Python 3 非程序员教程/数到 10
介绍我们的第一个控制结构。通常情况下，计算机从第一行开始，然后向下执行。控制结构改变语句执行的顺序，或决定是否执行某个语句。以下是一个使用 while 控制结构的程序的源代码
a = 0 # FIRST, set the initial value of the variable a to 0(zero).
while a < 10: # While the value of the variable a is less than 10 do the following:
a = a + 1 # Increase the value of the variable a by 1, as in: a = a + 1!
print(a) # Print to screen what the present value of the variable a is.
# REPEAT! until the value of the variable a is equal to 9!? See note.
# NOTE:
# The value of the variable a will increase by 1
# with each repeat, or loop of the 'while statement BLOCK'.
# e.g. a = 1 then a = 2 then a = 3 etc. until a = 9 then...
# the code will finish adding 1 to a (now a = 10), printing the
# result, and then exiting the 'while statement BLOCK'.
# --
# While a < 10: |
# a = a + 1 |<--[ The while statement BLOCK ]
# print (a) |
# --
以下是极其令人兴奋的输出
1 2 3 4 5 6 7 8 9 10
(你以为在将你的电脑变成一台五美元的计算器之后，情况还能更糟吗？)
那么程序到底做了什么呢？首先，它看到 a = 0 这行代码，并将 a 设置为零。然后它看到 while a < 10:，所以计算机检查是否 a < 10。计算机第一次看到这个语句时，a 为零，所以它小于 10。换句话说，只要 a 小于十，计算机就会运行缩进的语句。这最终使得 a 等于十（通过一次又一次地将 a 加一），while a < 10 就不再成立了。到达那个点，程序将停止运行缩进的行。
请始终记住在 while 语句行的末尾加上一个冒号“:”！
以下是以另一种方式使用 while 的示例
a = 1
s = 0
print('Enter Numbers to add to the sum.')
print('Enter 0 to quit.')
while a != 0:
print('Current Sum:', s)
a = float(input('Number? '))
s = s + a
print('Total Sum =', s)
Enter Numbers to add to the sum. Enter 0 to quit. Current Sum: 0 Number? 200 Current Sum: 200.0 Number? -15.25 Current Sum: 184.75 Number? -151.85 Current Sum: 32.9 Number? 10.00 Current Sum: 42.9 Number? 0 Total Sum = 42.9
注意 print('Total Sum =', s) 只在最后执行一次。while 语句只影响使用空格缩进的代码行。!= 表示不等于，所以 while a != 0: 表示只要 a 不等于零，就运行紧随其后的缩进语句。
请注意，a 是一个浮点数，并非所有浮点数都可以精确表示，因此对它们使用 != 有时不起作用。尝试在交互模式下输入 1.1。
现在我们有了 while 循环，就可以编写永远运行的程序了。一个简单的方法是编写一个像这样的程序
while 1 == 1:
print("Help, I'm stuck in a loop.")
运算符“==”用于测试运算符两侧表达式的相等性，就像之前用于“小于”的“<”一样（你将在下一章获得所有比较运算符的完整列表）。
这个程序会一直输出 Help, I'm stuck in a loop.，直到宇宙热寂或者你停止它，因为 1 会永远等于 1。停止它的方法是同时按下 Control（或 Ctrl）键和 C（字母）。这将终止程序。（注意：有时你需要在按下 Control-C 之后按下回车键。）在某些系统上，除了终止进程之外，没有任何方法可以停止它——所以请避免这种情况！
Fibonacci-method1.py
# This program calculates the Fibonacci sequence
a = 0
b = 1
count = 0
max_count = 20
while count < max_count:
count = count + 1
print(a, end=" ") # Notice the magic end=" " in the print function arguments
# that keeps it from creating a new line.
old_a = a # we need to keep track of a since we change it.
a = b
b = old_a + b
print() # gets a new (empty) line.
输出
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181
请注意，输出在一行上，这是因为 print 参数中添加了额外的参数 end=" "。
Fibonacci-method2.py
# Simplified and faster method to calculate the Fibonacci sequence
a = 0
b = 1
count = 0
max_count = 10
while count < max_count:
count = count + 1
print(a, b, end=" ") # Notice the magic end=" "
a = a + b
b = a + b
print() # gets a new (empty) line.
输出
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181
Fibonacci-method3.py
a = 0
b = 1
count = 0
maxcount = 20
#once loop is started we stay in it
while count < maxcount:
count += 1
olda = a
a = a + b
b = olda
print(olda,end=" ")
print()
输出
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181
Password.py
# Waits until a password has been entered. Use Control-C to break out without
# the password
#Note that this must not be the password so that the
# while loop runs at least once.
password = str()
# note that != means not equal
while password != "unicorn":
password = input("Password: ")
print("Welcome in")
示例运行
Password: auo Password: y22 Password: password Password: open sesame Password: unicorn Welcome in
编写一个程序，提示用户输入登录名和密码。然后，当他们输入“lock”时，需要输入用户名和密码来解锁程序。
编写一个程序，提示用户输入登录名和密码。然后，当他们输入“lock”时，需要输入用户名和密码来解锁程序。
name = input("What is your UserName: ")
password = input("What is your Password: ")
print("To lock your computer type lock.")
command = None
input1 = None
input2 = None
while command != "lock":
command = input("What is your command: ")
while input1 != name:
input1 = input("What is your username: ")
while input2 != password:
input2 = input("What is your password: ")
print("Welcome back to your system!")
如果你希望程序持续运行，只需在整个程序周围添加一个 while 1 == 1: 循环。当你将它添加到代码顶部时，需要将程序的其余部分缩进，但别担心，你不需要手动为每一行都这样做！只需突出显示要缩进的所有内容，然后在 python 窗口顶部的工具栏中点击“格式”下的“缩进”即可。
另一种方法是
name = input('Set name: ')
password = input('Set password: ')
while 1 == 1:
nameguess=""
passwordguess=""
key=""
while (nameguess != name) or (passwordguess != password):
nameguess = input('Name? ')
passwordguess = input('Password? ')
print("Welcome,", name, ". Type lock to lock.")
while key != "lock":
key = input("")
注意 while (nameguess != name) or (passwordguess != password) 中的 or，我们还没有介绍它。你可能已经猜到它是如何工作的了。