Python Closures

Closures

Counter local - not working

def counter():
    count = 0
    count += 1
    return count

print(counter())
print(counter())
print(counter())

1
1
1

Counter with global

• global

count = 0
def counter():
    global count
    count += 1
    return count

print(counter())
print(counter())
print(counter())

count = -42
print(counter())

1
2
3
-41

Create incrementors

In order to use in various map-expressions, we need a couple of functions that - for simplicity - need to increment a number:

def f3(x):
    return x + 3

def f7(x):
    return x + 7

def f23(x):
    return x + 23

print(f3(2))
print(f7(3))
print(f3(4))
print(f7(10))
print(f23(19))

5
10
7
17
42

Create internal function

def create_func():
    def internal():
        print("Hello world")
    internal()


func = create_func()
internal()

Hello world
Traceback (most recent call last):
  File "create_internal_func.py", line 8, in <module>
    internal()
NameError: name 'internal' is not defined

Create function by a function

def create_func():
    def internal():
        print("Hello world")
    #internal()

    return internal

func = create_func()
#internal()
func()

Hello world

Create function with parameters

def create_func(name):
    def internal():
        print(f"Hello {name}")

    return internal

foo = create_func("Foo")
foo()


bar = create_func("Bar")
bar()

Hello Foo
Hello Bar

Counter closure

• nonlocal

def create_counter():
    count = 0
    def internal():
        nonlocal count
        count += 1
        return count
    return internal

counter = create_counter()

print(counter())
print(counter())
print(counter())
print()

other = create_counter()
print(counter())
print(other())
print(counter())
print(other())

print()
print(count)

1
2
3

4
1
5
2

Traceback (most recent call last):
  File "counter.py", line 23, in <module>
    print(count)
NameError: name 'count' is not defined

Make incrementor with def (closure)

• closure

def make_incrementor(n):
    def inc(x):
        return x + n
    return inc

f3 = make_incrementor(3)
f7 = make_incrementor(7)

print(f3(2))
print(f7(3))
print(f3(4))
print(f7(10))

5
10
7
17

Make incrementor with lambda

def make_incrementor(n):
    return lambda x: x + n

f3 = make_incrementor(3)
f7 = make_incrementor(7)

print(f3(2))
print(f7(3))
print(f3(4))
print(f7(10))

5
10
7
17

Exercise: closure bank

• Create a closure that returns a function that holds a number (like a bank account) that can be incremented or decremented as follows:
• Allow for an extra paramter called prev that defaults to False. If True is passed then instead of returning the new balance, return the old balance.

bank = create_bank(20)

print(bank())    # 20
print(bank(7))   # 27
print(bank())    # 27
print(bank(-3))  # 24
print(bank())    # 24


print(bank(10, prev=True))   # 24
print(bank())    # 34

Exercise: counter with parameter

Change the counter example to accept a parameter and start counting from that number.

Solution: closure bank

def create_bank(n = 0):
    balance = n
    def bnk(change = 0, prev=False):
        nonlocal balance
        prev_balance = balance
        balance += change
        if prev:
            return prev_balance
        else:
            return balance
    return bnk


bank = create_bank(20)

print(bank())    # 20
print(bank(7))   # 27
print(bank())    # 27
print(bank(-3))  # 24
print(bank())    # 24


print(bank(10, prev=True))   # 24
print(bank())    # 34

20
27
27
24
24
24
34

Solution: counter with parameter

def create_counter(count=0):
    def internal():
        nonlocal count
        count += 1
        return count
    return internal

counter = create_counter()

print(counter())
print(counter())
print(counter())
print()

other = create_counter(42)
print(counter())
print(other())
print(counter())
print(other())

1
2
3

4
43
5
44