use join instead of + to concat a list of strings;
#bad
s = ""
for substring in list:
s += substring
#good
"".join(list)
#bad with direct concatenation
out = "<html>" + head + prologue + query + tail + "</html>"
#good with format output
out = "<html>%s%s%s%s</html>" % (head, prologue, query, tail)
Loops
use map cuz it is written in C, period.
#bad
newlist = []
for word in oldlist:
newlist.append(word.upper())
#good
newlist = map(str.upper, oldlist)
List comprehension and Generator expressions are very compact and super efficient way to write for loop:
#good: list comprehension
newlist = [s.upper() for s in oldlist]
#good: generator expression
iterator = (s.upper() for s in oldlist)
Avoiding dots
Both newlist.append and word.upper are function references that are reevaluated each time through the loop.
we can safely store these function in the first place, and just call them through the loop;
#bad:
newlist = []
for word in oldlist:
newlist.append(word.upper())
#good: store the refered functions
upper = str.upper #store the str.upper function
newlist = []
append = newlist.append #store the newlist.append function
for word in oldlist:
append(upper(word))
Local Variable(Function rocks)
Python accesses local variables much more efficiently than global variables
use local variables whenever possible;
how: creating functions to do a specific operation;
def func():
upper = str.upper
newlist = []
append = newlist.append
for word in oldlist:
append(upper(word))
return newlist
Time consumption of abovementioned methods:
Version Time (seconds)
Basic loop 3.47
Eliminate dots 2.45
Local variable & no dots 1.79
Using map function 0.54
Dict
dict's optimization is simple but plays an super vital role in python code optimization.
Suppose you are building a dictionary of word frequencies and you've already broken your text up into a list of words. You might execute something like:
#ordinary
wdict = {}
for word in words:
if word not in wdict:
wdict[word] = 0
wdict[word] += 1
#good
wdict = {}
get = wdict.get
for word in words:
wdict[word] = get(word, 0) + 1
#多使用大名鼎鼎的dict.get(key, defaultValue)方法
