Why can't I set a global variable in Python?
Question:
How do global variables work in Python? I know global variables are evil, I’m just experimenting.
This does not work in python:
G = None
def foo():
if G is None:
G = 1
foo()
I get an error:
UnboundLocalError: local variable 'G' referenced before assignment
What am I doing wrong?
Answers:
You need the global
statement:
def foo():
global G
if G is None:
G = 1
In Python, variables that you assign to become local variables by default. You need to use global
to declare them as global variables. On the other hand, variables that you refer to but do not assign to do not automatically become local variables. These variables refer to the closest variable in an enclosing scope.
Python 3.x introduces the nonlocal
statement which is analogous to global
, but binds the variable to its nearest enclosing scope. For example:
def foo():
x = 5
def bar():
nonlocal x
x = x * 2
bar()
return x
This function returns 10 when called.
You still have to declare G as global, from within that function:
G = None
def foo():
global G
if G is None:
G = 1
foo()
print G
which simply outputs
1
Define G as global in the function like this:
#!/usr/bin/python
G = None;
def foo():
global G
if G is None:
G = 1;
print G;
foo();
The above python prints 1
.
Using global variables like this is bad practice because: http://c2.com/cgi/wiki?GlobalVariablesAreBad
You need to declare G
as global
, but as for why: whenever you refer to a variable inside a function, if you set the variable anywhere in that function, Python assumes that it’s a local variable. So if a local variable by that name doesn’t exist at that point in the code, you’ll get the UnboundLocalError
. If you actually meant to refer to a global variable, as in your question, you need the global
keyword to tell Python that’s what you meant.
If you don’t assign to the variable anywhere in the function, but only access its value, Python will use the global variable by that name if one exists. So you could do:
G = None
def foo():
if G is None:
print G
foo()
This code prints None
and does not throw the UnboundLocalError.
How do global variables work in Python? I know global variables are evil, I’m just experimenting.
This does not work in python:
G = None
def foo():
if G is None:
G = 1
foo()
I get an error:
UnboundLocalError: local variable 'G' referenced before assignment
What am I doing wrong?
You need the global
statement:
def foo():
global G
if G is None:
G = 1
In Python, variables that you assign to become local variables by default. You need to use global
to declare them as global variables. On the other hand, variables that you refer to but do not assign to do not automatically become local variables. These variables refer to the closest variable in an enclosing scope.
Python 3.x introduces the nonlocal
statement which is analogous to global
, but binds the variable to its nearest enclosing scope. For example:
def foo():
x = 5
def bar():
nonlocal x
x = x * 2
bar()
return x
This function returns 10 when called.
You still have to declare G as global, from within that function:
G = None
def foo():
global G
if G is None:
G = 1
foo()
print G
which simply outputs
1
Define G as global in the function like this:
#!/usr/bin/python
G = None;
def foo():
global G
if G is None:
G = 1;
print G;
foo();
The above python prints 1
.
Using global variables like this is bad practice because: http://c2.com/cgi/wiki?GlobalVariablesAreBad
You need to declare G
as global
, but as for why: whenever you refer to a variable inside a function, if you set the variable anywhere in that function, Python assumes that it’s a local variable. So if a local variable by that name doesn’t exist at that point in the code, you’ll get the UnboundLocalError
. If you actually meant to refer to a global variable, as in your question, you need the global
keyword to tell Python that’s what you meant.
If you don’t assign to the variable anywhere in the function, but only access its value, Python will use the global variable by that name if one exists. So you could do:
G = None
def foo():
if G is None:
print G
foo()
This code prints None
and does not throw the UnboundLocalError.