【Python】【Python语言】Python3.7.2的关键字与保留字

本文是学习Python 3 (目前最新是3.7.3rc1版本) 的官方文档时形成的阅读与实验笔记。

Python关键字/保留字

Python 3文档中记述的关键字/保留字列表：https://docs.python.org/3/reference/lexical_analysis.html#keywords

保留字(reserved words)是Python语言预先保留的标识符(identifier)，这些标识符在Python程序中具有特定用途，不能被程序员作为常规变量名，函数名等使用。

关键字（keyword）是Python语言目前正在使用的保留字。目前未被使用的保留字，在未来版本中可能被使用，成为关键字。

注意：Python的保留字是大小写敏感的；除了True、False和None这三个字以外，其他的保留字都是小写单词。

打印Python关键字/保留字列表

从下面的列表中可以看出，相比Python 3.6版的33个关键字，Python 3.7版本中正式引入两个新的关键字async与await，共35个关键字。

import keyword
print(keyword.kwlist)

--- Python 3.6 Console Output ---
['False', 'None', 'True', 'and', 'as', 'assert',                   
'break', 'class', 'continue', 'def', 'del', 'elif', 
'else', 'except', 'finally', 'for', 'from', 'global', 
'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 
'not', 'or', 'pass', 'raise', 'return', 'try', 
'while', 'with', 'yield']

--- Python 3.7 Console Output --- 
['False', 'None', 'True', 'and', 'as', 'assert', 'async', 'await', 
'break', 'class', 'continue', 'def', 'del', 'elif', 
'else', 'except', 'finally', 'for', 'from', 'global', 
'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 
'not', 'or', 'pass', 'raise', 'return', 'try', 
'while', 'with', 'yield']

Python保留字简介

True与False

布尔类型变量的取指，True表示真，False表示假，例如

is_male = True
is_female = False

if、elif与else

交叉参考：else用于异常捕获语句中。
用于构造条件分支语句，例如

if is_male:
	print("You are a man.")
elif is_female:
	print("You are a woman.")
else
	print("You are in the 3rd-gender!")

in

用于测试某个值是否位于列表中。

5 in [5, 6, 7]

del

del用于删除一个变量，或者删除list中的某个元素。

a = "Hello"
b = a

del a # delete variable a
print(b) # we can still print b

l = [1, 3, 5, 7]
del l[1] # delete the 2nd element
print(l) # expect to print out [1, 5, 7]

--- Console Output ---
Hello
[1, 5, 7]

for/while与break

for与while用于构造循环语句，continue用于跳过循环内后续语句，立即开始执行下一次循环迭代，break用于退出循环语句（通常在循环语句正常结束前），例如

# for loop with continue/break
for i in [5, 6, 7, 8, 9]:
	if i == 5:
		continue # skip the first element
	if i == 8:
		break; # end the loop
	print(i)

# while loop with continue/break
i = 5
while i < 10:
    print(i)
    i += 1
    if i == 5:
        continue # skip the first element
    if i == 8:
        break # end the loop

and，or与not

and操作符是对两个布尔值做“与”操作，结果是布尔值
or操作符是对两个布尔值做“或”操作，结果是布尔值
not操作符是对布尔值做“取非”操作，结果是布尔值

is_male = True
is_tall = False

is_tall_and_male = is_male and is_tall
is_tall_or_male = is_male or is_tall
is_not_male = not is_male

print(is_tall_and_male, is_tall_or_male, is_not_male)

def, return与yield

def用于定义函数，return用于从函数中返回，可以带有一个值。
yield用于生成器，当函数中使用yield时，Python解释器会将函数解释为生成器。

# two functions with return
def foo():
    print("in function foo()")
    return # simple return

def square(num):
    return num*num # return a value 

foo()
print(square(5))

# a generator with yield
def fib(max):
	a, b = 1, 1
    while a < max:
        yield a # stop here and return a
        a, b = b, a + b # continue from here for next iteration

for n in fib(15):
    print (n)

class

class用于定义类，用于面向对象的程序设计。

class Dog:
    def __init__(self, name):
        self.__name = name

    def bark(self):
        print(self.__name + " is barking.")

mydog = Dog("Bob")
mydog.bark()

from, import与as

from $package.$module import $class as $new_class

表从 $package.$module 中导入 $class，as指定别名 $new_class，程序调用时，使用别名。

from math import pow as p
print(p(2,3))

assert

assert后面的表达式会计算出一个布尔值，如果为True，则断言成立，程序继续执行；如果为False，则立刻停止程序运行，打印AssertError，以及assert中指定的字符串消息。

assert语句通常用于程序的调试版本，用于断言本应该为True的条件。如果条件不成立，通常认为出现了bug，或者输入错误，assert语句可以在最近的地方停下来让程序员进行检查。注意：assert语句不是用来替代if判断语句的。

x = 4
assert x % 2 == 1, "x is not an odd number"

--- console output ---
Traceback (most recent call last):
  File "C:/Users/tsu5/PycharmProjects/HelloWorld/hello.py", line 2, in <module>
    assert x % 2 == 1, "x is not an odd number"
AssertionError: x is not an odd number

is

is主要用于判断两个变量是否引用同一个对象，即对象的id相同。如果是则返回True，否则返回False。

x = 4
y = 4
z = 5

print("x is at " + str(id(x)))
print("y is at " + str(id(y)))
print("z is at " + str(id(z)))

print("x is y:" + str(x is y))
print("y is z:" + str(y is z))
print("x is z:" + str(x is z))

--- Console Output --- 
x is at 1569055552
y is at 1569055552
z is at 1569055568
x is y:True
y is z:False
x is z:False

pass

pass是空语句，不做任何事情，一般用于做占位符，保持程序结构完整。

x = 5

if x > 3:
    print("x > 3")
else:
    pass # this is a placeholder for future 

None

None表示变量是空值，有点类似于C语言的NULL。None实际上是NoneType类的示例。

print(type(None))

x = None
y = 5
if x is None:
    print("x is None")

if y is not None:
    print("y is not None")

--- Console Output ---
<class 'NoneType'>
x is None
y is not None

try, except, else与finally

try用于捕获后面的语句可能产生的异常；当出现异常时，则执行except后面的语句；没有异常则执行else后面的语句；无论是否出现异常，都是执行finally后面的语句。
交叉参考：else用于if语句中。

x = 0
try:
    print("Let's try 0 / 0")
    x = x / 0
except:
    print("I caught an exception")
else:
    print("Luckily, there is no exception")
finally:
    print("This line is from always-executed finally block")

print("")
try:
    print("Let's try 0 / 1")
    x = x / 1
except:
    print("I caught an exception")
else:
    print("Luckily, there is no exception")
finally:
    print("This line is from always-executed finally block")

--- Console Output ---
Let's try 0 / 0
I caught an exception
This line is from always-executed finally block

Let's try 0 / 1
Luckily, there is no exception
This line is from always-executed finally block

with与as

with语句由Python 2.5开始引入，需要通过下面的import语句导入才可以使用。

from __future__ import with_statement

到了Python2.6，则正式引入，默认可用，无需额外的导入语句。

with语句与“上下文管理器”这个概念密切相关。只有支持上下文管理器的对象，才可以使用with语句进行处理。本文不详细介绍上下文管理器，仅说明支持上下文管理器的对象都会实现__enter__()与__exit()__方法。

with与as的基本语句格式如下：

with context_expression [as target(s)]:
	with-statements

当with语句进入时，会执行对象的__enter__()方法，该方法返回的值会赋值给as指定的目标；当with语句退出时，会执行对象的__exit__()方法，无论是否发生异常，都会进行清理工作。

# print out every line in /etc/fstab
with open(r'/etc/fstab') as f:
    for line in f:
        print(line, end="")

--- Console Output ---
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
#                
# / was on /dev/sda1 during installation
UUID=a586a207-1142-456e-aa2e-fe567327344b /               ext4    errors=remount-ro 0       1
# swap was on /dev/sda5 during installation
UUID=d507d3f1-20db-4ad6-b12e-e83ecee49398 none            swap    sw              0       0

global

global定义通知Python后面的变量是全局变量，不要定义出一个新的局部变量。

x = 6
def local_x():
    x = 7 # acutally defined a new local variable x
    print("x in func = " + str(x))

local_x()
print("global x = " + str(x))

print("")

def global_x():
    global x # tell python x is global
    x = 7 # not define a new local variable. changing global variable.
    print("x in func = " + str(x))

global_x()
print("global x = " + str(x))

--- Console Output ---
x in func = 7
global x = 6

x in func = 7
global x = 7

nonlocal

nonlocal是Python3新增的关键字，用于告知Python后面的变量定义在其他地方，不要在本函数中定义出一个新的局部变量。

def foo():
    x = 6 # this is a local variable in foo

    def change_foo_x():
        x = 7 # intend to change the x defined in foo(), but will fail

    change_foo_x()
    print("foo():x = " + str(x)) # actually failed to change foo():x

foo()

print("")
def foo2():
    x = 6 # this is a local variable in foo2

    def change_foo2_x():
        nonlocal x # search upwards for the x variable
        x = 7 # change the x defined in foo2(), and will succeed

    change_foo2_x()
    print("foo2():x = " + str(x))

foo2()

--- Console Output ---
foo():x = 6

foo2():x = 7

lambda

lambda用于定义一个表达式，实际上相当于定义了一个匿名函数。

# x, y are input parameters : return x + y
g = lambda x, y: x + y
print(g(1,2)) # number 1 + 2, return 3
print(g("2","4")) # string "2" + "4", return "24"
print(g(True, True)) # bool True + True, return 2

--- Console Output ---
3
24
2

await与async

廖雪峰老师的官方网站 aysnc/await介绍