59个Python使用技巧，从此你的Python与众不同！

i = 0
for item in iterable:
print i, item
i += 1

for i, item in enumerate(iterable):
print i, item

>>> list(enumerate('abc')) 
[(0, 'a'), (1, 'b'), (2, 'c')] 

>>> list(enumerate('abc', 1)) 
[(1, 'a'), (2, 'b'), (3, 'c')]

my_dict = {i: i * i for i in xrange(100)} 
my_set = {i * 15 for i in xrange(100)}

# There is only a difference of ':' in both

# 两者的区别在于字典推导中有冒号

from __future__ import division 
result = 1/2
# print(result)

# 0.5

import ast 
my_list = ast.literal_eval(expr) 

expr = "[1, 2, 3]"
my_list = eval(expr)

>>> a = [1,2,3,4]
>>> a[::-1]
[4, 3, 2, 1]

# This creates a new reversed list. 

# If you want to reverse a list in place you can do:


a.reverse()

>>> foo = "yasoob"
>>> foo[::-1]
'boosay'

[on_true] if [expression] else [on_false]
x, y = 50, 25
small = x if x < y else y

import copy
new_list = copy.copy(existing_list)

import copy
new_list_of_dicts = copy.deepcopy(existing_list_of_dicts)

copy(x)
Shallow copy operation on arbitrary Python objects.

deepcopy(x, memo=None, _nil=[])
Deep copy operation on arbitrary Python objects.

首先是C#中字符串的==和equal方法。

“==” :
对于内置值类型而言， == 判断两个内存值是否相等。
对于用户自定义的值类型而言(Struct)， == 需要重载，否则不能使用。
对于引用类型而言，默认是同一引用才返回true，但是系统重载了很多引用类型的 == （比如下文提到的string），所以c#中引用类型的比较并不建议使用 ==。

“equals” :
对于值类型而言， 内存相等才返回true。
对于引用类型而言，指向同一个引用才算相等。
但是比较特殊的是字符串String,是一个特殊的引用型类型，在C#语言中，重载了string的equals()方法，使string对象用起来就像是值类型一样。

python中的 ==

python中的对象包含三要素:id, type, value
id 用来标识唯一一个对象，type标识对象的类型，value用来设置对象的值。
is 判断是否是一个对象，使用id来判断的。
== 是判断a对象的值是否是b对象的值，默认调用它的__eq__方法。

def request(_argv): 

def __setitem__(self, key, value, dict_setitem=dict.__setitem__):
if key not in self:
root = self.__root
last = root[0]
last[1] = root[0] = self.__map[key] = [last, root, key]
return dict_setitem(self, key, value)

def not_list_or_dict(value):
return not (isinstance(value, dict) or isinstance(value, list))

def not_list_or_dict(value, _isinstance=isinstance, _dict=dict, _list=list):
return not (_isinstance(value, _dict) or _isinstance(value, _list))

$ python -m timeit -s 'def not_list_or_dict(value): return not (isinstance(value, dict) or isinstance(value, list))' 'not_list_or_dict(50)'
1000000 loops, best of 3: 0.48 usec per loop
$ python -m timeit -s 'def not_list_or_dict(value, _isinstance=isinstance, _dict=dict, _list=list): return not (_isinstance(value, _dict) or _isinstance(value, _list))' 'not_list_or_dict(50)'
1000000 loops, best of 3: 0.423 usec per loop

pelican==3.3
Markdown
pelican-extended-sitemap==1.0.0

Python 2.7.9 (default, Dec 19 2014, 06:05:48)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.56)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def generate_new_list_with(my_list=[], element=None):
... my_list.append(element)
... return my_list
...
>>> list_1 = generate_new_list_with(element=1)
>>> list_1
[1]
>>> list_2 = generate_new_list_with(element=2)
>>> list_2
[1, 2]
>>>

>>> _ Traceback (most recent call last): 
File "<stdin>", line 1, in <module> 
NameError: name '_' is not defined 
>>> 42
>>> _ 
42
>>> 'alright!' if _ else ':('
'alright!'
>>> _ 
'alright!'

n = 42
for _ in range(n): 
do_something()

from django.utils.translation import ugettext as _ 
from django.http import HttpResponse 
def my_view(request): 
output = _("Welcome to my site.") 
return HttpResponse(output)

>>> class A(object): 
... def _internal_use(self): 
... pass
... def __method_name(self): 
... pass
... 
>>> dir(A()) 
['_A__method_name', ..., '_internal_use']

def foo(x, y):
print x, y

alist = [1, 2]
adict = {'x': 1, 'y': 2}

foo(*alist) # 1, 2

foo(**adict) # 1, 2

>>> x = 3
>>> 1 < x < 5
True
>>> 4 > x >=3
True

>>> def foo(x=[]):
... x.append(1)
... print x
...
>>> foo()
[1]
>>> foo()
[1, 1]

>>> def foo(x=None):
... if x is None:
... x = []
... x.append(1)
... print x
...
>>> foo()
[1]
>>> foo()
[1]
>>>

>>> print "Hello %(name)s !" % {'name': 'James'}
Hello James !
>>> print "I am years %(age)i years old" % {'age': 18}
I am years 18 years old

>>> print "Hello {name} !".format(name="James")
Hello James !

>>> a = [1, 2, 3, 4, 5]
>>> a[::2]
[1, 3, 5]
>>> a[::-1]
[5, 4, 3, 2, 1]
>>>

[(i, j) for i in range(3) for j in range(i)]
[(1, 0), (2, 0), (2, 1)]

In[47]: a = 'abcd'

In[48]: [i+j+k for i in a for j in a.replace(i,'') for k in a.replace(i,'').replace(j,'')]
Out[48]: 
['abc',
'abd',
'acb',
'acd',
'adb',
'adc',
'bac',
'bad',
'bca',
'bcd',
'bda',
'bdc',
'cab',
'cad',
'cba',
'cbd',
'cda',
'cdb',
'dab',
'dac',
'dba',
'dbc',
'dca',
'dcb']

>>> print >> open("somefile", "w+"), "Hello World"

>>> a, b, *rest = range(10)
>>> a
0
>>> b
1
>>> rest
[2, 3, 4, 5, 6, 7, 8, 9]
>>>

>>> first, second, *rest, last = range(10)
>>> first
0
>>> second
1
>>> last
9
>>> rest
[2, 3, 4, 5, 6, 7, 8]

>>> pow(4, 2, 2)
0
>>> pow(4, 2, 3)
1

>>> lst = ["a", "b", "c"]
>>> list(enumerate(lst, 1))
[(1, 'a'), (2, 'b'), (3, 'c')]

>>> {1,2,3}
set([1, 2, 3])

>>> a = [1, 2, 3, 4, 5, 6, 7]
>>> a[1:4] = []
>>> a
[1, 5, 6, 7] 

>>> a = [0, 1, 2, 3, 4, 5, 6, 7]
>>> del a[::2]
>>> a
[1, 3, 5, 7]

>>> isinstance(1, (float, int))
True
>>> isinstance(1.3, (float, int))
True
>>> isinstance("1.3", (float, int))
False

import operator
somelist = [(1, 5, 8), (6, 2, 4), (9, 7, 5)]
somelist.sort(key=operator.itemgetter(0))
somelist
#Output = [(1, 5, 8), (6, 2, 4), (9, 7, 5)]

somelist.sort(key=operator.itemgetter(1))
somelist
#Output = [(6, 2, 4), (1, 5, 8), (9, 7, 5)]

somelist.sort(key=operator.itemgetter(2))
somelist

lowerlist = ['this', 'is', 'lowercase']
upper = str.upper
upperlist = []
append = upperlist.append
for word in lowerlist:
append(upper(word))
print(upperlist)
#Output = ['THIS', 'IS', 'LOWERCASE']

n = 16
myDict = {}
for i in range(0, n):
char = 'abcd'[i%4]
if char not in myDict:
myDict[char] = 0
myDict[char] += 1
print(myDict)

n = 16
myDict = {}
for i in range(0, n):
char = 'abcd'[i%4]
try:
myDict[char] += 1
except KeyError:
myDict[char] = 1
print(myDict)

num = [1, 4, -5, 10, -7, 2, 3, -1]
filtered_and_squared = []

for number in num:
if number > 0:
filtered_and_squared.append(number ** 2)
print filtered_and_squared

# [1, 16, 100, 4, 9]

num = [1, 4, -5, 10, -7, 2, 3, -1]
filtered_and_squared = map(lambda x: x ** 2, filter(lambda x: x > 0, num))
print filtered_and_squared

# [1, 16, 100, 4, 9]


## 更简化的一种写法    

num = [1, 4, -5, 10, -7, 2, 3, -1]
filtered_and_squared = [ x**2 for x in num if x > 0]
print filtered_and_squared

# [1, 16, 100, 4, 9]

num = [1, 4, -5, 10, -7, 2, 3, -1]
filtered_and_squared = ( x**2 for x in num if x > 0 )
print filtered_and_squared

# <generator object <genexpr> at 0x00583E18>


for item in filtered_and_squared:
print item

# 1, 16, 100 4,9

num = [1, 4, -5, 10, -7, 2, 3, -1]

def square_generator(optional_parameter):
return (x ** 2 for x in num if x > optional_parameter)

print square_generator(0)
# <generator object <genexpr> at 0x004E6418>


# Option I

for k in square_generator(0):
print k
# 1, 16, 100, 4, 9


# Option II

g = list(square_generator(0))
print g
# [1, 16, 100, 4, 9]

import os
def tree(top):
for path, names, fnames in os.walk(top):
for fname in fnames:
yield os.path.join(path, fname)

for name in tree('C:\Users\XXX\Downloads\Test'):
print name

import time
from functools import wraps

def timethis(func):
'''
   Decorator that reports the execution time.
   '''
@wraps(func)
def wrapper(*args, **kwargs):
start = time.time()
result = func(*args, **kwargs)
end = time.time()
print(func.__name__, end-start)
return result
return wrapper

@timethis
def countdown(n):
while n > 0:
n -= 1

countdown(100000)

# ('countdown', 0.006999969482421875)

import time
class demo:
def __init__(self, label):
self.label = label

def __enter__(self):
self.start = time.time()

def __exit__(self, exc_ty, exc_val, exc_tb):
end = time.time()
print('{}: {}'.format(self.label, end - self.start))

import time

class demo:
def __init__(self, label):
self.label = label

def __enter__(self):
self.start = time.time()

def __exit__(self, exc_ty, exc_val, exc_tb):
end = time.time()
print('{}: {}'.format(self.label, end - self.start))

with demo('counting'):
n = 10000000
while n > 0:
n -= 1

# counting: 1.36000013351

from contextlib import contextmanager
import time

@contextmanager
def demo(label):
start = time.time()
try:
yield
finally:
end = time.time()
print('{}: {}'.format(label, end - start))

with demo('counting'):
n = 10000000
while n > 0:
n -= 1

# counting: 1.32399988174

class Celsius(object):
def __init__(self, value=0.0):
self.value = float(value)
def __get__(self, instance, owner):
return self.value
def __set__(self, instance, value):
self.value = float(value)

class Temperature(object):
celsius = Celsius()

temp=Temperature()
temp.celsius #calls Celsius.__get__

>>> a = [1, 2, 3]
>>> b = ['a', 'b', 'c']
>>> z = zip(a, b)
>>> z
[(1, 'a'), (2, 'b'), (3, 'c')]
>>> zip(*z)
[(1, 2, 3), ('a', 'b', 'c')]

>>> a = [1, 2, 3, 4, 5, 6]

>>> # Using iterators

>>> group_adjacent = lambda a, k: zip(*([iter(a)] * k))
>>> group_adjacent(a, 3)
[(1, 2, 3), (4, 5, 6)]
>>> group_adjacent(a, 2)
[(1, 2), (3, 4), (5, 6)]
>>> group_adjacent(a, 1)
[(1,), (2,), (3,), (4,), (5,), (6,)]


>>> # Using slices

>>> from itertools import islice
>>> group_adjacent = lambda a, k: zip(*(islice(a, i, None, k) for i in range(k)))
>>> group_adjacent(a, 3)
[(1, 2, 3), (4, 5, 6)]
>>> group_adjacent(a, 2)
[(1, 2), (3, 4), (5, 6)]
>>> group_adjacent(a, 1)
[(1,), (2,), (3,), (4,), (5,), (6,)]

>>> from itertools import islice
>>> def n_grams(a, n):
... z = (islice(a, i, None) for i in range(n))
... return zip(*z)
...
>>> a = [1, 2, 3, 4, 5, 6]
>>> n_grams(a, 3)
[(1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 6)]
>>> n_grams(a, 2)
[(1, 2), (2, 3), (3, 4), (4, 5), (5, 6)]
>>> n_grams(a, 4)
[(1, 2, 3, 4), (2, 3, 4, 5), (3, 4, 5, 6)]

>>> m = {'a': 1, 'b': 2, 'c': 3, 'd': 4}
>>> m.items()
[('a', 1), ('c', 3), ('b', 2), ('d', 4)]
>>> zip(m.values(), m.keys())
[(1, 'a'), (3, 'c'), (2, 'b'), (4, 'd')]
>>> mi = dict(zip(m.values(), m.keys()))
>>> mi
{1: 'a', 2: 'b', 3: 'c', 4: 'd'}

>>> a = [[1, 2], [3, 4], [5, 6]]
>>> list(itertools.chain.from_iterable(a))
[1, 2, 3, 4, 5, 6]

>>> sum(a, [])
[1, 2, 3, 4, 5, 6]

>>> [x for l in a for x in l]
[1, 2, 3, 4, 5, 6]

>>> a = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]]
>>> [x for l1 in a for l2 in l1 for x in l2]
[1, 2, 3, 4, 5, 6, 7, 8]

>>> a = [1, 2, [3, 4], [[5, 6], [7, 8]]]
>>> flatten = lambda x: [y for l in x for y in flatten(l)] if type(x) is list else [x]
>>> flatten(a)
[1, 2, 3, 4, 5, 6, 7, 8]

>>> m = {x: x ** 2 for x in range(5)}
>>> m
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

>>> m = {x: 'A' + str(x) for x in range(10)}
>>> m
{0: 'A0', 1: 'A1', 2: 'A2', 3: 'A3', 4: 'A4', 5: 'A5', 6: 'A6', 7: 'A7', 8: 'A8', 9: 'A9'}

>>> def foo(bar=[]): # bar is optional and defaults to [] if not specified

... bar.append("baz") # but this line could be problematic, as we'll see...

... return bar

>>> foo()
["baz"]
>>> foo()
["baz", "baz"]
>>> foo()
["baz", "baz", "baz"]

>>> def foo(bar=None):
... if bar is None: # or if not bar:

... bar = []
... bar.append("baz")
... return bar
...
>>> foo()
["baz"]
>>> foo()
["baz"]
>>> foo()
["baz"]

>>> x = 10
>>> def foo():
... x += 1
... print x
...
>>> foo()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in foo
UnboundLocalError: local variable 'x' referenced before assignment

>>> lst = [1, 2, 3]
>>> def foo1():
... lst.append(5) # This works ok...

...
>>> foo1()
>>> lst
[1, 2, 3, 5]

>>> lst = [1, 2, 3]
>>> def foo2():
... lst += [5] # ... but this bombs!

...
>>> foo2()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in foo
UnboundLocalError: local variable 'lst' referenced before assignment

>>> odd = lambda x : bool(x % 2)
>>> numbers = [n for n in range(10)]
>>> for i in range(len(numbers)):
... if odd(numbers[i]):
... del numbers[i] # BAD: Deleting item from a list while iterating over it

...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
IndexError: list index out of range

>>> odd = lambda x : bool(x % 2)
>>> numbers = [n for n in range(10)]
>>> numbers[:] = [n for n in numbers if not odd(n)] # ahh, the beauty of it all

>>> numbers
[0, 2, 4, 6, 8]

import copy
a = range(100000)
%timeit -n 10 copy.copy(a) # 运行10次 copy.copy(a)

%timeit -n 10 copy.deepcopy(a)
10 loops, best of 3: 1.55 ms per loop
10 loops, best of 3: 151 ms per loop

%timeit -n 100 a = (i for i in range(100000))
%timeit -n 100 b = [i for i in range(100000)]
100 loops, best of 3: 1.54 ms per loop
100 loops, best of 3: 4.56 ms per loop

%timeit -n 10 for x in (i for i in range(100000)): pass
%timeit -n 10 for x in [i for i in range(100000)]: pass
10 loops, best of 3: 6.51 ms per loop
10 loops, best of 3: 5.54 ms per loop

x, y, z = 1,2,3
%timeit -n 1000000 if x < y < z:pass
%timeit -n 1000000 if x < y and y < z:pass
1000000 loops, best of 3: 101 ns per loop
1000000 loops, best of 3: 121 ns per loop

def while_1():
n = 100000
while 1:
n -= 1
if n <= 0: break
def while_true():
n = 100000
while True:
n -= 1
if n <= 0: break 

m, n = 1000000, 1000000 
%timeit -n 100 while_1()
%timeit -n 100 while_true()
100 loops, best of 3: 3.69 ms per loop
100 loops, best of 3: 5.61 ms per loop

%timeit -n 10000 c = pow(2,20)
%timeit -n 10000 c = 2**20
10000 loops, best of 3: 284 ns per loop
10000 loops, best of 3: 16.9 ns per loop

import cPickle
import pickle
a = range(10000)
%timeit -n 100 x = cPickle.dumps(a)
%timeit -n 100 x = pickle.dumps(a)
100 loops, best of 3: 1.58 ms per loop
100 loops, best of 3: 17 ms per loop

import json
import cPickle
a = range(10000)
s1 = str(a)
s2 = cPickle.dumps(a)
s3 = json.dumps(a)
%timeit -n 100 x = eval(s1)
%timeit -n 100 x = cPickle.loads(s2)
%timeit -n 100 x = json.loads(s3)
100 loops, best of 3: 16.8 ms per loop
100 loops, best of 3: 2.02 ms per loop
100 loops, best of 3: 798 µs per loop

python -mSimpleHTTPServer