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Python 3文档中记述的关键字/保留字列表：
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。
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 keywordprint(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']
布尔类型变量的取指，True表示真，False表示假，例如
is_male = Trueis_female = False
交叉参考：。
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!") 用于测试某个值是否位于列表中。
5 in [5, 6, 7]
del用于删除一个变量，或者删除list中的某个元素。
a = "Hello"b = adel a # delete variable aprint(b) # we can still print bl = [1, 3, 5, 7]del l[1] # delete the 2nd elementprint(l) # expect to print out [1, 5, 7]
--- Console Output ---Hello[1, 5, 7]
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/breakfor 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/breaki = 5while i < 10: print(i) i += 1 if i == 5: continue # skip the first element if i == 8: break # end the loop
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 = Trueis_tall = Falseis_tall_and_male = is_male and is_tallis_tall_or_male = is_male or is_tallis_not_male = not is_maleprint(is_tall_and_male, is_tall_or_male, is_not_male)
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 returndef foo(): print("in function foo()") return # simple returndef square(num): return num*num # return a value foo()print(square(5)) # a generator with yielddef 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 iterationfor n in fib(15): print (n)
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 $package.$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 pprint(p(2,3))
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 = 4assert 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, inassert x % 2 == 1, "x is not an odd number"AssertionError: x is not an odd number
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 = 4y = 4z = 5print("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 1569055552y is at 1569055552z is at 1569055568x is y:Truey is z:Falsex is z:False
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 = 5if x > 3: print("x > 3")else: pass # this is a placeholder for future 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 = Noney = 5if x is None: print("x is None")if y is not None: print("y is not None") --- Console Output ---x is Noney is not None
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：。x = 0try: print("Let's try 0 / 0") x = x / 0except: 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 / 1except: 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 / 0I caught an exceptionThis line is from always-executed finally blockLet's try 0 / 1Luckily, there is no exceptionThis line is from always-executed finally block
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/fstabwith 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 installationUUID=a586a207-1142-456e-aa2e-fe567327344b / ext4 errors=remount-ro 0 1# swap was on /dev/sda5 during installationUUID=d507d3f1-20db-4ad6-b12e-e83ecee49398 none swap sw 0 0
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 = 6def 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 = 7global x = 6x in func = 7global x = 7
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():xfoo()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 = 6foo2():x = 7
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 + yg = lambda x, y: x + yprint(g(1,2)) # number 1 + 2, return 3print(g("2","4")) # string "2" + "4", return "24"print(g(True, True)) # bool True + True, return 2 --- Console Output ---3242
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