python 控制語句

pyhton 控制語句

程序在一般情況下是按順序執行的，編程語言提供了各種控制結構，允許復雜的執行路徑。循環語句允許我們執行一個語句或語句多次

if 語句

Python條件語句是通過一條或多條語句的執行結果（True或者False）來決定執行的代碼塊，執行語句可以是單個語句或語句塊。判斷條件可以是任何表達式，任何非零、或非空（null）的值均為true

if True:
    print("Hello World")
print("Goog Bye")
------------------------
Hello World
Goog Bye


if False:
    print("Hello Python")
print("Goog Bye")
------------------------
Goog Bye

if ... else 語句

if False:
    print("No executed")
else:
    print("Executed")
------------------------
Executed

if..elif...else

age = int(input("Please your age>>:"))
if 0 < age and age <= 20:
    print("teenager")
elif 20 < age and age <= 40:
    print("Man")
elif 40 < age and age <= 60:
    print("Old")
else:
    print("Died")
------------------------
Please your age>>:30
Man

 if 嵌套

age = int(input("Please your age>>:"))
if age >= 0:
    if 0 < age and age <= 20:
        print("teenager")
    elif 20 < age and age <= 40:
        print("Man")
    elif 40 < age and age <= 60:
        print("Old")
    else:
        print("Died")
else:
    print("Your age error")
-------------------------
Please your age>>:48
Old

while 循環語句

Python 編程中 while 語句用於循環執行程序，即在某條件下，循環執行某段程序，以處理需要重復處理的相同任務。

Python中while語句的一般形式：
while 判斷條件：
    語句

sum = 0
count = 0
while count <= 100:
    sum += count
    count += 1
print(sum)
-------------------------
5050

while無限循環,可以使用 CTRL+C 來中斷循環
while  True:
    print("無限循環"）

for 語句

for循環可以遍歷任何序列的項目（一個列表或者一個字符串等）

for <variable> in <sequence>:
    <statements>
else:
    <statements>

languages = ["C", "C++", "Perl", "Python"]
for x in languages:
    print (x)

fruits = ['banana', 'apple', 'mango','tomato','pelar']
for fruit in range(len(fruits)):
    print('fruit: ',fruits[fruit])
------------------------------------------------
fruit:  banana
fruit:  apple
fruit:  mango
fruit:  tomato
fruit:  pelar

break 語句

break 語句可以跳出 for 和 while 的循環體。若遇到break而使得 for 或 while 循環中終止而且 else 塊將不執行

 

for megs in 'Hello Python':
    if megs == 'y':
        break
    print ('輸出的當前字母為 :',megs)
--------------------------------
輸出的當前字母為 : H
輸出的當前字母為 : e
輸出的當前字母為 : l
輸出的當前字母為 : l
輸出的當前字母為 : o
輸出的當前字母為 :  
輸出的當前字母為 : P

counts = 0
while counts < 6:
    print("counts:" ,counts)
    if counts == 3:
        break
    counts += 1
---------------------------------
counts: 0
counts: 1
counts: 2
counts: 3

continue語句

continue語句被用來告訴Python跳過當前循環塊中的剩余語句，然后繼續進行下一輪循環

for megs in 'Hello Python':
    if megs == 'y':
        continue
    print ('輸出的當前字母為 :',megs)
--------------------------------
輸出的當前字母為 : H
輸出的當前字母為 : e
輸出的當前字母為 : l
輸出的當前字母為 : l
輸出的當前字母為 : o
輸出的當前字母為 :  
輸出的當前字母為 : P
輸出的當前字母為 : t
輸出的當前字母為 : h
輸出的當前字母為 : o
輸出的當前字母為 : n

counts = 0
while counts < 6:
    counts += 1
    if counts == 3:
        continue
    print("counts:", counts)
--------------------------------
counts: 1
counts: 2
counts: 4
counts: 5
counts: 6

else子句

循環語句可以有 else 子句，它在窮盡列表(以for循環)或條件變為 false (以while循環)導致循環終止時被執行,但循環被break終止時不執行

for...else   

for 循環中使用 break 語句，break 語句用於跳出當前循環體,且不執行else子句，否則執行else子句

sites = ["Baidu", "Google","UC","Taobao"]
for site in sites:
    if site == "Baidu":
        print("李彥宏")
        break
    print("循環數據 " + site)
else:
    print("沒有循環數據!")
print("完成循環!")
--------------------------------
李彥宏
完成循環!

while...else語句

while … else 在條件語句為 false 時執行則else 的語句塊，若遇到break語句則不執行else子句

count = 0
while count < 10:
    print (count, " 小於 10")
    count = count + 1
else:
    print (count, " 大於或等於 10")
-------------------------
0  小於 10
1  小於 10
2  小於 10
3  小於 10
4  小於 10
5  小於 10
6  小於 10
7  小於 10
8  小於 10
9  小於 10
10  大於或等於 10

pass語句

Python pass是空語句，是為了保持程序結構的完整性。pass 不做任何事情，一般用做占位語句

for char in "Hello World":
    if char == 'W':
        pass
        print("執行pass語句")
    print("char: " ,char)
------------------------------
char:  H
char:  e
char:  l
char:  l
char:  o
char:   
執行pass語句
char:  W
char:  o
char:  r
char:  l
char:  d

 簡單示例

#!/usr/bin/env python
# -*- coding:utf-8 -*-
'''
用戶登錄、注銷、注冊
'''
import json

user_dict = {}
cmd_list = ['ls','move','cd','mkdir','rm','touch']

def login():
    while True:
        user_name = input("please enter login user name >>: ")
        with open('data.json','r') as f:
            data  = f.read()
        user_dict = json.loads(data)
        if user_name in user_dict:
            while True:
                user_pawd = input("please enter login user pawd >>: ")
                if user_pawd == user_dict[user_name]:
                    while True:
                        cmd = input("please enter command >>>: ")
                        if cmd in cmd_list:
                            print("Run command %s" %cmd)
                        elif cmd == "quit":
                            break
                        else:
                            print("please cmd error,please enter again!")
                    break
                else:
                    print("Enter login user pawd error,please enter again!")
            break
        else:
            print("Enter login user name error,please enter again!")

def registered():
    while True:
        with open('data.json','r') as f:
            data = f.read()
        user_dict = json.loads(data)
        user_name = input("please enter user name >>>: ")
        if user_name not in user_dict:
            user_pawd = input("please enter user pawd >>>: ")
            user_dict[user_name] = user_pawd
            data = json.dumps(user_dict)
            with open('data.json','w') as f:
                f.write(data)
            break
        else:
            print("Enter user name existed !")

def logout():
    while True:
        user_name = input("please enter delete user name >>: ")
        with open('data.json','r') as f:
            data = f.read()
        user_dict = json.loads(data)
        if user_name in user_dict:
            del user_dict[user_name]
            data = json.dumps(user_dict)
            with open('data.json','w') as f:
                f.write(data)
            break
        else:
            print("user name not existed!")

if __name__ == '__main__':
    while True:
        choice = input(" login \n regist \n quit \n logout \n >>:")
        if choice == "login":
            login()
        elif choice == "regist":
            registered()
        elif choice == "logout":
            logout()
        elif choice == "quit":
            break
        else:
            print("choice error!")