弄懂这56个Python使用技巧(轻松掌握Python高效开发)

i = 0for 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, 25small = x if x < y else y

import copynew_list = copy.copy(existing_list)

import copynew_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.

def request(_argv):

collections.OrderedDict类：
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==3.3Markdown

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 = 42for _ 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, 2foo(**adict) # 1, 2

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

> 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 !

可以用步长 -1 来反转链表：> 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)

> a0> b1> rest

[2, 3, 4, 5, 6, 7, 8, 9]

>

> first, second, *rest, last = range(10)

> first0> second1> last9> 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.upperupperlist = []

append = upperlist.appendfor word in lowerlist:

 append(upper(word))

 print(upperlist) #Output = ['THIS', 'IS', 'LOWERCASE']

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

 myDict[char] = 0

 myDict[char] += 1

 print(myDict)

n = 16myDict = {}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 Ifor k in square_generator(0):  print k# 1, 16, 100, 4, 9# Option IIg = 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 timefrom functools import wrapsdef 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@timethisdef countdown(n):

  while n > 0:

    n -= 1countdown(100000)# ('countdown', 0.006999969482421875)

import timeclass 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 timeclass 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 contextmanagerimport time@contextmanagerdef 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 fooUnboundLocalError: 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 fooUnboundLocalError: 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 copya = 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 loop10 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 loop100 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)]: pass10 loops, best of 3: 6.51 ms per loop10 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:pass1000000 loops, best of 3: 101 ns per loop1000000 loops, best of 3: 121 ns per loop

def while_1():

 n = 100000

 while 1:

 n -= 1

 if n <= 0: breakdef 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 loop100 loops, best of 3: 5.61 ms per loop

%timeit -n 10000 c = pow(2,20)

%timeit -n 10000 c = 2**2010000 loops, best of 3: 284 ns per loop10000 loops, best of 3: 16.9 ns per loop

import cPickleimport 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 loop100 loops, best of 3: 17 ms per loop

import jsonimport 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 loop100 loops, best of 3: 2.02 ms per loop100 loops, best of 3: 798 μs per loop