Python Advanced

Seifu, Deanne

Python Basics Recap

  1. Make a list of three employees named "bob", "sue", and "rick"
  2. Make a dictionary of three employees named "bob", "sue", and "rick" that have a position [waiter, host, cook] and a salary [3, 4, 8].
  3. Using the dictionary from above, raise the employees' salaries by 10%.
  4. Write a function that will output every employees information.

Variable Scope

Important to know where variables have values and when! 
class MenuItem:
    'Common base for all food items'
    def __init__(self, name, price):
            self.name = name
            self.price = price
    def displayMenuItem(self):
            print ("Name: ", self.name, ", Price: ", self.price)

if __name__ == "__main__":
    item1 = MenuItem("Rice",5)
    item1.displayMenuItem()

Exception Handling

Defending against code that may raise an exception.
  • try, except, else
  • try, except, else, finally
try:
    #You do your operations here
except ExceptionI:
    #If there is ExceptionI, then execute this block
except ExceptionII:
    #If there is ExceptionII, then execute this block
else:
    #If there is no exception then execute this block
try:
    fh = open("testfile", "w")
    fh.write("this is my test file for exception handling")
except IOError:
    print ("Error: can\'t find file or read data")
else:
    print ("Written content in the file successfully")
    fh.close()
try:
    #You do your operations here
    #If an exception occurs you skip remaining code
finally:
    #This is always exectued
Problem:

Write a script that sets a variable to any string. Next write a try-except block to convert the string to an integer using the int() function or print that "The argument does not contain numbers\n"

Comprehensions

Concise way to create lists. Great way to make a list from a list. 
mylist = [expression(i) for i in old_list if filter(i)]

Basic Example

# You can either use loops:
squares = []
for x in range(10):
    squares.append(x**2)

# Or you can use list comprehensions to get the same result:
squares2 = [x**2 for x in range(10)]

Nested Example

[x+y for x in [10,30,50] for y in [20,40,60]]
#[30, 50, 70, 50, 70, 90, 70, 90, 110]

Example using a filter (if)

[str(x) for x in range(9) if x%2 == 0]
 #['0', '2', '4', '6', '8']
Exercises
  1. Build a list of squares from 0 to 9 called S
  2. Build a list of powers of two from 0 to 12 called V
  3. Build a list where the number is in S and is even called M
  4. Make a list of all of the primes from 2 to 50
  5. Make a list of strings that has UPPER, lower, and length of word for /n "The quick brown fox jumps over the lazy dog"

Object Mutabilitiy

Objects are either mutable or immutable. An immutable object will not allow changes to it after it is created.

Examples of immutable objects are

  • numbers
  • Booleans
  • strings
  • tuples
  • frozensets

Why do we need mutability?

myStr = 'hello'
mystr[4] = 'O' #error!
Mutability Exercises 1
x = 3
y = x
x = x + y
#What is the value of x? y?
del x
#What is the value of y now?
Mutability Exercises 2
mystr1 = 'hello world'
mystr2 = mystr1
mystr1 = mystr1[:-1]
#What is the value of mystr1? mystr2?
del mystr1
#What is the value of mystr2 now?
Mutability Exercises 3
list1 = ['water', 'juice', 'soda']
list2 = list1
list1.append('beer')
#What are list1 and list2 now?
del list1
#What are list1 and list2 now?
Mutability Exercises 4
mytuple = ('a', 'b', 'c')
mytyple = 'b'
mytuple = mytyple
#what is the type of mytuple? What is the value?
Mutability Exercises 5
def myfunc(val):
    val += 'bar'
    print(val)
    return val

x = 'foo'
print (x) #what does it print?
x=myfunc(x)
print(val) #what does it print?
print (x) #what does it print?
Creating Classes

The class syntax will create a new class. The name of the class is preceeded immediately by the keyword class

class ClassName:
        #Optional class documentation string
        class_suite
Class Example

Using what you know develop an Employee class with two properties (name and salary) and a displayEmployee property that prints "Name: ", self.name, ", Salary: ", self.salary

Test it by making two employees named emp1 and emp2 with salaries 20 and 25.

Optional/Default Arguments in Functions

Arguments Example 1
class Employee:
    'Common base for all employees'

    def __init__(self, name, salary = 20):
        self.name = name
        self.salary = salary

    def displayEmployee(self):
        print "Name: ", self.name, ", Salary: ", self.salary

if __name__ == "__main__":
    emp1 = Employee('name')
    emp1.displayEmployee()
    emp2 = Employee('name2', 25)
    emp2.displayEmployee()

Variable-length Arguments

Arguments Example 2
Class Employee:
    'Common base for all employees'

    def __init__(self, name, salary = 20):
        self.name = name
        self.salary = salary
    def displayEmployee(self):
        print ("Name: ", self.name, ", Salary: ", self.salary)

def printEmployees(*varEmployees):
    for emp in varEmployees:
        if isinstance(emp, Employee):
            emp.displayEmployee()
if __name__ == "__main__":
    emp1 = Employee('name')
    emp2 = Employee('name2', 25)
    printEmployees(emp1, emp2)

Anonymous Functions and Uses

Some insights from the instructors.
Functions Examples
def f (x): return x**2
print f(8)
#or
g = lambda x: x**2
print g(8)

#guess the output here then try it!
def make_incrementor (n): return lambda x:x +n
f = make_incrementor(2)
g = make_incrementor(6)
print f(42), g(42)
print make_incrementor(22)(33)