# 30 个 Python 教程和技巧

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## 技巧1 就地交换两个数字

Python 提供了一种在一行中进行赋值和交换的直观方式。请参考下面的例子。

``````x, y = 10, 20
print(x, y)

x, y = y, x
print(x, y)

#1 (10, 20)
#2 (20, 10)
``````

## 技巧2 比较运算符的链接。

``````n = 10
result = 1 < n < 20
print(result)

# True

result = 1 > n <= 9
print(result)

# False
``````

## 技巧3 使用三元运算符进行条件赋值。

``````[on_true] if [expression] else [on_false]
``````

``````x = 10 if (y == 9) else 20
``````

``````x = (classA if y == 1 else classB)(param1, param2)
``````

``````def small(a, b, c):
return a if a <= b and a <= c else (b if b <= a and b <= c else c)

print(small(1, 0, 1))
print(small(1, 2, 2))
print(small(2, 2, 3))
print(small(5, 4, 3))

#Output
#0 #1 #2 #3
``````

``````[m**2 if m > 10 else m**4 for m in range(50)]

#=> [0, 1, 16, 81, 256, 625, 1296, 2401, 4096, 6561, 10000, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576, 625, 676, 729, 784, 841, 900, 961, 1024, 1089, 1156, 1225, 1296, 1369, 1444, 1521, 1600, 1681, 1764, 1849, 1936, 2025, 2116, 2209, 2304, 2401]
``````

## 技巧4 使用多行字符串

``````multiStr = "select * from multi_row \
where row_id < 5"
print(multiStr)

# select * from multi_row where row_id < 5
``````

``````multiStr = """select * from multi_row
where row_id < 5"""
print(multiStr)

#select * from multi_row
#where row_id < 5
``````

``````multiStr= ("select * from multi_row "
"where row_id < 5 "
"order by age")
print(multiStr)

#select * from multi_row where row_id < 5 order by age
``````

## 技巧5 将列表元素存储到新变量中

``````testList = [1,2,3]
x, y, z = testList

print(x, y, z)

#-> 1 2 3
``````

## 技巧6 打印导入模块的文件路径

``````import threading
import socket

print(socket)

#2- <module 'socket' from '/usr/lib/python2.7/socket.py'>
``````

## 技巧7 使用交互式“_”运算符

``````>>> 2 + 1
3
>>> _
3
>>> print _
3
``````

“_”引用上次执行的表达式的输出。

## 技巧8 字典/集合理解

``````testDict = {i: i * i for i in xrange(10)}
testSet = {i * 2 for i in xrange(10)}

print(testSet)
print(testDict)

#set([0, 2, 4, 6, 8, 10, 12, 14, 16, 18])
#{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
``````

## 技巧9 调试脚本

``````import pdb
pdb.set_trace()
``````

## 技巧10 设置文件共享

Python 允许运行 HTTP 服务器，您可以使用它从服务器根目录共享文件。下面是启动服务器的命令。

### Python 2

``````python -m SimpleHTTPServer
``````

### Python 3

``````python3 -m http.server
``````

## 技巧11 在 Python 中检查对象

``````test = [1, 3, 5, 7]
print( dir(test) )
``````
``````['__add__', '__class__', '__contains__', '__delattr__', '__delitem__', '__delslice__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', '__setattr__', '__setitem__', '__setslice__', '__sizeof__', '__str__', '__subclasshook__', 'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']
``````

## 技巧12 简化 if 语句

``````if m in [1,3,5,7]:
``````

``````if m==1 or m==3 or m==5 or m==7:
``````

## 技巧13 在运行时检测 Python 版本

``````import sys

#Detect the Python version currently in use.
if not hasattr(sys, "hexversion") or sys.hexversion != 50660080:
print("Sorry, you aren't running on Python 3.5\n")
sys.exit(1)

#Print Python version in a readable format.
print("Current Python version: ", sys.version)
``````

``````Python 2.7.10 (default, Jul 14 2015, 19:46:27)
[GCC 4.8.2] on linux

Sorry, you aren't running on Python 3.5

``````

``````Python 3.5.1 (default, Dec 2015, 13:05:11)
[GCC 4.8.2] on linux

Current Python version:  3.5.2 (default, Aug 22 2016, 21:11:05)
[GCC 5.3.0]
``````

## 技巧14 组合多个字符串

``````>>> test = ['I', 'Like', 'Python', 'automation']
``````

``````>>> print ''.join(test)
``````

## 技巧15 反转 string/list 的四种方法

### 反转列表本身

``````testList = [1, 3, 5]
testList.reverse()
print(testList)

#-> [5, 3, 1]
``````

### 在循环中迭代时反转

``````for element in reversed([1,3,5]): print(element)

#1-> 5
#2-> 3
#3-> 1
``````

### 反转一个字符串

``````"Test Python"[::-1]
``````

### 使用切片反转列表

``````[1, 3, 5][::-1]
``````

## 技巧16 玩枚举

``````testlist = [10, 20, 30]
for i, value in enumerate(testlist):
print(i, ': ', value)

#1-> 0 : 10
#2-> 1 : 20
#3-> 2 : 30
``````

## 技巧17 在 Python 中使用枚举。

``````class Shapes:
Circle, Square, Triangle, Quadrangle = range(4)

print(Shapes.Circle)
print(Shapes.Square)
print(Shapes.Triangle)

#1-> 0
#2-> 1
#3-> 2
#4-> 3
``````

## 技巧18 从函数返回多个值。

``````# function returning multiple values.
def x():
return 1, 2, 3, 4

# Calling the above function.
a, b, c, d = x()

print(a, b, c, d)
``````

#-> 1 2 3 4

## 技巧19 使用 splat 运算符解包函数参数。

splat 运算符提供了一种解压参数列表的艺术方式。为清楚起见，请参阅以下示例。

``````def test(x, y, z):
print(x, y, z)

testDict = {'x': 1, 'y': 2, 'z': 3}
testList = [10, 20, 30]

test(*testDict)
test(**testDict)
test(*testList)

#1-> x y z
#2-> 1 2 3
#3-> 10 20 30
``````

## 技巧20 使用字典来存储 switch。

``````stdcalc = {
'sum': lambda x, y: x + y,
'subtract': lambda x, y: x - y
}

print(stdcalc['sum'](9,3))
print(stdcalc['subtract'](9,3))

#1-> 12
#2-> 6
``````

## 技巧21 计算一行中任意数字的阶乘。

### Python 2.x.

``````result = (lambda k: reduce(int.__mul__, range(1,k+1),1))(3)
print(result)
#-> 6
``````

### Python 3.x.

``````import functools
result = (lambda k: functools.reduce(int.__mul__, range(1,k+1),1))(3)
print(result)
``````

## 技巧22 查找列表中出现频率最高的值。

``````test = [1,2,3,4,2,2,3,1,4,4,4]
print(max(set(test), key=test.count))

#-> 4
``````

## 技巧23 重置递归限制。

Python 将递归限制限制为 1000。我们可以重置它的值。

``````import sys

x=1001
print(sys.getrecursionlimit())

sys.setrecursionlimit(x)
print(sys.getrecursionlimit())

#1-> 1000
#2-> 1001
``````

## 技巧24 检查对象的内存使用情况。

### Python 2.7.

``````import sys
x=1
print(sys.getsizeof(x))

#-> 24
``````

### Python 3.5.

``````import sys
x=1
print(sys.getsizeof(x))

#-> 28
``````

## 技巧25 使用 __slots__ 减少内存开销。

``````import sys
class FileSystem(object):

def __init__(self, files, folders, devices):
self.files = files
self.folders = folders
self.devices = devices

print(sys.getsizeof( FileSystem ))

class FileSystem1(object):

__slots__ = ['files', 'folders', 'devices']

def __init__(self, files, folders, devices):
self.files = files
self.folders = folders
self.devices = devices

print(sys.getsizeof( FileSystem1 ))

#In Python 3.5
#1-> 1016
#2-> 888
``````

## 技巧26 Lambda 模仿打印功能。

``````import sys
lprint=lambda *args:sys.stdout.write(" ".join(map(str,args)))
lprint("python", "tips",1000,1001)

#-> python tips 1000 1001
``````

## 技巧27 从两个相关序列创建字典。

``````t1 = (1, 2, 3)
t2 = (10, 20, 30)

print(dict (zip(t1,t2)))

#-> {1: 10, 2: 20, 3: 30}
``````

## 技巧28 在线搜索字符串中的多个前缀。

``````print("http://www.baidu.com".startswith(("http://", "https://")))
print("https://juejin.cn".endswith((".com", ".cn")))

#1-> True
#2-> True
``````

## 技巧29 形成一个统一的列表，不使用任何循环。

``````import itertools
test = [[-1, -2], [30, 40], [25, 35]]
print(list(itertools.chain.from_iterable(test)))

#-> [-1, -2, 30, 40, 25, 35]
``````

``````def unifylist(l_input, l_target):
for it in l_input:
if isinstance(it, list):
unifylist(it, l_target)
elif isinstance(it, tuple):
unifylist(list(it), l_target)
else:
l_target.append(it)
return l_target

test =  [[-1, -2], [1,2,3, [4,(5,[6,7])]], (30, 40), [25, 35]]

print(unifylist(test,[]))

#Output => [-1, -2, 1, 2, 3, 4, 5, 6, 7, 30, 40, 25, 35]
``````

``````import more_itertools

test = [[-1, -2], [1, 2, 3, [4, (5, [6, 7])]], (30, 40), [25, 35]]

print(list(more_itertools.collapse(test)))

#Output=> [-1, -2, 1, 2, 3, 4, 5, 6, 7, 30, 40, 25, 35]
``````

## 技巧30 在 Python 中实现真正的 switch-case 语句。

``````def xswitch(x):
return xswitch._system_dict.get(x, None)

xswitch._system_dict = {'files': 10, 'folders': 5, 'devices': 2}

print(xswitch('default'))
print(xswitch('devices'))

#1-> None
#2-> 2
``````