Download General The null object: None Tuple (): immutable and for

General The null object: None Tuple (): immutable and for heterogeneous data List []: mutable and for homogeneous data Dict {}: associative array, e.g., {'one':1, 'two':2} Non-­‐keyworded var-­‐length arg list: *args Keyworded var-­‐length arg list: **kwargs Swapping: a,b=b,a
zip([1,2,3],['a','b','c']) returns the list of tuples [(1,
'a'),(2,'b'),(3,'c')]
Different ways of looping through a list L: for index in range(len(L)): for item in L:
for index, item in enumerate(L):
it = iter(L)
it.next()
# raises exception if no more elements
Looping through multiple lists: for x,y in zip(lst1,lst2)
for x,y in map(None,lst1,lst2)
Retrieving values from a dict:
map(somedict.get, keys)
or [somedict[key] for key in keys] Creating a list of lists: [[0 for col in range(nCols)] \
for row in range(nRols)]
note that the following duplicates references to the first col:
[[0]*nCols]*nRows # DO NOT use
Cloning a list which may contain sub-­‐lists: copy.deepcopy Adding item to dictionary value that is a list: somedict.setdefault(key,[]).append(val)
Reduction is underused and powerful, e.g., the multiplication equivalent of sum: import operator
reduce(operator.mul, [1,2,3,4])
Exception handling try:
if len(args) == 0:
raise Exception('No args')
except Exception as exc:
print exc.message
else:
...
finally:
...
Data conversion Byte list → string: array('B', lst).tostring() String → byte list: array('B', s).tolist() Text processing Unicode string: u"Hello World" Multi-­‐line strings: "line 1\
line2"
"""line1
line2"""
Joining strings: '\n'.join(listoflines)
Process characters: map(func, s)
Check if string contains any character from some set: from functools import reduce
from operator import and_, or_
reduce(or_, map(s.__contains__, someset))
Alignment: ljust, rjust, center, e.g.
'Hello world'.ljust(width)
Outputting Unicode strings: sys.stdout=codecs.lookup('utf-8')[-1](sys.stdout)
print(s) Regular expression x* matches 0 or more repetitions of x x+ matches 1 or more repetitions of x x? matches 0 or 1 appearance of x *?, +?, ?? make the qualifiers non-­‐greedy ^ matches the start of a string x{m} matches m copies of x x{m,n} matches m to n copies of x x{m,n}? matches the least number of copies of x Lookahead assertion:
(?=x) matches if x matches next (?!x) matches if x does not match next Lookbehind assertion: (?<=x) matches if x matches previously (?<!x) matches if x does not match previously Searching and sorting Python used to use a hybrid of samplesort (a quicksort variant) for big lists and binary insertion sort for small lists; it now uses timsort Decorate-­‐sort-­‐undecorate: # sort by surname, then
orglst = ['Peter Pan', 'Peter Parker']
auxlst = map(lambda x: (x.split()[-1], x), orglst)
auxlst.sort()
# sort a list of tuples
orglst = map(lambda x: x[-1], auxlst)
To reverse-­‐sort strings, reverse the strings first (Python 2.x): import string
allchars = string.maketrans('','')
revlst = list(allchars)
revlst.reverse()
revchars = ''.join(revlst)
revs = 'abcdef'.translate(string.maketrans(\
allchars, revchars))
Priority queue: import bisect
q = []
# list of tuples of task ID and priority
tasks = [('Task 1',1), ('Task 2',3), ('Task 3',2)]
for t in tasks:
bisect.insort(q, (t[1], t[0]))
Itertools module Iterators: count, cycle, repeat Object-­‐oriented programming class SomeClass:
def __init__(self):
self.data = init_data
def SomeFunc(self, d):
self.data = d
Check/get for attribute: hasattr(o,attr), getattr(o,attr)
Files Writing to file: print >> file, sometext
file.write(sometext) Networking TCP server: from socket import *
sin = socket(AF_INET, SOCK_STREAM)
sin.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
sin.bind(('localhost',someport))
sin.listen(1)
try:
while 1:
sconn, addr = sin.accept()
while 1:
recvdata = sconn.recv(1024)
if not recvdata: break
sconn.close()
finally:
sin.close()
Threads, processes and synchronization Extending and embedding References David Ascher, Anna Martelli Ravenscroft, and Alex Martelli, “Python Cookbook,” 2nd ed., O'Reilly Media, 2005.