Scope & arguments
Python Basics Lesson 11 3:04 English narration · English + 中文 subtitles burned in
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Transcript
Think of your program as a room, and every function call as a smaller room inside it.
把你的程序想象成一个房间，每一次函数调用是里面的一个小房间。
A variable made inside the function belongs to that small room: it is created when the call starts and thrown away when the call ends.
在函数里创建的变量属于那个小房间：调用开始时它被创建，调用结束时被丢掉。
Outside, that name simply does not exist, and asking for it gives a NameError.
在外面，那个名字根本不存在，去访问它会得到 NameError。
This is good news, not a restriction — it means two functions can both use a variable called total without ever interfering.
这是好事，不是限制——它意味着两个函数都可以使用一个叫 total 的变量， 而彼此永远不会打架。
A name defined at the top level of the file is global, and a function can read it happily — a pass mark or a tax rate is a fair thing to look up.
在文件最外层定义的名字是全局的，函数可以放心地读它—— 像及格线或者税率这种东西，去查一下是很合理的。
But if you assign to it inside a function, Python does something surprising: it makes a brand-new local name with the same spelling, and the global is untouched.
但如果你在函数里面给它赋值，Python 会做一件让人意外的事： 它会新建一个拼写相同的局部名字，而全局的那个毫发无损。
So the habit worth building is to pass it in as a parameter and return the result.
所以值得养成的习惯是：把它作为参数传进去，再把结果返回出来。
Then a function's answer depends only on what you handed it.
这样函数的答案就只取决于你交给它的东西。
A parameter can carry a fallback value, written with an equals sign in the definition.
形参可以带一个默认值，在定义里用等号写出来。
Then the caller may leave it out, and the fallback is used — greet of Mia gives Hi, Mia.
这样调用者可以把它省略，用的就是默认值——greet("Mia") 得到 "Hi, Mia!"。
Or they may supply your own greeting, and that wins.
调用者也可以自己给一个问候语，那就用他给的。
One rule of grammar: parameters with defaults must come last in the list, because Python matches the ones without defaults by position first.
有一条语法规则：带默认值的形参必须放在最后， 因为 Python 会先按位置去匹配那些没有默认值的形参。
Now the part that quietly surprises people.
现在说那个悄悄让人吃惊的地方。
Pass a number and the function gets a copy: adding one inside changes nothing outside, so x is still five.
传一个数字，函数拿到的是副本： 在里面加 1，外面什么也没变，x 还是 5。
Pass a list and the function gets the same list, not a copy — appending inside really does change the caller's list.
传一个列表，函数拿到的就是同一个列表，不是副本—— 在里面 append，调用者的列表真的会被改变。
That is often exactly what you want, and it is a bug when you did not expect it.
这常常正是你想要的，而在你没预料到时它就是个 bug。
If you need to keep the original, make a copy first.
如果需要保留原来的列表，先复制一份。
Now the lesson's third task: a function that says whether x lies between low and high, boundaries included.
现在做课程里的第三道题：写一个函数，判断 x 是否在 low 和 high 之间，包含边界。
The body is one line — low at most x, and x at most high — and you return the comparison itself, because a comparison already IS True or False.
函数体只有一行——low 小于等于 x，并且 x 小于等于 high—— 而且你直接把这个比较返回出去，因为比较的结果本身就是 True 或 False。
Test three values: one comfortably inside gives True, one on the boundary also gives True because the test uses at-most, and one just outside gives False.
测三个值：一个稳稳在里面的，得到 True； 一个正好落在边界上的，也得到 True，因为用的是小于等于； 一个刚刚超出去的，得到 False。
Four things to take with you.
带走四点。
One: a name made inside a function dies with the call, which is why two functions never clash.
第一：函数里创建的名字随着这次调用一起消失， 所以两个函数永远不会撞名。
Two: a global can be read inside a function, but assigning to it makes a local instead — prefer passing it in.
第二：全局变量可以在函数里读，但给它赋值只会造出一个局部变量—— 更好的做法是把它传进去。
Three: a default value lets a caller leave an argument out.
第三：默认值让调用者可以省略某个实参。
Four: numbers and strings are copied into a call, while a list is shared, so a function can change it.
第四：数字和字符串是被复制进调用的，而列表是共享的，所以函数能改动它。
Now do the three tasks.
现在去做那三道题。