Function Scoping, Functions as Variables

By default variables used in a function definition are local.

i = 5
def printi():
   i=4
   print i
printi() # outputs 4
print i #outputs 5
#note without the i=4 assignment would get i=5

To be able to modify a variable in global scope we must use the keyword global:

def assign_i():
  global i
  i=3
print i #now get 3

Function names can be treated as references to code in memory, so we can assign them to variables. i.e., We have function pointers and functions can be passed to other functions, etc:

a = printi
a() # prints 4

Python also supports anonymous function definitions:

Consider:
def f(x): return x**3
f(10) #returns 1000

g = lambda x: x**3 #notice don't use return with lambda
g(10) #returns 1000

def make_adder (n): return lambda x: x + n
f = make_adder(2)
g = make_adder(6)
print f(42), g(42)

Anonymous function are often used to write "one-off" functions like callback handlers in GUI's and things.

Generators

We can generate sequences one element at a time using the yield construct:

def countdown(n):
    while n > 0:
       yield n
       n -=1
c = countdown(5)
print c.next() #prints 5
print c.next() #prints 4
print c.next() #prints 3

The next() method runs to the next yield statement

Here's how we could get a function which monitors a log file and returns lines as they appear:

import time
def tail_log(f):
    f.seek(0, 2) # 2nd arg: 0 means start of file, 1 means current pos, 2 means end of file
    while True:
        line = f.readline()
        if not line:
            time.sleep(0.1) # sleep 1/10 of a second
            continue
        yield line

Coroutines

Generators are functions which output a sequence of values when their next() method is called
One can imagine the opposite situation where a function receives a sequence of values one at a time and computes something.

Such a function is called a coroutine and can also be defined using yield:

def print_matches(text):
    print "Trying to find", text
    while True:
        line = (yield)
        if text in line:
            print line
p = print_matches("hello")
p. next()  #prints 'Trying to find hello'
p.send("lalalala la") #doesn't print anything
p.send("hello world") #prints hello world

Quiz

Which of the following is true?

Strings enclosed in two single quotes '' ... '' can span several lines in Python.
The evaluation function used for Greedy Best First Search has the form `c(n) + h(n)` where `h` is a heuristic, and `c` is the cost to the given node.
The Java code for(i = 0; i< 10; i++) {/* do something */} could be written in Python as:
```
    for i in range(0, 10):
       #do something
```

Objects and Classes

We have already seen that lists, dicts, etc are objects and we can invoke methods on them much as we do in other languages. For instance, my_list.append("hello")

Given an object we can see its methods by calling dir(my_obj). For example, a=[]; dir(a); gives:

['__add__', '__class__', '__contains__', '__delattr__', '__delitem__', 
'__delslice__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', 
'__getitem__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__', 
'__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', 
'__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', 
'__setattr__', '__setitem__', '__setslice__', '__sizeof__', '__str__', 
'__subclasshook__', 'append', 'count', 'extend', 'index', 'insert', 'pop', 
'remove', 'reverse', 'sort']

You can see append in the list. Special methods such as operator overloads have '__' around them. For example, __add__ in the above corresponds to the + operator. We can actually use the + operator in a cumbersome way as: a.__add__([1, 2]).
If you were writing your own class overriding __add__ would give you operator overloading of "+" for operations of the form my_obj + something_else. To override something_else + my_obj you would need to implement __radd__. To override += use overrider __iadd__, etc.
__init__ represents the constructor.

To define new classes we using the keyword class. For example,

class Stack(object): #this says stack inherits from object
    a_class_variable = 5 # this var behaves like a Java static var
    def __init__(self): #self = this in Java
        self.stack = [] #now stack is a field variable of Stack
        #in general using self.field_var is how we declare and 
        #instantiate a instance variable
    def push(self, object): #the first argument of any method 
        self.stack.append(object) # is the object itself
    def pop(self):
        return self.stack.pop()
    @property #properties are attributes that computes its value when accessed
    def length(self):
        return len(self.stack)

To instantiate an instance of this class and use it one could do:

my_instance = Stack()
my_instance.push("hello")
print my_instance.length()
print isinstance(my_instance, object) #returns True
print issubclass(Stack, object) #returns True
#type(my_instance) returns Stack
...
#etc

Inheritance, static methods, abstract classes.

Stack above is almost identical to the built-in list object except for the push method. We could have instead inherited from list using:

class Stack(list):
    def push(self, object):
        #could refer to parent by using syntax list.some_method_of_list
        #or use super(list, self).some_method_of_list
        self.append(object)

Python supports multiple inheritance: class MyClass(A, B, ...):
We can use the super mechanism to distinguish between parent members. Python also uses name mangling on methods beginning with "__". The method __foo in class A would be mangled to _A__foo and so would be different from a __foo in class B.

Static methods of classes can be created and used using the syntax:

class MyClass:
   @staticmethod
   def my_method():
      #some code

MyClass.my_method() #similar to Java

Similarly, abstract methods and properties can be declared as:

#load in abstract class module
from abc import ABCMeta, abstractmethod, abstractproperty
class MyClass:
    __metaclass__ = ABCMeta # a metaclass is a class object that knows how to
                            # create other class objects. The default metaclass
                            # is type (Python 3, types.ClassType in Python 2). 
                            # Here we are assignment ABCMeta
                            # to be used rather than type
                            # In Python 3, write MyClass(metaclass=ABCmeta)
    @abstractmethod
    def my_abstract_method(self):
       pass
    @abstractproperty
    def my_abstract_property(self):
       pass

Tic-tac-toe Board Example

Here is another example of creating and using a class:

class Board:
    def __init__(self):
        self.board = ["...", "...", "..."]
    def add(self, player_piece, x, y):
        if player_piece != "x" and player_piece != "o":
            raise RuntimeError("Piece must be x or o")
        row = ""
        for i in range(3):
            if i == y:
                row += player_piece
            else:
                row += self.board[x][y]
        self.board[x] = row
    def draw(self):
        for i in range(3):
            print self.board[i]

b = Board()
b.add("x", 1, 1)
b.draw()
print
b.add("o", 0, 1)
b.draw()

Python Objects, Classes