Python关键字和标识符

关键字是Python中一些预定义和保留的词，具有特殊的含义。关键字用于定义编码的语法。关键字不能用作标识符、函数和变量名。除了True和False之外， Python中的所有关键字都是小写的。 Python 3.7 中共有 33 个关键字，让我们一一介绍。

Python关键字总数

No.	Keywords	Description
1	and	This is a logical operator it returns true if both the operands are true else return false.
2	Or	This is also a logical operator it returns true if anyone operand is true else return false.
3	not	This is again a logical operator it returns True if the operand is false else return false.
4	if	This is used to make a conditional statement.
5	elif	Elif is a condition statement used with if statement the elif statement is executed if the previous conditions were not true
6	else	Else is used with if and elif conditional statement the else block is executed if the given condition is not true.
7	for	This is created for a loop.
8	while	This keyword is used to create a while loop.
9	break	This is used to terminate the loop.
10	as	This is used to create an alternative.
11	def	It helps us to define functions.
12	lambda	It used to define the anonymous function.
13	pass	This is a null statement that means it will do nothing.
14	return	It will return a value and exit the function.
15	True	This is a boolean value.
16	False	This is also a boolean value.
17	try	It makes a try-except statement.
18	with	The with keyword is used to simplify exception handling.
19	assert	This function is used for debugging purposes. Usually used to check the correctness of code
20	class	It helps us to define a class.
21	continue	It continues to the next iteration of a loop
22	del	It deletes a reference to an object.
23	except	Used with exceptions, what to do when an exception occurs
24	finally	Finally is use with exceptions, a block of code that will be executed no matter if there is an exception or not.
25	from	The form is used to import specific parts of any module.
26	global	This declares a global variable.
27	import	This is used to import a module.
28	in	It’s used to check if a value is present in a list, tuple, etc, or not.
29	is	This is used to check if the two variables are equal or not.
30	None	This is a special constant used to denote a null value or avoid. It’s important to remember, 0, any empty container(e.g empty list) do not compute to None
31	nonlocal	It’s declared a non-local variable.
32	raise	This raises an exception
33	yield	It’s ends a function and returns a generator.

标识符：标识符是用于标识变量、函数、类、模块等的名称。标识符是字符数字和下划线的组合。标识符应以字符或下划线开头，然后使用数字。字符是 AZ 或 az、下划线 (_) 和数字 (0-9)。我们不应在标识符中使用特殊字符（#、@、$、%、!）。

有效标识符的示例：

1. 变量1
2. _var1
3. _1_var
4. var_1

无效标识符的例子

1. !var1
2. 1var
3. 1_var
4. 变量#1

示例 1： and、or、not、True、False 关键字示例。

Python

print("example of True, False, and, or not keywords")
 
#  compare two operands using and operator
print(True and True)
 
# compare two operands using or operator
print(True or False)
 
# use of not operator
print(not False)

输出：

example of True, False, and, or not keywords
True
True
True

示例 2：中断、继续的示例。

Python

# execute for loop
for i in range(1, 11):
     
    # print the value of i
    print(i)
     
    # check the value of i is less then 5
    # if i lessthen 5 then continue loop
    if i < 5: 
        continue
         
    # if i greather then 5 then break loop
    else: 
        break

输出：

1
2
3
4
5

示例 3： for、in、if、elif 和 else 关键字示例。

Python

# run for loop
for t in range(1, 5):
  # print one of t ==1
    if t == 1:
        print('One')
   # print two if t ==2
    elif t == 2:
        print('Two')
    else:
        print('else block execute')

输出：

One
Two
else block execute
else block execute

示例 4： def、if 和 else 关键字示例。

Python

# define GFG() function using def keyword
def GFG():
    i=20
    # check i is odd or not
    # using if and else keyword
    if(i % 2 == 0):
        print("given number is even")
    else:
        print("given number is odd")   
     
# call GFG() function   
GFG()

输出：

given number is even

示例 5：示例 try、except、raise。

Python

def fun(num):
    try:
        r = 1/num
    except:
        print('Exception raies')
        return
    return r
 
print(fun(10))
print(fun(0))

输出：

0.1
Exception raies
None

示例 6： lambda 关键字示例。

Python

# define a anonymous using lambda keyword
# this lambda function increment the value of b
a = lambda b: b+1
 
# run a for loop
for i in range(1, 6):
    print(a(i))

输出：

2
3
4
5
6

示例 7：使用 return 关键字。

Python

# define a function
def fun():
  # declare a variable
    a = 5
    # return the value of a
    return a
# call fun method and store
# it's return value in a variable 
t = fun()
# print the value of t
print(t)

输出：

5

示例 8：使用 del 关键字。

Python

# create a list
l = ['a', 'b', 'c', 'd', 'e']
 
# print list before using del keyword
print(l)
 
del l[2]
 
# print list after using del keyword
print(l)

输出：

['a', 'b', 'c', 'd', 'e']
['a', 'b', 'd', 'e']

示例 9 ：使用 global 关键字。

Python

# declare a variable
gvar = 10
 
# create a function
def fun1():
  # print the value of gvar
    print(gvar)
 
# declare fun2()
def fun2():
  # declare global value gvar
    global gvar
    gvar = 100
 
# call fun1()
fun1()
 
# call fun2()
fun2()

输出：

10

示例 10： yield 关键字示例。

Python

def Generator():
    for i in range(6):
        yield i+1
 
t = Generator()
for i in t:
    print(i)

输出：

1
2
3
4
5
6

示例 10： assert 关键字示例。

蟒蛇3

def sumOfMoney(money):
    assert len(money) != 0,"List is empty."
    return sum(money)
 
money = []
print("sum of money:",sumOfMoney(money))

输出：

AssertionError: List is empty.