In Python 3.3.1, this works:
i = 76
def A():
global i
i += 10
print(i) # 76
A()
print(i) # 86
This also works:
def enclosing_function():
i = 76
def A():
nonlocal i
i += 10
print(i) # 76
A()
print(i) # 86
enclosing_function()
But this doesn't work:
i = 76
def A():
nonlocal i # "SyntaxError: no binding for nonlocal 'i' found"
i += 10
print(i)
A()
print(i)
The documentation for the nonlocal
keyword states (emphasis added):
The nonlocal statement causes the listed identifiers to refer to previously bound variables in the nearest enclosing scope .
In the third example, the "nearest enclosing scope" just happens to be the global scope. So why doesn't it work?
I do notice that the documentation goes on to state (emphasis added):
The [
nonlocal
] statement allows encapsulated code to rebind variables outside of the local scope besides the global (module) scope .
but, strictly speaking, this doesn't mean that what I'm doing in the third example shouldn't work.
The search order for names is LEGB, ie Local, Enclosing, Global, Builtin. So the global scope is not an enclosing scope.
EDIT
From the docs :
The nonlocal statement causes the listed identifiers to refer to previously bound variables in the nearest enclosing scope. This is important because the default behavior for binding is to search the local namespace first. The statement allows encapsulated code to rebind variables outside of the local scope besides the global (module) scope.
The answer is that the global scope does not enclose anything - it is global to everything. Use the global
keyword in such a case.
why is a module's scope considered global and not an enclosing one? It's still not global to other modules (well, unless you do
from module import *
), is it?
If you put some name into module
's namespace; it is visible in any module that uses module
ie, it is global for the whole Python process.
In general, your application should use as few mutable globals as possible. See Why globals are bad? :
Therefore It would be bad if nonlocal
allowed to create globals by accident. If you want to modify a global variable; you could use global
keyword directly.
global
is the most destructive: may affect all uses of the module anywhere in the program nonlocal
is less destructive: limited by the outer() function scope (the binding is checked at compile time) You can read about history and motivation behind nonlocal
in PEP: 3104 Access to Names in Outer Scopes .
It depends upon the Boundary cases:
nonlocals come with some senstivity areas which we need to be aware of. First, unlike the global statement, nonlocal names really must have previous been assigned in an enclosing def's scope when a nonlocal is evaluated or else you'll get an error-you cannot create them dynamically by assigning them anew in the enclosing scope. In fact, they are checked at function definition time before either or nested function is called
>>>def tester(start):
def nested(label):
nonlocal state #nonlocals must already exist in enclosing def!
state = 0
print(label, state)
return nested
SyntaxError: no binding for nonlocal 'state' found
>>>def tester(start):
def nested(label):
global state #Globals dont have to exits yet when declared
state = 0 #This creates the name in the module now
print(label, state)
return nested
>>> F = tester(0)
>>> F('abc')
abc 0
>>> state
0
Second, nonlocal restricts the scope lookup to just enclosing defs; nonlocals are not looked up in the enclosing module's global scope or the built-in scope outside all def's, even if they are already there:
for example:-
>>>spam = 99
>>>def tester():
def nested():
nonlocal spam #Must be in a def, not the module!
print('current=', spam)
spam += 1
return nested
SyntaxError: no binding for nonlocal 'spam' found
These restrictions make sense once you realize that python would not otherwise generally know enclosing scope to create a brand-new name in. In the prior listing, should spam be assigned in tester , or the module outside? Because this is ambiguous, Python must resolve nonlocals at function creation time, not function call time.
In 2.x, nonlocal
didn't exist yet. It wasn't considered necessary to be able to modify enclosing, non-global scopes; the global scope was seen as a special case. After all, the concept of a "global variable" is a lot easier to explain than lexical closures.
Because functions are objects, and in particular because a nested function could be return
ed from its enclosing function (producing an object that persists after the call to the enclosing function), Python needs to implement lookup into enclosing scopes differently from lookup into either local or global scopes. Specifically, in the reference implementation of 3.x, Python will attach a __closure__
attribute to the inner function, which is a tuple of cell
instances that work like references (in the C++ sense) to the closed-over variables. (These are also references in the reference-counting garbage-collection sense; they keep the call frame data alive so that it can be accessed after the enclosing function return
s.)
By contrast, global lookup works by doing a chained dictionary lookup: there's a dictionary that implements the global scope, and if that fails, a separate dictionary for the builtin scope is checked. (Of course, writing a global only writes to the global dict, not the builtin dict; there is no builtin
keyword.)
Theoretically, of course, there's no reason why the implementation of nonlocal
couldn't fall back on a lookup in the global (and then builtin) scope, in the same way that a lookup in the global scope falls back to builtins. Stack Overflow is not the right place to speculate on the reason behind the design decision. I can't find anything relevant in the PEP , so it may simply not have been considered.
The best I can offer is: like with local variable lookup, nonlocal
lookup works by determining at compile time what the scope of the variable will be. If you consider builtins as simply pre-defined, shadow-able globals (ie the only real difference between the actual implementation and just dumping them into the global scope ahead of time, is that you can recover access to the builtin with del
), then so does global
lookup. As they say, "simple is better than complex" and "special cases aren't special enough to break the rules"; so, no fallback behaviour.
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