Python中的关键字（python中的关键字是什么意思）

Python中的关键字（python中的关键字是什么意思）

Python关键字是python编程语言的保留字。这些关键字不能用于其他目的。

Python中有35个关键字-下面列出了它们的用法。

Keyword

Description

and

A logical AND operator. Return True if both statements are True.

x = (5 > 3 and 5 < 10)

print(x)    # True

or

A logical OR operator. Returns True if either of two statements is true. If both statements are false, the returns False.

x = (5 > 3 or 5 > 10)

print(x)    # True

as

It is used to create an alias.

import calendar as c

print(c.month_name[1])  #January

assert

It can be used for debugging the code. It tests a condition and returns True , if not, the program will raise an AssertionError.

x = "hello"

assert x == "goodbye", "x should be 'hello'"  # AssertionError

async

It is used to declare a function as a coroutine, much like what the @asyncio.coroutine decorator does.

async def ping_server(ip):

await

It is used to call async coroutine.

async def ping_local():

    return await ping_server('192.168.1.1')

class

It is used to create a class.

class User:

  name = "John"

  age = 36

def

It is used to create or define a function.

def my_function():

  print("Hello world !!")

my_function()

del

It is used to delete objects. In Python everything is an object, so the del keyword can also be used to delete variables, lists, or parts of a list, etc.

x = "hello"

del x

if

It is used to create conditional statements that allows us to execute a block of code only if a condition is True.

x = 5

if x > 3:

  print("it is true")

elif

It is used in conditional statements and is short for else if.

i = 5

if i > 0:

    print("Positive")

elif i == 0:

    print("ZERO")

else:

    print("Negative")

else

It decides what to do if the condition is False in if..else statement.

i = 5

if i > 0:

    print("Positive")

else:

    print("Negative")

It can also be use in try...except blocks.

x = 5

try:

    x > 10

except:

    print("Something went wrong")

else:

    print("Normally execute the code")

try

It defines a block of code ot test if it contains any errors.

except

It defines a block of code to run if the try block raises an error.

try:

    x > 3

except:

    print("Something went wrong")

finally

It defines a code block which will be executed no matter if the try block raises an error or not.

try:

    x > 3

except:

    print("Something went wrong")

finally:

     print("I will always get executed")

raise

It is used to raise an exception, manually.

x = "hello"

if not type(x) is int:

    raise TypeError("Only integers are allowed")

False

It is a Boolean value and same as 0.

True

It is a Boolean value and same as 1.

for

It is used to create a for loop. A for loop can be used to iterate through a sequence, like a list, tuple, etc.

for x in range(1, 9):

    print(x)

while

It is used to create a while loop. The loop continues until the conditional statement is false.

x = 0

while x < 9:

    print(x)

    x = x + 1

break

It is used to break out a for loop, or a while loop.

i = 1

while i < 9:

    print(i)

    if i == 3:

        break

    i += 1

continue

It is used to end the current iteration in a for loop (or a while loop), and continues to the next iteration.

for i in range(9):

    if i == 3:

        continue

    print(i)

import

It is used to import modules.

import datetime

from

It is used to import only a specified section from a module.

from datetime import time

global

It is used to create global variables from a no-global scope, e.g. inside a function.

def myfunction():

    global x

    x = "hello"

in

1. It is used to check if a value is present in a sequence (list, range, string etc.).

2. It is also used to iterate through a sequence in a for loop.

fruits = ["apple", "banana", "cherry"]

if "banana" in fruits:

    print("yes")

for x in fruits:

    print(x)

is

It is used to test if two variables refer to the same object.

a = ["apple", "banana", "cherry"]

b = ["apple", "banana", "cherry"]

c = a

print(a is b)   # False

print(a is c)   # True

lambda

It is used to create small anonymous functions. They can take any number of arguments, but can only have one expression.

x = lambda a, b, c : a + b + c

print(x(5, 6, 2))

None

It is used to define a null value, or no value at all. None is not the same as 0, False, or an empty string.

None is a datatype of its own (NoneType) and only None can be None.

x = None

if x:

  print("Do you think None is True")

else:

  print("None is not True...")      # Prints this statement

nonlocal

It is used to declare that a variable is not local. It is used to work with variables inside nested functions, where the variable should not belong to the inner function.

def myfunc1():

    x = "John"

    def myfunc2():

        nonlocal x

        x = "hello"

    myfunc2()

    return x

print(myfunc1())

not

It is a logical operator and reverses the value of True or False.

x = False

print(not x)    # True

pass

It is used as a placeholder for future code. When the pass statement is executed, nothing happens, but you avoid getting an error when an empty code is not allowed.

Empty code is not allowed in loops, function definitions, class definitions, or in if statements.

for x in [0, 1, 2]:

            pass

return

It is to exit a function and return a value.

def myfunction():

            return 3+3

with

Used to simplify exception handling

yield

To end a function, returns a generator

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