Python代码调试

Question

I'm wondering what's the problem with this piece of code:

list_=[]

def inicialize_list_(list_):
    list_=[]
    return list_

def create_list_(list_):

   list_=inicialize_list_(list_)

   i=0

   for num in range(8):
       list_.append(num)
       i=i+1

   return list_

create_list_(list_)

print list_

create_list_(list_)

print list_

This is the output I'm getting:

[]
[]

But this is the expected output:

[0, 1, 2, 3, 4, 5, 6, 7]
[0, 1, 2, 3, 4, 5, 6, 7]

Answer 1

You're not assigning the return value of create_list_ back to list_ .

If you write:

list_ = create_list_(list_)

It'll work.

Answer 2

I have the impression there is a fundamental missunderstanding how function arguments and variables work (forgive me if I am wrong).

I try to explain:

first, you define a global variable "list_" and initialize it to an empty list. So far. But this is a completely different variable from those use in the functions.

Basically, if you define a function with an argument list like

def f(arg):
    ...

Any value passed to the function can be referenced inside f using the name arg.

The argument arg is a placeholder for any value you pass to the function when you invoke it by calling it like

some_var = f("whatever")

This will pass the sting "whatever" to f. Inside f, the local variable arg will then have the value "whatever" just for this invocation. Another infocation of f may be f(42), where arg will have the value 42.

So, arg is only accessible locally (and valid) inside f (that's why it's called a local variable). Any change to arg will also be just local; arg does not even exist outside f.

Even worse, if you have a global variable (defined outside f) with the same name (that's what you did with list_), the local variable "shadows" the global variable of the same name (which is bad practice btw.).

So, the argument-list in a function definition does not list the variables which are accessed inside the function, but are basically just some placeholders.

To output a local value to the outer world, a function has to return a value. That is also called the function result . Python (like C/C++, ...) the keyword "return" for this:

def square(x):
    return x * x

This will take any numerical value and return the square of it:

>>> square(2)
4
>>> square(8)
64
...

A good way to try this is to play a little with the Python interactive console. You can define functions and type any kind of code there as if you just type it into en editor and run the file through the interpreter.

Hope that gives you the right direction. This is actually not Python-specific, but very basic programming techniques which are definitively required to be understood well for all programming tasks.