How to use (access or modify) variables from different scopes in Python

On nonlocal, global and closures

The aim of this page📝is to describe the use of nonlocal keyword, its relation to an analogous global, its difference to closures, its indifference to LEGB rule and finally on the error

local variable '<binding>' referenced before assignment

1. NONLOCAL VS CLOSURES

• the following piece of code creates a counter starting at given value
• nonlocal introduces-for-reassignment an existing binding from enclosed scope

def make_counter(n):
    def incr():
        nonlocal n      #introduce `n` from L1
        n = n + 1       #re-assign/modify that value from enclosing scope
        print(n)
    return incr

• on the other hand, the following one prints the value of enclosed binding
• closures introduce-for-reference — objects close over values defined in enclosed/global scopes

def enclosing():        
    x = 'closed over'   
    def local_func():
        print(x)        # refer to `x` from L2; no assignment operator (`=`)
    return local_func

2. LEGB AND ASSIGNMENTS

• NOTE: LEGB does not apply when we make/assign a new name binding
• LEGB is designed for referring not for (re)assigning
• below, local() creates an entirely new name binding which has nothing to do with other scopes
• below, local() creates a new frame/scope with the named binding message applying only there
• the presence of = is critical in this dynamic

message = "global"


def enclosing():
    message = "enclosing"

    def local():
        message = "local"

    print("enclosing message:", message)
    local()
    print("enclosing mesage", message)


print("global message", message)
enclosing()
print("global message", message)

""" OUTPUT """
 # global message global
# enclosing message: enclosing
# enclosing mesage enclosing  
# global message global

3. LOCAL VARIABLE ‘’ REFERENCED BEFORE ASSIGNMENT

• CONDITIONS:

1. even if a local function contains a call to an existing binding in an enclosed/global scope
2. and if there also is an assignment of that exact name

• RESULT: the call will fail with

local variable 'message' referenced before assignment
message = "global"


def enclosing():
    message = "enclosing"

    def local():
        print("from local", message) # THIS WILL NOT PRINT "enclosing" BECAUSE OF THE NEXT LINE
        message = "local"            # DELETE THIS AND THE ABOVE LINE WILL PRINT "enclosing"s

    print("enclosing message:", message)
    local()
    print("enclosing mesage", message)


print("global message", message)
enclosing()
print("global message", message)

""" OUTPUT """
# global message global
# enclosing message: enclosing      
# Traceback (most recent call last):
#   ...
# UnboundLocalError: local variable 'message' referenced before assignment

4. GLOBAL KEYWORD

• Note — there already is Global-Keyword that introduces bindings from global scope into a local scope
• once introduced, the global binding can be used/accessed/modified locally

message = "global"


def enclosing():
    message = "enclosing"

    def local():
        global message
        message = "local"

    print("enclosing message:", message)
    local()
    print("enclosing mesage", message)


""" CALL """
print("global message", message)
enclosing()
print("global message", message)
""" OUTPUT """
# global message global
# enclosing message: enclosing
# enclosing mesage enclosing
# global message local <<< THE GLOBAL BINDING WAS CHANGED IN LOCAL FUNCTION

5. NONLOCAL KEYWORD

• inserts a name binding from an enclosing scope into the local namespace
• searches enclosing scopes from innermost > outermost
• as soon as it finds a match, that binding is introduced to the scope where nonlocal is invoked
• from that point, the local function is modifying binding in enclosing scopes
• nonlocal is used to keep state / create a form of persistent storage

message = "global"


def enclosing():
    message = "enclosing"

    def local():
        nonlocal message
        message = "local"

    print("enclosing message:", message)
    local()
    print("enclosing mesage", message)


""" CALL """
print("global message", message)
enclosing()
print("global message", message)
""" OUTPUT """
# global message global
# enclosing message: enclosing
# enclosing mesage local <<< MODIFIED BY `nonlocal` KEYWORD
# global message global

• NOTE: If no matching nonlocal binding is found, Python returns a syntax error