Notes on Python variable scope
Example 1: The difference between global and local variables¶Global variables are accessible inside and outside of functions. Local variables are only accessible inside the function. In the example below, the function can access both the global and the local variable. However, trying to access the local variable outside the function produces an error.
global_var = 'foo' def ex1(): local_var = 'bar' print global_var print local_var ex1() print global_var print local_var # this gives an error
foo bar foo Traceback (most recent call last): File "nested_scope.py", line 12, in
print local_var # this gives an error NameError: name 'local_var' is not defined
Example 2: How *not* to set a global variable¶*Setting* a global variable from within a function is not as simple. If I set a variable in a function with the same name as a global variable, I am actually creating a new local variable. In the example below,
'foo'even after the function is called.
var = 'foo' def ex2(): var = 'bar' print 'inside the function var is ', var ex2() print 'outside the function var is ', var
inside the function var is bar outside the function var is foo
Example 3: How to set a global variable¶To set the global variable inside a function, I need to use the
globalstatement. This declares the inner variable to have module scope. Now
'bar'after the function is called.
var = 'foo' def ex3(): global var var = 'bar' print 'inside the function var is ', var ex3() print 'outside the function var is ', var
inside the function var is bar outside the function var is bar
Example 4: Nested functions¶Scoping for nested functions works similarly. In the example below, the inner function can access both
var_inner. However, the outer function cannot access
var_inner. Side note: the inner function is considered a closure if it makes reference to a non-global outside variable.
def ex4(): var_outer = 'foo' def inner(): var_inner = 'bar' print var_outer print var_inner inner() print var_outer print var_inner # this gives an error ex4()
foo bar foo Traceback (most recent call last): File "nested_scope.py", line 53, in
ex3() File "nested_scope.py", line 51, in ex3 print var_inner # this gives an error NameError: global name 'var_inner' is not defined
Example 5: How *not* to set an outer variable¶Like Example 2, setting a variable in the inner function creates a new local variable instead of modifying the outer variable. In the example below,
varin the outer function does not get changed to
def ex5(): var = 'foo' def inner(): var = 'bar' print 'inside inner, var is ', var inner() print 'inside outer function, var is ', var ex5()
inside inner, var is bar inside outer function, var is foo
Example 6: Another way to *not* set an outer variable¶However, using the
globalkeyword won't work in this case.
globalcause a variable to have module scope, but I want my variable to have the scope of the outer function. Per the Python 3000 Status Update, Python 3000 will have a
nonlocalkeyword to solve this problem. See PEP 3104 for more information about
nonlocaland nested scopes. In the example below,
varis still not changed to
'bar'in the outer function.
def ex6(): var = 'foo' def inner(): global var var = 'bar' print 'inside inner, var is ', var inner() print 'inside outer function, var is ', var ex6()
inside inner, var is bar inside outer function, var is foo
Example 7: A workaround until Python 3000 arrives¶A workaround is to create an empty class to use as an additional namespace. Now the variable in the outer function can be set to
class Namespace: pass def ex7(): ns = Namespace() ns.var = 'foo' def inner(): ns.var = 'bar' print 'inside inner, ns.var is ', ns.var inner() print 'inside outer function, ns.var is ', ns.var ex7()
inside inner, ns.var is bar inside outer function, ns.var is bar
Example 8: Alternative to Example 7¶Update 2010-03-01: According to Alexander's comment below, this is not a good way to do things.
I learned about this method from Nihiliad's comment on my recursion example. To me, this seems like a more elegant alternative to the solution in Example 7.
def ex8(): ex8.var = 'foo' def inner(): ex8.var = 'bar' print 'inside inner, ex8.var is ', ex8.var inner() print 'inside outer function, ex8.var is ', ex8.var ex8()
inside inner, ex8.var is bar inside outer function, ex8.var is bar
Core Python Programming, Second Edition, Ch 11
- How to use *args and **kwargs in Python — posted 2008-01-03
26 Comments — Comments feed for this post
#1 Anonymous commented on 2008-03-20:
Many thanks for this post! I could not find the solution to setting global variables anywhere in all my python documentation.
#2 Anonymous commented on 2008-04-14:
Another workaround is to wrap the variable in a mutable:
... var = ['foo']
... def inner():
... var = 'bar'
... print 'inside inner, var is ', var
... print 'inside outer function, var is ', var
inside inner, var is ['bar']
inside outer function, var is ['bar']
#3 Asif commented on 2008-05-15:
Can anybody tell me what's wrong in the below code:
argref_list = 
for entry in arg_l: # processing #nnnn($Fmm)
the output is:
note that 'NONE' is printed (strange) and secondly throws the error that NONETYPE has no append function.
I am fade up with PYTHON!
#4 Anonymous commented on 2008-06-26:
Nice post, told me exactly what I needed to know!
#5 Brendan commented on 2009-02-26:
Thanks for the clear and concise examples. The python tutorial left me confused about variable scope. This cleared it up.
#6 wvh commented on 2009-03-25:
This confused me for a long time, but your explanation is understandable. Thanks very much!
#7 pythonkhmer commented on 2009-04-21:
It's clear explanation about variables scope.that python official documentation aborted these. thanks,thanks,for your sharing.
#8 roger.ngeuf commented on 2009-04-25:
Thanks a lot. I did wrote this code in my programm a wonder why he didn't work (I have hear that python support closure. my code was :
def main(): count = 0 def counting(): for i in range(10): count += 1 counting() return count
which throw an exception. if first change it to :
def main(): count = 0 def counting(count): for i in range(10): count += 1 counting() return count
but thanks to you, i update it to :
def main(): main.count = 0 def counting(): for i in range(10): main.count += 1 counting() return main.count
Nice Post and keep posting stuff like this.
#9 roger.ngeuf commented on 2009-04-25:
Sorry but your editor didn't keep the formatting
#10 Eliot commented on 2009-04-25:
roger, Thanks for the comment. I fixed the formatting for your code. To format code blocks, put 4 spaces before each line and a blank line before and after the block.
#11 Oli commented on 2009-05-13:
firstly great article, really really useful.
I was wondering... in a code I am writing, I assign a global variable a value from a function, but I then want to delete that global variable from another function in which identically named local variables are defined. For this reason, "del variable" deletes the local not the global. How would I go about deleting the global variable?
#12 Zach commented on 2009-08-19:
Thanks, this was helpful.
I do have a question about scope within a class. I am using Python 2.5
def class foo(object): def test(self): test.var = 1 print test.var f = foo() f.test()
Result is "global name test is not defined"
I can get around this by using a "namespace" or mutable wrapper as described but I was just curious why this didn't work
#13 Eliot commented on 2009-08-19:
I think you get this error message because
test is a method of
foo and not a global. However, the following code gives an error as well:
class foo(object): def test(self): self.test.var = 1 print self.test f = foo() f.test()
AttributeError: 'instancemethod' object has no attribute 'var'
#14 akaihola commented on 2009-10-23:
A variation of Example 7:
def ex7b(): class ns: pass ns.var = 'foo' def inner(): ns.var = 'bar' print 'inside inner, ns.var is ', ns.var inner() print 'inside outer function, ns.var is ', ns.var ex7b()
#15 Eliot commented on 2009-10-23:
akaihola, thanks. it does seem to make more sense to put the namespace within the outer function. btw, nice blog-- i see we have some similar interests (twisted, scribus, emacs, django)
#16 Leo commented on 2010-02-24:
I think the snippet you posted is trying to access 'test', a method of 'foo', before its declaration is complete (within the same block), thus the error. Did you mean
self.var = 3
foo.var = 3
#17 Leo commented on 2010-02-24:
Can someone help me understand what is wrong with this example?
class T: A = range(2) B = range(4) s = sum(i*j for i in A for j in B)
It produces the exception:
<type 'exceptions.NameError'>: global name 'j' is not defined
The exception above is especially confusing when the following similar example (I just replaced the generator by an explicit array) works:
class T: A = range(2) B = range(4) s = sum([(i*j) for i in A for j in B])
(BTW, the class scope declarations are intentional).
#18 Alexander Khodyrev commented on 2010-02-28:
Example 8 is not a very good way to do things. ex8's properties are effectively global variables. That means that something like http://www.paulgraham.com/accgen.html won't work this way. (inner functions defined this way are not closures)
#19 Eliot commented on 2010-03-01:
Leo: sorry, I've run in to this also, but don't know the answer.
Alexander: thank you for alerting me to this. I added a quick note to example 8 above.
#20 Gerrat commented on 2010-03-01:
As for the question in #17, see: http://docs.python.org/reference/executionmodel.html#naming-and-binding
"A scope defines the visibility of a name within a block. If a local variable is defined in a block, its scope includes that block. If the definition occurs in a function block, the scope extends to any blocks contained within the defining one, unless a contained block introduces a different binding for the name. The scope of names defined in a class block is limited to the class block; it does not extend to the code blocks of methods – this includes generator expressions since they are implemented using a function scope. This means that the following will fail:"
class A: a = 42 b = list(a + i for i in range(10))
#21 Beni Cherniavsky-Paskin commented on 2010-08-31:
#18: ex8 vars are indeed effectively global, but you can make it work if you use attributes of the inner function. Here is Paul Graham's accgen:
def foo(n): def inner(i): inner.n += i return inner.n inner.n = n return inner >>> f = foo(10) >>> print f(1), f(2), f(3) 11 13 16 >>> f = foo(20) >>> print f(1), f(2), f(3) 21 23 26
#23 Rui commented on 2012-10-31:
#25 Elliott commented on 2013-03-26:
Thank you! This explained my closure woes.
#26 theKashyap commented on 2013-09-26:
Nice post.. gave me very useful info, not quite clear from python error messages.. But I find it incomplete as it's missing description of how scope works in classes. Thanks none the less..