从if语句嵌套在for循环中使变量成为全局变量

Question

I have a typical newbie problem of getting the result of a function into the global scope and I can usually follow how local and global variables work in simple examples but I can having a trouble understanding what happens when there are if statements nested in for loops.

Below is the original code I am working with. I am trying to the the result of thisItem in to the global scope.

def getTheFirstPoint(selection):
    for thisItem in selection:
        if type(thisItem) == GSNode:
            print 'LOCAL', thisItem
            return thisItem
    return None

I have been trying things like this:

thisItem = ''

def getTheFirstPoint(selection):
    global thisItem
    for thisItem in selection:
        if type(thisItem) == GSNode:
            print 'LOCAL', thisItem
            #return thisItem
    #return None

getTheFirstPoint(thisItem)
print 'GLOBAL:', thisItem

I've seen at times the global variable doesn't need to be explicitly set outside of the function – do I need the " thisItem = ' ' "?

Are the returns necessary?

What do I need to do to access thisItem globally?

Any help will be appreciated.

Answer 1

If you run code:

thiItem = ''

Inside function definition the new local variable will be created. If you do run code below in function

global thisItem
thisItem = ''

The global variable will be modified. When you run for loop you create new local variable for [newVarName] in [iterable] . After you have defined thisItem as global do not use this name second time in the for loop.

If you wish to modify global variable on condition the code below will do the work:

thisItem = ''

def getTheFirstPoint(selection):
    global thisItem
    for item in selection:
        if type(item) == GSNode:
            print 'LOCAL', item
            # Modify global variable.
            thisItem = item
            #return thisItem
    #return None

getTheFirstPoint(thisItem)
print 'GLOBAL:', thisItem

PS I have a feeling that the Python Generators may be use full for you, also consider going more closely to the information about global vs local variables .

Answer 2

I've seen at times the global variable doesn't need to be explicitly set outside of the function – do I need the " thisItem = ' ' "? If you create a variable thisItem inside a function a local variable will be created in that function. If you use

previous_function():
    thisItem = ''
new_function():
    global thisItem 
    thisItem = 'updated value'

Then the value will be overwritten when you call it in new_function. That's why you need to define it outside any other function.

Are the returns necessary? No, the returns are not necessary. As you can call your global variable from any other function. If you have another variable with the same name, you need to specify that you want to use the global definition and not the local one as in above example.

What do I need to do to access thisItem globally? You can use it like below:

thisItem = ''
def getTheFirstPoint(selection):
    for thisItem in selection:
        if type(thisItem) == GSNode:
            print 'GLOBAL', thisItem
def some_new_function():
    global thisItem
    thisItem = 'modified value'