Class Notes: Functions Redux

1. Variable length args (*args)
2. Default args
   1. Default args example
   2. Do not use mutable default args
   3. One workaround for mutable default args
3. Functions as parameters
4. Lambda functions
5. Keyword args (**kwargs)
6. Functions inside functions
7. Closures + Non-local variables
8. Non-local variables fail on setting
9. Functions that return functions
10. Function decorators

Variable length args (*args)

  video


def longestWord(*args):
    if (len(args) == 0): return None
    result = args[0]
    for word in args:
        if (len(word) > len(result)):
            result = word
    return result

print(longestWord("this", "is", "really", "nice")) # really

mywords = ["this", "is", "really", "nice"]

print(longestWord(mywords))  # ['this', 'is', 'really', 'nice']
print(longestWord(*mywords)) # really

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Default args

  video


Default args example

def f(x, y=10): return (x,y)
print(f(5))   # (5, 10)
print(f(5,6)) # (5, 6)

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Do not use mutable default args

def f(x, L=[ ]):
    L.append(x)
    return L

print(f(1))
print(f(2)) # why is this [1, 2]?

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One workaround for mutable default args

def f(x, L=None):
    if (L == None):
        L = [ ]
    L.append(x)
    return L

print(f(1))
print(f(2)) # [2] (that's better)

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Functions as parameters

  video


def derivative(f, x):
    h = 10**-8
    return (f(x+h) - f(x))/h

def f(x): return 4*x + 3
print(derivative(f, 2)) # about 4

def g(x): return 4*x**2 + 3
print(derivative(g, 2)) # about 16 (8*x at x==2)

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Lambda functions

  video


print(derivative(lambda x:3*x**5 + 2, 2)) # about 240, 15*x**4 at x==2

myF = lambda x: 10*x + 42
print(myF(5)) # 92
print(derivative(myF, 5)) # about 10

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Keyword args (**kwargs)

  video


def f(x=1, y=2): return (x,y)
print(f()) # (1, 2)
print(f(3)) # (3, 2)
print(f(y=3)) # (1, 3) [here is where we use a keyword arg]

def f(x, **kwargs): return (x, kwargs)
print(f(1)) # (1, { })
print(f(2, y=3, z=4)) # (2, {'z': 4, 'y': 3})

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Functions inside functions

  video


def f(L):
    def squared(x): return x**2
    return [squared(x) for x in L]
print(f(range(5)))
try:
    print(squared(5))
except:
    print("squared is not defined outside f")

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Closures + Non-local variables

  video


def f(L):
    myMap = dict()
    def squared(x):
        result = x**2
        myMap[x] = result
        return result
    squaredList = [squared(x) for x in L]
    return myMap
print(f(range(5)))

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Non-local variables fail on setting (use nonlocal)

  video


def brokenF(L):
    lastX = 0
    def squared(x):
        result = x**2
        lastX = x
        return result
    squaredList = [squared(x) for x in L]
    return lastX
print(brokenF(range(5)))

def fixedF(L):
    lastX = 0
    def squared(x):
        nonlocal lastX
        result = x**2
        lastX = x
        return result
    squaredList = [squared(x) for x in L]
    return lastX
print(fixedF(range(5)))

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Functions that return functions

  video


def derivativeFn(f):
    def g(x):
        h = 10**-5
        return (f(x+h) - f(x))/h
    return g

def f(x): return 5*x**3 + 10
fprime1 = derivativeFn(f)
fprime2 = derivativeFn(fprime1)
print(f(3))    # 145, 5*x**3 + 10 evaluated at x == 3
print(fprime1(3)) # about 135, 15*x**2 evaluated at x == 3
print(fprime2(3)) # about 90, 30*x evaluated at x == 3

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Function decorators

  video


@derivativeFn
def h(x): return 5*x**3 + 10
print(h(3)) # 135, matches fprime1 from above.

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