1 python control of flow and defining classes ling 5200 computational corpus linguistics martha...
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Python Control of Flow and Defining Classes
LING 5200Computational Corpus LinguisticsMartha Palmer
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For Loops
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides3
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 <item> in <collection>:<statements>
When <collection> is a list or a tuple, then the loop steps through each element of the container.
When <collection> is a string, then the loop steps through each character of the string. for someChar in “Hello World”: print someChar
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides4
For Loops 2
The <item> part of the for loop can also be more complex than a single variable name. When the elements of a container <collection>
are also containers, then the <item> 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
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides5
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 say:for x in range(5): print x
LING 5200, 2006 BASED on Matt Huenerfauth’s
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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
LING 5200, 2006 BASED on Matt Huenerfauth’s
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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.
LING 5200, 2006 BASED on Matt Huenerfauth’s
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If Statements – Another Ex.
x = ‘killer rabbit’if x == ‘roger’:
print “how’s jessica?”elif x == ‘bugs’:
print “What’s up, doc?”else:
print “Run away! Run away!”
Run away! Run away!
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides9
While Loops
x = 3
while x < 10:
x = x + 1
print “Still in the loop.”
print “Outside of the loop.”
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides10
While Loops, more examples
While 1:
print “Type Ctrl-C to stop me!”
x = ‘spam’
While x:
print x
x = x[1:]
‘spam’ ‘pam’ ‘am’ ‘m’
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides11
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.
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides12
While Loop with Break
while 1:
name = raw_input(‘Enter name:’)
if name == ‘stop’: break
age = raw_input(‘Enter age:’)
print ‘Hello’, name, ‘=>’,\
int(age)**2
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides13
While Loop with Break, cont.
Enter name: melEnter age: 20Hello mel => 400
Enter name: bobEnter age: 30Hello, bob => 900
Enter name: stop
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides14
While Loop with Continue
x = 10
while x:
x = x-1
if x % 2 != 0: continue #Odd? Skip print
print x
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides15
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 < 5)
if number_of_players >= 5:
STOP
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Logical Expressions
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides17
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: ==, !=, <, <=, 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
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides18
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.
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides19
Special Properties of And and Or
Actually ‘and’ and ‘or’ don’t 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.
LING 5200, 2006 BASED on Matt Huenerfauth’s
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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 won’t work.
I wouldn’t use this trick yourself, but you should be able to understand it if you see it
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Object Oriented Programmingin Python
LING 5200, 2006 BASED on Matt Huenerfauth’s
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It’s all objects…
Everything in Python is really an object. We’ve seen hints of this already…“hello”.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
LING 5200, 2006 BASED on Matt Huenerfauth’s
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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 doesn’t use separate class interface
definitions as in some languages. You just define the class and then use it.
LING 5200, 2006 BASED on Matt Huenerfauth’s
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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.
We’ll talk about both later…
LING 5200, 2006 BASED on Matt Huenerfauth’s
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Definition of student
class student:“““A 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
LING 5200, 2006 BASED on Matt Huenerfauth’s
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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.
LING 5200, 2006 BASED on Matt Huenerfauth’s
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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.
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides30
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…
LING 5200, 2006 BASED on Matt Huenerfauth’s
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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.’
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides32
Self
Although you must specify self explicitly when defining the method, you don’t 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): >>> x.set_age(23)
self.age = num
LING 5200, 2006 BASED on Matt Huenerfauth’s
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No Need to “free”
When you are done with an object, you don’t 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. There’s also no “destructor” method for
classes.
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Access to Attributes and Methods
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides35
Definition of student
class student:“““A class representing a student.”””def __init__(self,n,a): self.full_name = n self.age = adef get_age(self): return self.age
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides36
Traditional Syntax for Access
>>> f = student (“Bob Smith”, 23)
>>> f.full_name # Access an attribute.
“Bob Smith”
>>> f.get_age() # Access a method.
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LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides37
Accessing unknown members
What if you don’t 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)?
LING 5200, 2006 BASED on Matt Huenerfauth’s
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getattr(object_instance, string)
>>> f = student(“Bob Smith”, 23)
>>> getattr(f, “full_name”)“Bob Smith”
>>> getattr(f, “get_age”) <method get_age of class studentClass at 010B3C2>
>>> getattr(f, “get_age”)() # We can call this.23
>>> getattr(f, “get_birthday”) # Raises AttributeError – No method exists.
LING 5200, 2006 BASED on Matt Huenerfauth’s
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hasattr(object_instance,string)
>>> f = student(“Bob Smith”, 23)
>>> hasattr(f, “full_name”)True
>>> hasattr(f, “get_age”)True
>>> hasattr(f, “get_birthday”)False
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Attributes
LING 5200, 2006 BASED on Matt Huenerfauth’s
Python Slides41
Two Kinds of Attributes
The non-method data stored by objects are called attributes. There’s 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.
LING 5200, 2006 BASED on Matt Huenerfauth’s
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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 teacher:“A class representing teachers.”def __init__(self,n): self.full_name = ndef print_name(self): print self.full_name
LING 5200, 2006 BASED on Matt Huenerfauth’s
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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: >>> a = sample()x = 23 >>> a.increment()def increment(self): >>> a.__class__.x self.__class__.x += 1 24
LING 5200, 2006 BASED on Matt Huenerfauth’s
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Data vs. Class Attributes
class counter:overall_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
>>> a = counter()>>> b = counter()>>> a.increment()>>> b.increment()>>> b.increment()>>> a.my_total1>>> a.__class__.overall_total3>>> b.my_total2>>> b.__class__.overall_total3