声明全局变量

Question

I have 2 question regarding global variables:

1. Why can't I declare a list as a global variable as so: global list_ex = [] ?

2. I have already defined a global variable that I am trying to use in a function, but can't:

    global column def fx_foo(cols): common = set(cols).intersection(set(column)) #Error Here!! 

When I try to access column inside the function, I get an error:

NameError: global name 'column' is not defined

Answer 1

You are not using global correctly. You don't need to use it at all .

You need to actually set a global column variable, there is none right now. global does not make the variable available. Just create a global column first:

column = []

then refer to it in your function. That is what the NameError exception is trying to tell you; Python cannot find the global column variable, you didn't assign anything to the name so it doesn't exist.

You only need to use global if you want to assign to a global column in your function :

def somefunction():
    global column
    column = [1, 2, 3]

Here the global keyword is needed to distinguish column from a local variable in the function.

Compare:

>>> foo = 1
>>> def set_foo():
...     foo = 2
...
>>> set_foo()
>>> foo
1

to

>>> foo = 1
>>> def set_foo():
...     global foo
...     foo = 2
...
>>> set_foo()
>>> foo
2

The first form only set a local variable, the second form set the global variable instead.

Answer 2

The keyword global means you are explicitly using a variable declared outside the scope of a function.

Your variable must be declared normally:

column = []

and declared global in the function that uses it

def fx_foo(cols):
    global column
    common = set(cols).intersection(set(column))

It is used to allow python to distinguish between new local variables and reused global variables.

Answer 3

this will work :

column =[]

def fx_foo(cols):
    global column
    common = set(cols).intersection(set(column)) 

but this will work even without global as column will be considered as nonlocal here

column =[]
def fx_foo(cols):
    common = set(cols).intersection(set(column)) 

I think It is more interesting to assign data to column if You want to display global feature (as You can use column from nonlocals without global declaration if you don't assign anything to it)

column =[]
def fx_foo(cols):   
    global column     
    column = set(cols).intersection(set(column)) 

or

def fx_foo(cols): 
    column =[]          
    global column 
    column = set(cols).intersection(set(column)) 

Answer 4

Global variables need to be declared global inside of the function not on the outside. A variable declared outside a function is by default global.

Functions don't need to declare a variable as global if they only need to access it. The global declaration is just if you need to modify the global variable.

Therefore, in your case, I suggest that you look into what column actually is when you're passing it to the function, because I think the problem may have something to do with that.