Python Control of Flow and Defining Classes

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Python Control of Flow and Defining Classes

• LING 5200
• Computational Corpus Linguistics
• Martha Palmer

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For Loops
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For Loops 1

• A for-loop steps through each of the items in a
  list, tuple, string, or any other type of object
  which the language considers an iterator.
• for ltitemgt in ltcollectiongtltstatementsgt
• When ltcollectiongt is a list or a tuple, then the
  loop steps through each element of the container.
• When ltcollectiongt is a string, then the loop
  steps through each character of the string.
• for someChar in Hello World
• print someChar

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For Loops 2

• The ltitemgt part of the for loop can also be more
  complex than a single variable name.
• When the elements of a container ltcollectiongt are
  also containers, then the ltitemgt part of the for
  loop can match the structure of the elements.
• This multiple assignment can make it easier to
  access the individual parts of each element.
• for (x, y) in (a,1), (b,2), (c,3), (d,4)
• print x

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For loops and range() function

• Since we often want to range a variable over some
  numbers, we can use the range() function which
  gives us a list of numbers from 0 up to but not
  including the number we pass to it.
• range(5) returns 0,1,2,3,4
• So we could sayfor x in range(5) print x

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Control of Flow

• There are several Python expressions that control
  the flow of a program. All of them make use of
  Boolean conditional tests.
• If Statements
• While Loops
• Assert Statements

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If Statements

• if x 3
• print X equals 3.
• elif x 2
• print X equals 2.
• else
• print X equals something else.
• print This is outside the if.
• Be careful! The keyword if is also used in the
  syntax of filtered list comprehensions.

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If Statements Another Ex.

• x killer rabbit
• if x roger
• print hows jessica?
• elif x bugs
• print Whats up, doc?
• else
• print Run away! Run away!
• Run away! Run away!

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While Loops

• x 3
• while x lt 10
• x x 1
• print Still in the loop.
• print Outside of the loop.

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While Loops, more examples

• While 1
• print Type Ctrl-C to stop me!
• x spam
• While x
• print x
• x x1
• spam pam am m

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Break and Continue

• You can use the keyword break inside a loop to
  leave the while loop entirely.
• You can use the keyword continue inside a loop to
  stop processing the current iteration of the loop
  and to immediately go on to the next one.

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While Loop with Break

• while 1
• name raw_input(Enter name)
• if name stop break
• age raw_input(Enter age)
• print Hello, name, gt,\
• int(age)2

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While Loop with Break, cont.

• Enter name mel
• Enter age 20
• Hello mel gt 400
• Enter name bob
• Enter age 30
• Hello, bob gt 900
• Enter name stop

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While Loop with Continue

• x 10
• while x
• x x-1
• if x 2 ! 0 continue Odd? Skip print
• print x

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Assert

• An assert statement will check to make sure that
  something is true during the course of a program.
  
• If the condition if false, the program stops.
• assert(number_of_players lt 5)
• if number_of_players gt 5
• STOP

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Logical Expressions
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True and False

• True and False are constants in Python.
• Generally, True equals 1 and False equals 0.
• Other values equivalent to True and False
• False zero, None, empty container or object
• True non-zero numbers, non-empty objects.
• Comparison operators , !, lt, lt, etc.
• X and Y have same value X Y
• X and Y are two names that point to the same
  memory reference X is Y

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Boolean Logic Expressions

• You can also combine Boolean expressions.
• True if a is true and b is true a and b
• True if a is true or b is true a or b
• True if a is false not a
• Use parentheses as needed to disambiguate complex
  Boolean expressions.

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Special Properties of And and Or

• Actually and and or dont return True or
  False. They return the value of one of their
  sub-expressions (which may be a non-Boolean
  value).
• X and Y and Z
• If all are true, returns value of Z.
• Otherwise, returns value of first false
  sub-expression.
• X or Y or Z
• If all are false, returns value of Z.
• Otherwise, returns value of first true
  sub-expression.

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The and-or Trick

• There is a common trick used by Python
  programmers to implement a simple conditional
  using and and or. result test and expr1 or
  expr2
• When test is True, result is assigned expr1.
• When test is False, result is assigned expr2.
• Works like (test ? expr1 expr2) expression of
  C.
• But you must be certain that the value of expr1
  is never False or else the trick wont work.
• I wouldnt use this trick yourself, but you
  should be able to understand it if you see it

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Object Oriented Programmingin Python
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Its all objects

• Everything in Python is really an object.
• Weve seen hints of this alreadyhello.upper()
  list3.append(a)dict2.keys()
• You can also design your own objects in
  addition to these built-in data-types.
• In fact, programming in Python is typically done
  in an object oriented fashion.

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Defining Classes
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Defining a Class

• A class is a special data type which defines how
  to build a certain kind of object.
• The class also stores some data items that are
  shared by all the instances of this class.
• Instances are objects that are created which
  follow the definition given inside of the class.
• Python doesnt use separate class interface
  definitions as in some languages. You just
  define the class and then use it.

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Methods in Classes

• You can define a method in a class by including
  function definitions within the scope of the
  class block.
• Note that there is a special first argument self
  in all of the method definitions.
• Note that there is usually a special method
  called __init__ in most classes.
• Well talk about both later

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Definition of student

• class studentA class representing a
  student.def __init__(self,n,a)
  self.full_name n self.age adef
  get_age(self) return self.age

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Creating and Deleting Instances
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Instantiating Objects

• There is no new keyword as in Java.
• You merely use the class name with () notation
  and assign the result to a variable.
• b student(Bob Smith, 21)
• The arguments you pass to the class name are
  actually given to its .__init__() method.

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Constructor __init__

• __init__ acts like a constructor for your class.
  
• When you create a new instance of a class, this
  method is invoked. Usually does some
  initialization work.
• The arguments you list when instantiating an
  instance of the class are passed along to the
  __init__ method. b student(Bob, 21)
  So, the __init__ method is passed Bob and 21.

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Constructor __init__

• Your __init__ method can take any number of
  arguments.
• Just like other functions or methods, the
  arguments can be defined with default values,
  making them optional to the caller.
• However, the first argument self in the
  definition of __init__ is special

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Self

• The first argument of every method is a reference
  to the current instance of the class.
• By convention, we name this argument self.
• In __init__, self refers to the object currently
  being created so, in other class methods, it
  refers to the instance whose method was called.
• Similar to the keyword this in Java or C.
• But Python uses self more often than Java uses
  this.

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Self

• Although you must specify self explicitly when
  defining the method, you dont include it when
  calling the method.
• Python passes it for you automatically.
• Defining a method Calling a method
• (this code inside a class definition.)
• def set_age(self, num) gtgtgt x.set_age(23)self.ag
  e num

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No Need to free

• When you are done with an object, you dont have
  to delete or free it explicitly.
• Python has automatic garbage collection.
• Python will automatically detect when all of the
  references to a piece of memory have gone out of
  scope. Automatically frees that memory.
• Generally works well, few memory leaks.
• Theres also no destructor method for classes.

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Access to Attributes and Methods
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Definition of student

• class studentA class representing a
  student.def __init__(self,n,a)
  self.full_name n self.age adef
  get_age(self) return self.age

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Traditional Syntax for Access

• gtgtgt f student (Bob Smith, 23)
• gtgtgt f.full_name Access an attribute.
• Bob Smith
• gtgtgt f.get_age() Access a method.
• 23

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Accessing unknown members

• What if you dont know the name of the attribute
  or method of a class that you want to access
  until run time
• Is there a way to take a string containing the
  name of an attribute or method of a class and get
  a reference to it (so you can use it)?

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getattr(object_instance, string)

• gtgtgt f student(Bob Smith, 23)
• gtgtgt getattr(f, full_name)
• Bob Smith
• gtgtgt getattr(f, get_age)
• ltmethod get_age of class studentClass at
  010B3C2gt
• gtgtgt getattr(f, get_age)() We can call this.
• 23
• gtgtgt getattr(f, get_birthday)
• Raises AttributeError No method exists.

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hasattr(object_instance,string)

• gtgtgt f student(Bob Smith, 23)
• gtgtgt hasattr(f, full_name)
• True
• gtgtgt hasattr(f, get_age)
• True
• gtgtgt hasattr(f, get_birthday)
• False

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Attributes
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Two Kinds of Attributes

• The non-method data stored by objects are called
  attributes. Theres two kinds
• Data attribute Variable owned by a particular
  instance of a class.Each instance can have its
  own different value for it.These are the most
  common kind of attribute.
• Class attributes Owned by the class as a whole.
  All instances of the class share the same value
  for it.Called static variables in some
  languages. Good for class-wide constants or for
  building counter of how many instances of the
  class have been made.

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Data Attributes

• You create and initialize a data attribute inside
  of the __init__() method.
• Remember assignment is how we create variables in
  Python so, assigning to a name creates the
  attribute.
• Inside the class, you refer to data attributes
  using self for example, self.full_name
• class teacherA class representing
  teachers.def __init__(self,n)
  self.full_name ndef print_name(self)
  print self.full_name

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Class Attributes

• All instances of a class share one copy of a
  class attribute, so when any of the instances
  change it, then the value is changed for all of
  them.
• We define class attributes outside of any method.
• Since there is one of these attributes per class
  and not one per instance, we use a different
  notation
• We access them using self.__class__.name
  notation.

class sample gtgtgt a sample()x 23 gtgtgt
a.increment()def increment(self) gtgtgt
a.__class__.x self.__class__.x 1 24
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Data vs. Class Attributes

• class counteroverall_total 0 class
  attribute def __init__(self) self.my_total
  0 data attributedef increment(self)
  counter.overall_total \
  counter.overall_total 1 self.my_total \
  self.my_total 1

gtgtgt a counter()gtgtgt b counter()gtgtgt
a.increment()gtgtgt b.increment()gtgtgt
b.increment()gtgtgt a.my_total1gtgtgt
a.__class__.overall_total3gtgtgt b.my_total2gtgtgt
b.__class__.overall_total3