Actually, a concise rule for Python Scope resolution, from Learning Python, 3rd. Ed.. (These rules are specific to variable names, not attributes. If you reference it without a period, these rules apply)
LEGB Rule.
L, Local — Names assigned in any way within a function (def or lambda)), and not declared global in that function.
E, Enclosing function locals — Name in the local scope of any and all enclosing functions (def or lambda), from inner to outer.
G, Global (module) — Names assigned at the top-level of a module file, or declared global in a def within the file.
B, Built-in (Python) — Names preassigned in the built-in names module : open,range,SyntaxError,...
So, in the case of
code1
class Foo: code2 def spam..... code3 for code4..: code5 x()
The for loop does not have its own namespace. In LEGB order, the scopes would be
L : local, in the current def.
E : Enclosed function, any enclosing functions (if def spam was in another def)
G : Global. Were there any declared globally in the module?
Essentially, the only thing in Python that introduces a new scope is a function definition. Classes are a bit of a special case in that anything defined directly in the body is placed in the class's namespace, but they are not directly accessible from within the methods (or nested classes) they contain.
In your example there are only 3 scopes where x will be searched in:
spam's scope - containing everything defined in code3 and code5 (as well as code4, your loop variable)
The global scope - containing everything defined in code1, as well as Foo (and whatever changes after it)
The builtins namespace. A bit of a special case - this contains the various Python builtin functions and types such as len() and str(). Generally this shouldn't be modified by any user code, so expect it to contain the standard functions and nothing else.
More scopes only appear when you introduce a nested function (or lambda) into the picture. These will behave pretty much as you'd expect however. The nested function can access everything in the local scope, as well as anything in the enclosing function's scope. eg.
def foo(): x=4 def bar(): print x # Accesses x from foo's scope bar() # Prints 4 x=5 bar() # Prints 5
Restrictions:
Variables in scopes other than the local function's variables can be accessed, but can't be rebound to new parameters without further syntax. Instead, assignment will create a new local variable instead of affecting the variable in the parent scope. For example:
global_var1 = []
global_var2 = 1
def func(): # This is OK: It's just accessing, not rebinding global_var1.append(4) # This won't affect global_var2. Instead it creates a new variable global_var2 = 2 local1 = 4 def embedded_func(): # Again, this doen't affect func's local1 variable. It creates a # new local variable also called local1 instead. local1 = 5 print local1 embedded_func() # Prints 5 print local1 # Prints 4
In order to actually modify the bindings of global variables from within a function scope, you need to specify that the variable is global with the global keyword. Eg:
Currently there is no way to do the same for variables in enclosing function scopes, but Python 3 introduces a new keyword, "nonlocal" which will act in a similar way to global, but for nested function scopes.
There was no thorough answer concerning Python3 time, so I made an answer here.
As provided in other answers, there are 4 basic scopes, the LEGB, for Local, Enclosing, Global and Builtin. In addition to those, there is a special scope, the class body, which does not comprise an enclosing scope for methods defined within the class; any assignments within the class body make the variable from there on be bound in the class body.
Especially, no block statement, besides def and class, create a variable scope. In Python 2 the list comprehension does not create a variable scope, however in Python 3 the loop variable is created in a new scope.
To demonstrate the peculiarities of the class body
x = 0
class X(object): y = x x = x + 1 # x is now a variable z = x def method(self): print(self.x) # -> 1 print(x) # -> 0, the global x print(y) # -> NameError: global name 'y' is not defined
inst = X()
print(inst.x, inst.y, inst.z, x) # -> (1, 0, 1, 0)
Thus unlike in function body, you can reassign the variable to the same name in class body, to get a class variable with the same name; further lookups on this name resolve to the class variable instead.
One of the greater surprises to many newcomers to Python is that a for loop does not create a variable scope. In Python 2 the list comprehensions do not create a scope either (while generators and dict comprehensions do!) Instead they leak the value in the function or the global scope:
>>> [ i for i in range(5) ]
>>> i
4
The comprehensions can be used as a cunning (or awful if you will) way to make modifiable variables within lambda expressions in Python 2 - a lambda expression does create a variable scope, like the def statement would, but within lambda no statements are allowed. Assignment being a statement in Python means that no variable assignments in lambda are allowed, but a list comprehension is an expression...
This behaviour has been fixed in Python 3 - no comprehension expressions or generators leak variables.
The global really means the module scope; the main python module is the __main__; all imported modules are accessible through the sys.modules variable; to get access to __main__ one can use sys.modules['__main__'], or import __main__; it is perfectly acceptable to access and assign attributes there; they will show up as variables in the global scope of the main module.
If a name is ever assigned to in the current scope (except in the class scope), it will be considered belonging to that scope, otherwise it will be considered to belonging to any enclosing scope that assigns to the variable (it might not be assigned yet, or not at all), or finally the global scope. If the variable is considered local, but it is not set yet, or has been deleted, reading the variable value will result in UnboundLocalError, which is a subclass of NameError.
x = 5
def foobar() print(x) # UnboundLocalError! x += 1 # assignment here makes x a local variable!
The scope can declare that it explicitly wants to modify the global (module scope) variable, with the global keyword:
x = 5
def foobar(): global x print(x) # -> 5 x += 1
foobar()
print(x) # -> 6
This also is possible even if it was shadowed in enclosing scope:
x = 5
y = 13
def make_closure(): x = 42 y = 911 def func(): global x # sees the global value print(x, y) x += 1 return func
func = make_closure()
func() # -> print 5 911
print(x, y) # -> 6 13
In python 2 there is no easy way to modify the value in the enclosing scope; usually this is simulated by having a mutable value, such as a list with length of 1:
However in python 3, the nonlocal comes to rescue:
def make_closure(): value = 0 def get_next_value(): nonlocal value value += 1 return value return get_next_value
get_next = make_closure() # identical behavior to the previous example.
Any variable that is not deemed to be local to the current scope, or any enclosing scope, is a global variable. A global name is looked up in the module global dictionary; if not found, the global is then looked up from the builtins module; the name of the module was changed from python 2 to python 3; in python 2 it was __builtin__ and in python 3 it is now called builtins. If you assign to an attribute of builtins module, it will be visible thereafter to any module as a readable global variable, unless that module shadows them with its own global variable with the same name.
Reading the builtin module can also be useful; suppose that you want the python 3 style print function in some parts of file, but other parts of file still use the print statement, if your python version is >= 2.6, you can get the new style function as:
The from __future__ import print_function actually does not import the print function anywhere in Python 2 - instead it just disables the parsing rules for print statement in the current module, handling print like any other variable identifier, and thus allowing the print the function be looked up in the builtins.
The scoping rules for Python 2.x have been outlined already in other answers. The only thing I would add is that in Python 3.0, there is also the concept of a non-local scope (indicated by the 'nonlocal' keyword). This allows you to access outer scopes directly, and opens up the ability to do some neat tricks, including lexical closures (without ugly hacks involving mutable objects).
EDIT: Here's the PEP with more information on this.
x = 100
print "1. Global x:", x
class Test(object): y = x print "2. Enclosed y:", y x = x + 1 print "3. Enclosed x:", x z = x def method(self): print "4. Enclosed self.x", self.x print "5. Global x", x try: print y except NameError, e: print "6.", e def method_local_ref(self): try: print x except UnboundLocalError, e: print "7.", e x = 200 print "8. Local x", x
inst = Test()
inst.method()
inst.method_local_ref()
Python resolves your variables with -- generally -- three namespaces available.
At any time during execution, there are at least three nested scopes whose namespaces are directly accessible: the innermost scope, which is searched first, contains the local names; the namespaces of any enclosing functions, which are searched starting with the nearest enclosing scope; the middle scope, searched next, contains the current module's global names; and the outermost scope (searched last) is the namespace containing built-in names.
There are two functions: globals and locals which show you the contents two of these namespaces.
Namespaces are created by packages, modules, classes, object construction and functions. There aren't any other flavors of namespaces.
In this case, the call to a function named x has to be resolved in the local name space or the global namespace.
Local in this case, is the body of the method function Foo.spam.
Global is -- well -- global.
The rule is to search the nested local spaces created by method functions (and nested function definitions), then search global. That's it.
There are no other scopes. The for statement (and other compound statements like if and try) don't create new nested scopes. Only definitions (packages, modules, functions, classes and object instances.)
Inside a class definition, the names are part of the class namespace. code2, for instance, must be qualified by the class name. Generally Foo.code2. However, self.code2 will also work because Python objects look at the containing class as a fall-back.
An object (an instance of a class) has instance variables. These names are in the object's namespace. They must be qualified by the object. (variable.instance.)
From within a class method, you have locals and globals. You say self.variable to pick the instance as the namespace. You'll note that self is an argument to every class member function, making it part of the local namespace.
x is not found as you haven't defined it. :-) It could be found in code1 (global) or code3 (local) if you put it there.
code2 (class members) aren't visible to code inside methods of the same class — you would usually access them using self. code4/code5 (loops) live in the same scope as code3, so if you wrote to x in there you would be changing the x instance defined in code3, not making a new x.
Python is statically scoped, so if you pass ‘spam’ to another function spam will still have access to globals in the module it came from (defined in code1), and any other containing scopes (see below). code2 members would again be accessed through self.
lambda is no different to def. If you have a lambda used inside a function, it's the same as defining a nested function. In Python 2.2 onwards, nested scopes are available. In this case you can bind x at any level of function nesting and Python will pick up the innermost instance:
fun3 sees the instance x from the nearest containing scope, which is the function scope associated with fun2. But the other x instances, defined in fun1 and globally, are not affected.
Before nested_scopes — in Python pre-2.1, and in 2.1 unless you specifically ask for the feature using a from-future-import — fun1 and fun2's scopes are not visible to fun3, so S.Lott's answer holds and you would get the global x:
