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Closures

Closures

  • formed by local functions
  • maintain references to objects from earlier scopes
  • implemented in Python with special attribute __closure__
def enclosing():
    x = 'closed over'
    def local_func():
        print(x)
    return local_func

lf = enclosing()
lf()
#=> closed over
lf.__closure__
#=> (<cell at 0x10ea95cc8: str object at 0x10eac19f0>,)

Function Factory

  • function that returns new, specialized functions
  • simple example
def raise_to(exp):
    def raise_to_exp(x):
        return pow(x, exp)
    return raise_to_exp

The nonlocal Keyword

  • LEGB does not apply when making new bindings
  • nonlocal keyword introduces names from enclosing namespace into local namespace

💡 IMPORTANT 💡 using nonlocal with a name that doesn't exist will result in error

message = 'global'

def enclosing():
    message = 'enclosing'

    def local():
        # does not affect `global` or `enclosing` bindings
        message = 'local'

    print('enclosing message before `local()` call: ', message)
    local()
    print('enclosing message after `local()` call: ', message)

print('global message before `enclosing()` call: ', message)
enclosing()
print('global message after `enclosing()` call: ', message)

#=> global message before `enclosing()` call: global
#=> enclosing message before `local()` call: enclosing
#=> enclosing message after `local()` call: enclosing
#=> global message after `enclosing()` call: global

Changing local binding using global keyword

message = 'global'

def enclosing():
    message = 'enclosing'

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

    print('enclosing message before `local()` call: ', message)
    local()
    print('enclosing message after `local()` call: ', message)

print('enclosing message before `enclosing()` call: ', message)
enclosing()
print('enclosing message after `enclosing()` call: ', message)

#=> global message before `enclosing()` call: global
#=> enclosing message before `local()` call: enclosing
#=> enclosing message after `local()` call: enclosing
#=> global message after `enclosing()` call: local

Changing local binding using nonlocal keyword

message = 'global'

def enclosing():
    message = 'enclosing'

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

    print('enclosing message before `local()` call: ', message)
    local()
    print('enclosing message after `local()` call: ', message)

print('enclosing message before `enclosing()` call: ', message)
enclosing()
print('enclosing message after `enclosing()` call: ', message)

#=> global message before `enclosing()` call: global
#=> enclosing message before `local()` call: enclosing
#=> enclosing message after `local()` call: local
#=> global message after `enclosing()` call: global

Practical Example

# make_timer.py
import time


def make_timer():
    last_called = None

    def elapsed():
        nonlocal last_called
        now = time.time()
        if last_called is None:
            last_called = now
            return None
        result = now - last_called
        last_called = now
        return result

    return elapsed
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