Python Programming

7.1 Variable Scope

Variable Scope (变量的作用域)

In Python, module, class, def, lambda can introduce new variable scope, if/elif/else/, try/except, for/while will not introduce new variable scope.

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

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

Global and Local

msg_loc = "Shanghai" # Global
def test():
    msg = 'I am from Shanghai' # Local

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

mutable vs. immutable

a = [1,2,3]
def test():
    print(a)
    a = [1,2,3,4]
test() 

UnboundLocalError: local variable 'a' 
referenced before assignment

a = [1,2,3]
def test():
    print(a)
    a.append(4)
test() 
print(a)

[1, 2, 3]
[1, 2, 3, 4]

a = {"color": "green"}
def test():
    print(a)
    a["color"] = "red"
    a["position"] = "left"
test() 
print(a)

{'color': 'green'}
{'color': 'red', 'position': 'left'}

Parameter and Argument

• For immutable objects (number, string, tuple), the assignment inside a function cannot change the value of the variable outside the function even if the variable name inside is the same with the variable name outside.

def ChangeInt(a):
    a = 10
    return a

b = 2
print(ChangeInt(b))
print(b) 

10
2

def ChangeInt(b):
    b = 10
    return b

b = 2
x = ChangeInt(b)
print(x, b) 

10 2

Examples

def ChangeInt(a):
    a = 10
    print(a)
    return a

b = 2
ChangeInt(b)
print(b) 
print(ChangeInt(b))

10
2
10
10

def ChangeInt(a):
    a = 10
    print(a)

b = 2
ChangeInt(b)
print(b) 
print(ChangeInt(b))

10
2
10
None

• For mutable objects (list, dictionary), the assignment inside a function cannot change the value of the variable outside the function. (the same with immutable objects!)

def changeme(mylist):
   mylist = [1,2]
   print("inside: ", mylist)
 
x = [10,20]
changeme(x)
print("outside: ", x)

inside:  [1, 2]
outside:  [10, 20]

• However, the modification can change the value of the variable outside.

def changeme(mylist):
   mylist.extend([1,2])
   print ("inside: ", mylist)
 
x = [10,20]
changeme(x)
print ("outside: ", x)

inside:  [10, 20, 1, 2]
outside:  [10, 20, 1, 2]

7.2 Nested Functions

Nested Functions

def outer():
    def inner():
        print(100)
    inner()
outer()

def outer():
    def inner():
        num = 100
        print(num)
    inner()
outer()

• Local$\rightarrow$Enclosing$\rightarrow$Global$\rightarrow$ Build-in (LEGB)

num = 3           # Global
def outer():
    num = 10      # Enclosing
    def inner():
        num = 100 # Local
        print(num)
    inner()
    print(num)
outer()

The nonlocal keyword

def outer():
    num = 10
    def inner():
        nonlocal num   
        # nonlocal keyword
        num = 100
        print(num)
    inner()
    print(num)
outer()

100
100

def outer():
    num = 10
    def inner():
        global num  
        # global does not work
        num = 100
        print(num)
    inner()
    print(num)
outer()

100 
10

7.3 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):
    profile = {}
    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

Summary

• Functions
• Reading: Python for Everybody, Chapter 4
• Reading: Python Crash Course, Chapter 8