Please look at this code:
def x():
a.append(1)
def y():
print a
a = []
x()
y()
which outputs:
[1]
and this:
def x():
a = [1]
def y():
print a
a = []
x()
y()
which outputs:
[]
I don't understand why I can't explicitly set global list variable a in a function, without using list methods, like .append(), but just setting the value?
In your second example, a = [1] is creating a new local variable named a, not referencing the global variable a. In order to reference the outer scoped a you could do:
def x():
global a
a = [1]
See the Python documentation for the global keyword, specifically:
It would be impossible to assign to a global variable without global,
although free variables may refer to globals without being declared
global.
In your first example, a uses the global variable because you are referencing a variable that does not exist as a local in the x() function, but does exist in the global variables.
In version 1, when you use a in x and y Python "goes looking" for an a to use, and finds it in the outer scope.
In version 2, you define a local a inside x, and assign to that; there's no need for Python to look in other scopes. If you want to use the "outside a", you have to be explicit about your intentions:
def x():
global a
a = [1]
the second example isn't a global variable. You are declaring a new local variable a, and assigning it to [1]
Related
This question already has answers here:
Using global variables in a function
(25 answers)
Closed 5 months ago.
I know I should avoid using global variables in the first place due to confusion like this, but if I were to use them, is the following a valid way to go about using them? (I am trying to call the global copy of a variable created in a separate function.)
x = "somevalue"
def func_A ():
global x
# Do things to x
return x
def func_B():
x = func_A()
# Do things
return x
func_A()
func_B()
Does the x that the second function uses have the same value of the global copy of x that func_a uses and modifies? When calling the functions after definition, does order matter?
If you want to simply access a global variable you just use its name. However to change its value you need to use the global keyword.
E.g.
global someVar
someVar = 55
This would change the value of the global variable to 55. Otherwise it would just assign 55 to a local variable.
The order of function definition listings doesn't matter (assuming they don't refer to each other in some way), the order they are called does.
Within a Python scope, any assignment to a variable not already declared within that scope creates a new local variable unless that variable is declared earlier in the function as referring to a globally scoped variable with the keyword global.
Let's look at a modified version of your pseudocode to see what happens:
# Here, we're creating a variable 'x', in the __main__ scope.
x = 'None!'
def func_A():
# The below declaration lets the function know that we
# mean the global 'x' when we refer to that variable, not
# any local one
global x
x = 'A'
return x
def func_B():
# Here, we are somewhat mislead. We're actually involving two different
# variables named 'x'. One is local to func_B, the other is global.
# By calling func_A(), we do two things: we're reassigning the value
# of the GLOBAL x as part of func_A, and then taking that same value
# since it's returned by func_A, and assigning it to a LOCAL variable
# named 'x'.
x = func_A() # look at this as: x_local = func_A()
# Here, we're assigning the value of 'B' to the LOCAL x.
x = 'B' # look at this as: x_local = 'B'
return x # look at this as: return x_local
In fact, you could rewrite all of func_B with the variable named x_local and it would work identically.
The order matters only as far as the order in which your functions do operations that change the value of the global x. Thus in our example, order doesn't matter, since func_B calls func_A. In this example, order does matter:
def a():
global foo
foo = 'A'
def b():
global foo
foo = 'B'
b()
a()
print foo
# prints 'A' because a() was the last function to modify 'foo'.
Note that global is only required to modify global objects. You can still access them from within a function without declaring global.
Thus, we have:
x = 5
def access_only():
return x
# This returns whatever the global value of 'x' is
def modify():
global x
x = 'modified'
return x
# This function makes the global 'x' equal to 'modified', and then returns that value
def create_locally():
x = 'local!'
return x
# This function creates a new local variable named 'x', and sets it as 'local',
# and returns that. The global 'x' is untouched.
Note the difference between create_locally and access_only -- access_only is accessing the global x despite not calling global, and even though create_locally doesn't use global either, it creates a local copy since it's assigning a value.
The confusion here is why you shouldn't use global variables.
You can directly access a global variable inside a function. If you want to change the value of that global variable, use "global variable_name". See the following example:
var = 1
def global_var_change():
global var
var = "value changed"
global_var_change() #call the function for changes
print var
Generally speaking, this is not a good programming practice. By breaking namespace logic, code can become difficult to understand and debug.
As others have noted, you need to declare a variable global in a function when you want that function to be able to modify the global variable. If you only want to access it, then you don't need global.
To go into a bit more detail on that, what "modify" means is this: if you want to re-bind the global name so it points to a different object, the name must be declared global in the function.
Many operations that modify (mutate) an object do not re-bind the global name to point to a different object, and so they are all valid without declaring the name global in the function.
d = {}
l = []
o = type("object", (object,), {})()
def valid(): # these are all valid without declaring any names global!
d[0] = 1 # changes what's in d, but d still points to the same object
d[0] += 1 # ditto
d.clear() # ditto! d is now empty but it`s still the same object!
l.append(0) # l is still the same list but has an additional member
o.test = 1 # creating new attribute on o, but o is still the same object
Here is one case that caught me out, using a global as a default value of a parameter.
globVar = None # initialize value of global variable
def func(param = globVar): # use globVar as default value for param
print 'param =', param, 'globVar =', globVar # display values
def test():
global globVar
globVar = 42 # change value of global
func()
test()
=========
output: param = None, globVar = 42
I had expected param to have a value of 42. Surprise. Python 2.7 evaluated the value of globVar when it first parsed the function func. Changing the value of globVar did not affect the default value assigned to param. Delaying the evaluation, as in the following, worked as I needed it to.
def func(param = eval('globVar')): # this seems to work
print 'param =', param, 'globVar =', globVar # display values
Or, if you want to be safe,
def func(param = None)):
if param == None:
param = globVar
print 'param =', param, 'globVar =', globVar # display values
You must use the global declaration when you wish to alter the value assigned to a global variable.
You do not need it to read from a global variable. Note that calling a method on an object (even if it alters the data within that object) does not alter the value of the variable holding that object (absent reflective magic).
What is a global statement? And how is it used? I have read Python's official definition;
however, it doesn't make a lot of sense to me.
Every "variable" in python is limited to a certain scope. The scope of a python "file" is the module-scope. Consider the following:
#file test.py
myvariable = 5 # myvariable has module-level scope
def func():
x = 3 # x has "local" or function level scope.
Objects with local scope die as soon as the function exits and can never be retrieved (unless you return them), but within a function, you can access variables in the module level scope (or any containing scope):
myvariable = 5
def func():
print(myvariable) # prints 5
def func2():
x = 3
def func3():
print(x) # will print 3 because it picks it up from `func2`'s scope
func3()
However, you can't use assignment on that reference and expect that it will be propagated to an outer scope:
myvariable = 5
def func():
myvariable = 6 # creates a new "local" variable.
# Doesn't affect the global version
print(myvariable) # prints 6
func()
print(myvariable) # prints 5
Now, we're finally to global. The global keyword is the way that you tell python that a particular variable in your function is defined at the global (module-level) scope.
myvariable = 5
def func():
global myvariable
myvariable = 6 # changes `myvariable` at the global scope
print(myvariable) # prints 6
func()
print(myvariable) # prints 6 now because we were able
# to modify the reference in the function
In other words, you can change the value of myvariable in the module-scope from within func if you use the global keyword.
As an aside, scopes can be nested arbitrarily deeply:
def func1():
x = 3
def func2():
print("x=",x,"func2")
y = 4
def func3():
nonlocal x # try it with nonlocal commented out as well. See the difference.
print("x=",x,"func3")
print("y=",y,"func3")
z = 5
print("z=",z,"func3")
x = 10
func3()
func2()
print("x=",x,"func1")
func1()
Now in this case, none of the variables are declared at the global scope, and in python2, there is no (easy/clean) way to change the value of x in the scope of func1 from within func3. That's why the nonlocal keyword was introduced in python3.x . nonlocal is an extension of global that allows you to modify a variable that you picked up from another scope in whatever scope it was pulled from.
mgilson did a good job but I'd like to add some more.
list1 = [1]
list2 = [1]
def main():
list1.append(3)
#list1 = [9]
list2 = [222]
print list1, list2
print "before main():", list1, list2
>>> [1] [1]
main()
>>> [1,3] [222]
print list1, list2
>>> [1, 3] [1]
Inside a function, Python assumes every variable as local variable
unless you declare it as global, or you are accessing a global variable.
list1.append(2)
was possible because you are accessing the 'list1' and lists are mutable.
list2 = [222]
was possible because you are initializing a local variable.
However if you uncomment #list1 = [9], you will get
UnboundLocalError: local variable 'list1' referenced before assignment
It means you are trying to initialize a new local variable 'list1' but it was already referenced before,
and you are out of the scope to reassign it.
To enter the scope, declare 'list1' as global.
I strongly recommend you to read this even though there is a typo in the end.
a = 1
def f():
a = 2 # doesn't affect global a, this new definition hides it in local scope
a = 1
def f():
global a
a = 2 # affects global a
Basically it tells the interpreter that the variable its given should be modified or assigned at the global level, rather than the default local level.
You can use a global variable in other functions by declaring it as global in each function that modifies it
Python wants to make sure that you really know that's what you're playing with by explicitly requiring the global keyword.
See this answer
This question already has answers here:
UnboundLocalError trying to use a variable (supposed to be global) that is (re)assigned (even after first use)
(14 answers)
Short description of the scoping rules?
(9 answers)
Closed 6 months ago.
If I run the following code:
x = 1
class Incr:
print(x)
x = x + 1
print(x)
print(x)
It prints:
1
2
1
Okay no problems, that's exactly what I expected. And if I do the following:
x = 1
class Incr:
global x
print(x)
x = x + 1
print(x)
print(x)
It prints:
1
2
2
Also what I expected. No problems there.
Now if I start making an increment function as follows:
x = 1
def incr():
print(x)
incr()
It prints 1 just as I expected. I assume it does this because it cannot find x in its local scope, so it searches its enclosing scope and finds x there. So far no problems.
Now if I do:
x = 1
def incr():
print(x)
x = x + 1
incr()
This gives me the following error in the traceback:
UnboundLocalError: local variable 'x' referenced before assignment.
Why does Python not just search the enclosing space for x when it cannot find a value of x to use for the assignment like my class Incr did? Note that I am not asking how to make this function work. I know the function will work if I do the following:
x = 1
def incr():
global x
print(x)
x = x + 1
print(x)
incr()
This will correctly print:
1
2
just as I expect. All I am asking is why it doesn't just pull x from the enclosing scope when the keyword global is not present just like it did for my class above. Why does the interpreter feel the need to report this as an UnboundLocalError when clearly it knows that some x exists. Since the function was able to read the value at x for printing, I know that it has x as part of its enclosing scope...so why does this not work just like the class example?
Why is using the value of x for print so different from using its value for assignment? I just don't get it.
Classes and functions are different, variables inside a class are actually assigned to the class's namespace as its attributes, while inside a function the variables are just normal variables that cannot be accessed outside of it.
The local variables inside a function are actually decided when the function gets parsed for the first time, and python will not search for them in global scope because it knows that you declared it as a local variable.
So, as soon as python sees a x = x + 1(assignment) and there's no global declared for that variable then python will not look for that variable in global or other scopes.
>>> x = 'outer'
>>> def func():
... x = 'inner' #x is a local variable now
... print x
...
>>> func()
inner
Common gotcha:
>>> x = 'outer'
>>> def func():
... print x #this won't access the global `x`
... x = 'inner' #`x` is a local variable
... print x
...
>>> func()
...
UnboundLocalError: local variable 'x' referenced before assignment
But when you use a global statement then python for look for that variable in global scope.
Read: Why am I getting an UnboundLocalError when the variable has a value?
nonlocal: For nested functions you can use the nonlocal statement in py3.x to modify a variable declared in an enclosing function.
But classes work differently, a variable x declared inside a class A actually becomes A.x:
>>> x = 'outer'
>>> class A:
... x += 'inside' #use the value of global `x` to create a new attribute `A.x`
... print x #prints `A.x`
...
outerinside
>>> print x
outer
You can also access the class attributes directly from global scope as well:
>>> A.x
'outerinside'
Using global in class:
>>> x = 'outer'
>>> class A:
... global x
... x += 'inner' #now x is not a class attribute, you just modified the global x
... print x
...
outerinner
>>> x
'outerinner'
>>> A.x
AttributeError: class A has no attribute 'x'
Function's gotcha will not raise an error in classes:
>>> x = 'outer'
>>> class A:
... print x #fetch from globals or builitns
... x = 'I am a class attribute' #declare a class attribute
... print x #print class attribute, i.e `A.x`
...
outer
I am a class attribute
>>> x
'outer'
>>> A.x
'I am a class attribute'
LEGB rule: if no global and nonlocal is used then python searches in this order.
>>> outer = 'global'
>>> def func():
enclosing = 'enclosing'
def inner():
inner = 'inner'
print inner #fetch from (L)ocal scope
print enclosing #fetch from (E)nclosing scope
print outer #fetch from (G)lobal scope
print any #fetch from (B)uilt-ins
inner()
...
>>> func()
inner
enclosing
global
<built-in function any>
From Python scopes and namespaces:
It is important to realize that scopes are determined textually: the global scope of a function defined in a module is that module’s namespace, no matter from where or by what alias the function is called. On the other hand, the actual search for names is done dynamically, at run time — however, the language definition is evolving towards static name resolution, at “compile” time, so don’t rely on dynamic name resolution! (In fact, local variables are already determined statically.)
Which means that, the scope for x = x + 1 is determined statically, before the function is called. And since this is an assignment, then 'x' becomes a local variable, and not looked up globally.
That is also the reason why from mod import * is disallowed in functions. Because the interpreter won't import modules for you in compile time to know the names you are using in the function. i.e, it must know all names referenced in the function at compile time.
It's the rule Python follows - get used to it ;-) There is a practical reason: both the compiler and human readers can determine which variables are local by looking only at the function. Which names are local has nothing to do with the context in which a function appears, and it's generally a Very Good Idea to follow rules that limit the amount of source code you have to stare at to answer a question.
About:
I assume it does this because it cannot find x in its local scope, so
it searches its enclosing scope and finds x.
Not quite: the compiler determines at compile time which names are and aren't local. There's no dynamic "hmm - is this local or global?" search going on at runtime. The precise rules are spelled out here.
As to why you don't need to declare a name global just to reference its value, I like Fredrik Lundh's old answer here. In practice, it is indeed valuable that a global statement alerts code readers to that a function may be rebinding a global name.
Because that's the way it was designed to work.
Basically, if you have an assignment anywhere in your function, then that variable becomes local to that function (unless you've used global, of course).
Because it would lead to very hard to track down bugs!
When you type that x = x + 1, you could have meant to increment an x in the enclosing scope... or you could have simply forgotten that you already used x somewhere else and been trying to declare a local variable.
I would prefer the interpreter to only allow you to change the parent namespace if you intend to- this way you can't do it by accident.
I can try and make an educated guess why it works this way.
When Python encounters a string x = x + 1 in your function, it has to decide where to look up the x.
It could say "the first occurrence of x is global, and the second one is local", but this is quite ambiguous (and therefore against Python philosophy). This could be made part of the syntax, but it potentially leads to tricky bugs. Therefore it was decided to be consistent about it, and treat all occurrences as either global or local variables.
There is an assignment, therefore if x was supposed to be global, there would be a global statement, but none is found.
Therefore, x is local, but it is not bound to anything, and yet it is used in the expression x + 1. Throw UnboundLocalError.
As an extra example to the newly created A.x created within the class. The reassigning of x to 'inner' within the class does not update the global value of x because it is now a class variable.
x = 'outer'
class A:
x = x
print(x)
x = 'inner'
print(x)
print(x)
print(A.x)
As a follow up to this question
Why can I change the global value of x.a within def A? I am guessing it has to do with the fact that it is a class since it would not work with a regular variable because it would be redefined in the local scope of def A, but I am not quite understanding what is happening.
Case 1
class X:
def __init__(self):
self.a = 1
self.b = 2
self.c = 3
class Y:
def A(self):
print(x.a,x.b,x.c)
x.a = 4
x = X()
y = Y()
y.A()
print(x.a,x.b,x.c)
If you set x = Y() in A scope it would create a local x in that scope. In this case however, you are not setting x, you are setting x.a. Looking up variables takes into account global variables too. Imagine you are doing this instead setattr(x, "a", 4) and it will make more sense.
Also if I remember correctly you can "import" global variables into a function scope by using the global keyword. (see Use of "global" keyword in Python)
Global names can be read within functions:
x = 5
def read_x():
print(x)
Globals that reference mutable types can also be mutated within functions:
x = [1, 2, 3]
def mutate_x():
x[0] = 'One'
What you can't do to a global within the scope of a function is assignment:
x = 5
def set_x():
# this simply assigns a variable named x local to this function -- it doesn't modify the global x
x = 3
# now, back outside the scope of set_x, x remains 5
print(x)
5
Unless you explicitly declare the global within the scope of the function:
x = 5
def set_x():
global x
x = 3
# back outside the function's scope
print(x)
3
What you're doing in your example is "mutating" -- modifying an attribute of an object. Assigning a value to an attribute of a user-defined type is one example of mutation, just like modifying an element of a list or a dictionary. That's why this works.
The class A doesn't have an init method that defines x, so the method A, when you try to access the attribute a of x, try to find the x object in the local scope, and since it can't find it then it looks the outside scope where an object name x is present, it grabs that object and override the attribute a.
So basically what you are doing is modify the actual attribute a of the object x that you create before you call y.A().
It's the very basic foundation of a closure: access a variable that is defined outside the local scope.
What is a global statement? And how is it used? I have read Python's official definition;
however, it doesn't make a lot of sense to me.
Every "variable" in python is limited to a certain scope. The scope of a python "file" is the module-scope. Consider the following:
#file test.py
myvariable = 5 # myvariable has module-level scope
def func():
x = 3 # x has "local" or function level scope.
Objects with local scope die as soon as the function exits and can never be retrieved (unless you return them), but within a function, you can access variables in the module level scope (or any containing scope):
myvariable = 5
def func():
print(myvariable) # prints 5
def func2():
x = 3
def func3():
print(x) # will print 3 because it picks it up from `func2`'s scope
func3()
However, you can't use assignment on that reference and expect that it will be propagated to an outer scope:
myvariable = 5
def func():
myvariable = 6 # creates a new "local" variable.
# Doesn't affect the global version
print(myvariable) # prints 6
func()
print(myvariable) # prints 5
Now, we're finally to global. The global keyword is the way that you tell python that a particular variable in your function is defined at the global (module-level) scope.
myvariable = 5
def func():
global myvariable
myvariable = 6 # changes `myvariable` at the global scope
print(myvariable) # prints 6
func()
print(myvariable) # prints 6 now because we were able
# to modify the reference in the function
In other words, you can change the value of myvariable in the module-scope from within func if you use the global keyword.
As an aside, scopes can be nested arbitrarily deeply:
def func1():
x = 3
def func2():
print("x=",x,"func2")
y = 4
def func3():
nonlocal x # try it with nonlocal commented out as well. See the difference.
print("x=",x,"func3")
print("y=",y,"func3")
z = 5
print("z=",z,"func3")
x = 10
func3()
func2()
print("x=",x,"func1")
func1()
Now in this case, none of the variables are declared at the global scope, and in python2, there is no (easy/clean) way to change the value of x in the scope of func1 from within func3. That's why the nonlocal keyword was introduced in python3.x . nonlocal is an extension of global that allows you to modify a variable that you picked up from another scope in whatever scope it was pulled from.
mgilson did a good job but I'd like to add some more.
list1 = [1]
list2 = [1]
def main():
list1.append(3)
#list1 = [9]
list2 = [222]
print list1, list2
print "before main():", list1, list2
>>> [1] [1]
main()
>>> [1,3] [222]
print list1, list2
>>> [1, 3] [1]
Inside a function, Python assumes every variable as local variable
unless you declare it as global, or you are accessing a global variable.
list1.append(2)
was possible because you are accessing the 'list1' and lists are mutable.
list2 = [222]
was possible because you are initializing a local variable.
However if you uncomment #list1 = [9], you will get
UnboundLocalError: local variable 'list1' referenced before assignment
It means you are trying to initialize a new local variable 'list1' but it was already referenced before,
and you are out of the scope to reassign it.
To enter the scope, declare 'list1' as global.
I strongly recommend you to read this even though there is a typo in the end.
a = 1
def f():
a = 2 # doesn't affect global a, this new definition hides it in local scope
a = 1
def f():
global a
a = 2 # affects global a
Basically it tells the interpreter that the variable its given should be modified or assigned at the global level, rather than the default local level.
You can use a global variable in other functions by declaring it as global in each function that modifies it
Python wants to make sure that you really know that's what you're playing with by explicitly requiring the global keyword.
See this answer