How to reset global variable in python?

SOME_VARIABLE = [] def some_fun: append in SOME_VARIABLE s = [] s = SOME_VARIABLE SOME_VARIABLE = [] // Not setting to empty list. return s 

How to reset SOME_VARIABLE to empty.

4 Answers

If you read a variable, Python looks for it in the entire scope chain. This mean that:

GLOB_VAR = "Some string" def some_fun(): print GLOB_VAR 

will print Some string

Now, if you write to a variable, Python looks for it in the local scope, and if it cannot find a variable with the name you gave at the local level, then it creates one.

This means that in your example, you have created a variable named SOME_VARIABLE local to your some_fun function, instead of updating the global SOME_VARIABLE. This is a classic python gotcha.

If you want to write to your global, you have to explicitly tell Python that you are talking about a global variable that already exists. To do so, you need to use the global keyword. So, the following:

GLOB_VAR = "Some string" def some_fun(): global GLOB_VAR GLOB_VAR = "Some other string" some_fun() print GLOB_VAR 

will print Some other string.

Note: I see it as a way of encouraging people to keep global variables read-only, or at least to think about what they're doing.

The behaviour is the same (just a bit more surprising) when you try to read first and then write to a global. The following:

GLOB_VAR = False def some_fun(): if GLOB_VAR: GLOB_VAR = False some_fun() 

will raise:

Traceback (most recent call last): File "t.py", line 7, in <module> some_fun() File "t.py", line 4, in some_fun if GLOB_VAR: UnboundLocalError: local variable 'GLOB_VAR' referenced before assignment 

because since we will modify GLOB_VAR, it is considered a local variable.

Update: Ely Bendersky has a related in-depth post about this that is worth a read for more formal details.

You need to tell the interpreter that you are talking about a global variable:

def some_fun: global SOME_VARIABLE ... SOME_VARIABLE = [] 

if you don't need the SOME_VARIABLE anymore you could use:

del SOME_VARIABLE 

if you want a empty list:

del SOME_VARIABLE[:] 

SOME_VARIABLE is global, so rebinding it won't take effect unless you use global. But since it's a mutable object, just mutate it appropriately.

del SOME_VARIABLE[:]