6.1 Functions (2)

Variable Scope

g_count = 0  # Global
def outer():
i_count = 2  # Local
• In Python, module, class, def, lambda can introduce new variable scope, if/elif/else/, try/except, for/while will not introduce new variable scope.

• Local$\rightarrow$Global

if True:
msg = 'I am from Shanghai'
print(msg) # We can use msg.

def test():
msg = 'I am from Shanghai'
print(msg) # error

New Assignment

msg = 'I am from Shanghai'
def test():
msg = 'I am from Beijing'
print(msg)
test()
print(msg)

I am from Beijing
I am from Shanghai

Reference

msg = 'I am from Shanghai'
def test():
print(msg)
test()

I am from Shanghai

Modification

msg = 'I am from Shanghai'
def test():
print(msg)
msg = 'I am from Beijing'
test()

UnboundLocalError: local variable 'msg'
referenced before assignment

• How to modify the variable outside? The global keyword

num = 1
def fun():
global num
num = 123
print(num)
fun()
print(num)

123
123

num = 1
def fun():
print(num)
global num
num = 123
print(num)
fun()

SyntaxError: name 'num' is used
prior to global declaration

a = 10
def test():
a = a + 1
print(a)
test() # error

a = 10
def test():
global a
a = a + 1
print(a)
test() # output: 11

a = 10
def test():
a=10
a = a + 1
print(a)
test() # output: 11
print(a) # output: 10

6.2 Functions (3)

arbitrary arguments

def make_pizza(toppings):
print(toppings)

make_pizza('pepperoni')
• If we want to pass multiple arguments, we may create a list at first.

x=['mushrooms', 'green peppers', 'extra cheese']
def make_pizza(toppings):
for y in toppings:
print(y, end=', ')

make_pizza(x)

def make_pizza(topping_1, topping_2, topping_3):
print(topping_1, topping_2, topping_3)

make_pizza('mushrooms', 'green peppers', 'extra cheese')
• However, sometimes we are not sure about the exact number of the arguments.

• We can pass multiple arguments at once in the following way.

def make_pizza(*toppings):
print(toppings)

make_pizza('pepperoni')
make_pizza('mushrooms', 'green peppers', 'extra cheese')
• Note that Python packs the arguments into a tuple, even if the function receives only one value.

def make_pizza(*toppings):
for topping in toppings:
print("- " + topping)

make_pizza('pepperoni')
make_pizza('mushrooms', 'green peppers', 'extra cheese')

- pepperoni
- mushrooms
- green peppers
- extra cheese
• Mixing Positional and Arbitrary Arguments

• If you want a function to accept several different kinds of arguments, the parameter that accepts an arbitrary number of arguments must be placed last in the function definition.

def make_pizza(size, *toppings):
print(str(size) + "-inch pizza:")
for topping in toppings:
print("- " + topping)

make_pizza(16, 'pepperoni')
make_pizza(12, 'mushrooms', 'green peppers', 'extra cheese')

16-inch pizza:
- pepperoni
12-inch pizza:
- mushrooms
- green peppers
- extra cheese
• Using Arbitrary Keyword Arguments

def build_profile(**user_info):
print(user_info)

build_profile(location='Shanghai',field='Management')

{'location': 'Shanghai', 'field': 'Management'}
• Using Arbitrary Keyword Arguments

def build_profile(first, last, **user_info):
for key, value in user_info.items():
profile[key] = value
return first, last, profile

user_profile = build_profile('albert', 'einstein',
location='princeton',
field='physics')
print(user_profile)

('albert', 'einstein',
{'location': 'princeton', 'field': 'physics'})

def build_profile(*name, **user_info):
print(name)
print(user_info)

build_profile('albert', 'einstein',
location='princeton',
field='physics')

('albert', 'einstein')
{'location': 'princeton', 'field': 'physics'}
• As a tradition, we often use *args and **kw

• Example

def test(x,y=1,*a,**b):
print(x,y,a,b)

test(1)
test(1,2)
test(1,2,3)
test(1,2,3,4)
test(x=1,y=2)
test(1,a=2)
test(1,2,3,a=4)
test(1,2,3,y=4)

• Result

1 1 () {}
1 2 () {}
1 2 (3,) {}
1 2 (3, 4) {}
1 2 () {}
1 1 () {'a': 2}
1 2 (3,) {'a': 4}
TypeError: test() got multiple values

6.3 Functions (4)

Comprehension Syntax
• Four ways of creating a list

def test1():
l = []
for i in range(100):
l = l + [i]

def test2():
l = []
for i in range(100):
l.append(i)

def test3():
l = list(range(100))

def test4():
l = [i for i in range(100)]

[ expression for value in iterable if condition ]

>>> [x * x for x in range(1, 11)]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

>>> [x * x for x in range(1, 11) if x % 2 == 0]
[4, 16, 36, 64, 100]

>>> [m + n for m in 'ABC' for n in 'XYZ']
['AX', 'AY', 'AZ', 'BX', 'BY', 'BZ', 'CX', 'CY', 'CZ']

>>> d = {'x': 'A', 'y': 'B', 'z': 'C' }
>>> [k + '=' + v for k, v in d.items()]
['y=B', 'x=A', 'z=C']

>>> L = ['Hello', 'World', 'IBM', 'Apple']
>>> [s.lower() for s in L]
['hello', 'world', 'ibm', 'apple']
• map()

def f(x):
return x * x

y = map(f, [1, 2, 3, 4, 5, 6, 7, 8, 9])
print(list(y))

[1, 4, 9, 16, 25, 36, 49, 64, 81]

>>> print(list(map(str, [1, 2, 3, 4, 5, 6, 7, 8, 9])))
['1', '2', '3', '4', '5', '6', '7', '8', '9']

Anonymous function: lambda

return x + y

lambda x, y: x + y

lambda x, y = 2: x + y
lambda *z: z

>>> a = lambda x, y: x + y
>>> a( 1, 3 )
4
>>> b = lambda x, y = 2: x + y
>>> b( 1 )
3
>>> b( 1, 3 )
4
>>> c = lambda *z: z
>>> c( 10, 'test')
(10, 'test')
• Sometimes the anonymous function is convenient.

• It has only one expression. (You do not have to use return)

sum = lambda arg1, arg2: arg1 + arg2

print ("The total is : ", sum( 10, 20 ))
print ("The total is : ", sum( 20, 20 ))

>>> list(map(lambda x: x * x, [1, 2, 3, 4, 5, 6, 7, 8, 9]))
[1, 4, 9, 16, 25, 36, 49, 64, 81]

Summary

• Functions
• Reading: Python for Everybody, Chapter 10.1-10.5, 10.7-10.8
• Reading: Python Crash Course, Chapter 8