract subset of a Dictionary p1 = dict((key,value) for key,value in prices.items() if value > 200)

SA tips

  • Terminating a program with an error message raise SystemExit('It failed')

  • Putting limits on Memory and CPU Usage

import signal
import resource
import os

def time_exceed(signo, frame):
    priint('Time is up!')
    raise SystemExit(1)

def set_max_runtime(seconds):
    soft, hard = resource.getrlimit(resource.RLIMIT_CPU)
    resource.setrlimit(resource.RLIMIT_CPU, (seconds, hard));
    signal.signal(signal.SIGXCPU, time_exceed)

if __name__ == '__main__':
    set_max_runtime(15)
        while True:
            pass


def limit_memory(maxsize):
    soft, hard = resource.getrlimit(resource.RLIMIT_AS)
    resource.setrlimit(resource.RLIMIT_AS, (maxsize,hard))
Written on January 4, 2014