CSE 399-004:Python Programming

Lecture 2: Data, Classes, and ModulesJanuary 22, 2007

http://www.seas.upenn.edu/~cse39904/

Administrative things

Teaching assistant

• Brian Summa (bsumma @ seas.upenn.edu)

• Office hours in Moore 100A (Linux lab)• Thursday 6 – 7pm• Friday 5:30 – 6:30pm

• He'll be doing a lot of the grading on homeworks• Talk to Brian S. first if you have a question about grading• If that doesn't work out, talk to me next

• Feel free to ask him questions, as well as me

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Other announcements

• Homework 2 was due this morning

• Homework plans:• Homework 3 (Othello board) out by tomorrow• Homework 4 (Othello AI) out by next week• Both will be due two weeks from today

• My office hours: Wednesday 11–noon, 4:30–6pm

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cse39904submit

• You must submit all of your files every single timeyou use cse39904submit to turn in something

• You cannot submit things in bits and pieces

• cse39904submit -i hwX will list the files you have submitted for hwX

• cse39904submit -h tells you how to use it

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Quick poll

• Would you rather there be a bulletin board for discussions, as in CSE 1xx, or would you rather use the email list?

• I'm going to insist that all discussions happen on exactly one of these things

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The view from here

So far in this course

• One lecture: Crash-course introduction to Python• Basic datatypes• Basic control flow• How to run Python programs

• Two homeworks, which threw you in the ocean and asked you to swim (not so kind of me to do that…)

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The plan from here

• Next 3 lectures: Talk about Python itself• Everyone here comes from a wide range of backgrounds

• That leaves 9 lectures to talk about…• Python itself some more• Various Python libraries

• Somewhere in here, you'll start working on projects• Not sure what exactly will happen to homeworks• Lectures will continue

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Data in Python

What is a variable?

• All data in Python is represented by objects

• Then variables must all be references to objects

• That is, the memory a variable takes up is merely a pointer to some other part of memory that actually has the object

• We usually think of the thing being pointed to as the value of the variable, not the pointer itself

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>>> q = [2, 3]>>> p = [1, q, 4]>>> print p[1, [2, 3], 4]>>> q[0] = 42>>> print q[42, 3]>>> print p[1, [42, 3], 4]

q

2 3

The integers should also be pointers, but they're immutable, so it doesn't

really matter.

>>> q = [2, 3]>>> p = [1, q, 4]>>> print p[1, [2, 3], 4]>>> q[0] = 42>>> print q[42, 3]>>> print p[1, [42, 3], 4]

q

2 3

p

1 4

>>> q = [2, 3]>>> p = [1, q, 4]>>> print p[1, [2, 3], 4]>>> q[0] = 42>>> print q[42, 3]>>> print p[1, [42, 3], 4]

q

42 3

p

1 4

>>> q = [2, 3]>>> p = (1, q, 4)>>> print p(1, [2, 3], 4)>>> q[0] = 42>>> print q[42, 3]>>> print p(1, [42, 3], 4)>>> p[2] = 100(... error messages ...)TypeError: 'tuple' object does not support item assignment

Notice what it means for a tuple to be immutable!

q

42 3

p

1 4

Classes: The basics

Object-oriented programming (OOP)

• Based on the survey results, it seems safe to assume that everyone here knows what OOP is

• Terminology like the following should be familiar:• Class• Inheritance• Subclass / superclass• Instances / objects• Methods (instance and static)• Instance variables• Derived class / Base class

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Some differences from Java

• Everything in a class is public

• No overloading — Use optional arguments• Don't confuse this with overriding

• Derived classes may have multiple base classes• This is known as "multiple inheritance"• You should avoid this when possible

• No super

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Aside: New-style versus Classic-style

• Basic idea: there should be no distinction between built-in types and user-defined classes

• I'll try to highlight some of the differences between the two styles of classes

• It's unlikely you'll come out today with any real appreciation of what makes them different

• Just be aware that Python is in the process of transitioning to the new-style classes

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Basic structure of a class definition

• Defines a class Derived which inherits from base classes Base1 and Base2

• Derived will be a new-style class if it inherits from a new-style class and classic-style otherwise

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class Derived(Base1, Base2): "Documentation string." <statements>

Basic structure of a class definition

• You can specify no base classes, in which case you get a classic-style class

• You can specify the base class object if you want a new-style class that inherits as little as possible

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class Derived(Base1, Base2): "Documentation string." <statements>

Constructors

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class Pair(): def __init__(self, i, j): self.fst = i self.snd = j

>>> a = Pair(2, 3)>>> print a<__main__.Pair instance at 0x67300>

Constructors

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class Pair(): def __init__(self, i, j): self.fst = i self.snd = j

>>> a = Pair(2, 3)>>> print a<__main__.Pair instance at 0x67300>

like Java, but no new needed

Constructors

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class Pair(): def __init__(self, i, j): self.fst = i self.snd = j

>>> a = Pair(2, 3)>>> print a<__main__.Pair instance at 0x67300>

the constructor of a class is called __init__

Constructors

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class Pair(): def __init__(self, i, j): self.fst = i self.snd = j

>>> a = Pair(2, 3)>>> print a<__main__.Pair instance at 0x67300>

Did you notice that __init__ takes 3 arguments, but we constructed an instance by only giving 2? And no optional arguments either!

Self

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• The first argument of the constructor or a method in a class is usually called self

• It corresponds to this in Java

• Since Python has no variable declarations, you need an explicit means of assigning to instance variables

• self provides you means of doing exactly that• It also lets you call methods on the current instance

Self (continued)

• The assignments here assign to two variables fst and snd which are local to __init__

• Calling get_first() would result in an error

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class Pair(): def __init__(self, i, j): fst = i snd = j def get_first(): return fst

Self (continued)

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class Pair(): def __init__(self, i, j): fst = i snd = j def get_first(): return fst

>>> a = Pair(2, 3)>>> a.get_first()(... error messages ...)TypeError: get_first() takes no arguments (1 given)

Self (continued)

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class Pair(): def __init__(self, i, j): fst = i snd = j def get_first(): return fst

>>> a = Pair(2, 3)>>> a.get_first()(... error messages ...)TypeError: get_first() takes no arguments (1 given)

method invocation uses familiar dot notation

Self (continued)

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class Pair(): def __init__(self, i, j): fst = i snd = j def get_first(): return fst

>>> a = Pair(2, 3)>>> a.get_first()(... error messages ...)TypeError: get_first() takes no arguments (1 given)

no self argument

namely, the instance

Self (continued)

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class Pair(): def __init__(self, i, j): fst = i snd = j def get_first(self): return fst

>>> a = Pair(2, 3)>>> a.get_first()(... error messages ...)NameError: global name 'fst' is not defined

Self (continued)

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class Pair(): def __init__(self, i, j): fst = i snd = j def get_first(self): return fst

>>> a = Pair(2, 3)>>> a.get_first()(... error messages ...)NameError: global name 'fst' is not defined

should get this from self

should assign these via self

Self (continued)

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class Pair(): def __init__(self, i, j): self.fst = i self.snd = j def get_first(self): return self.fst

>>> a = Pair(2, 3)>>> a.get_first()2>>> a.fst2

Self (continued)

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class Pair(): def __init__(self, i, j): self.fst = i self.snd = j def get_first(self): return self.fst

>>> a = Pair(2, 3)>>> a.get_first()2>>> a.fst2

everything is public

Basic rules of thumb

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• Instance variables are referred to through self• For example: self.foo

• Methods and constructors must take at least one argument, which is called self

Basic rules of thumb

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• Instance variables are referred to through self• For example: self.foo

• Methods and constructors must take at least one argument, which is called self

Technically, you can call it anything you want. The convention is to call it self.Also ignoring static methods (for now).

instanceof

• instanceof(obj, cls)• Return True if obj is an instance of cls• Returns False otherwise

• Works pretty much as you might expect

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Single inheritance

• Works much as it did in Java

• Catch #1: Super class constructor is not called automatically (call it yourself if need be)

• Catch #2: No super in Python• Use BaseClass.foo to refer to superclass attributes• If you call super class methods like this, you have to pass

in self yourself!

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Single inheritance (example)

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class Pair(): def __init__(self, i, j): self.fst = i self.snd = j def get_first(self): return self.fst

class SillyPair(Pair): def get_first(self): return Pair.get_first(self) + 50 def get_second(self): return self.get_first() + self.snd

>>> a = SillyPair(2, 3)>>> a.get_second()55>>> a.get_first()52

Single inheritance (example)

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class Pair(): def __init__(self, i, j): self.fst = i self.snd = j def get_first(self): return self.fst

class SillyPair(Pair): def get_first(self): return Pair.get_first(self) + 50 def get_second(self): return self.get_first() + self.snd

>>> a = SillyPair(2, 3)>>> a.get_second()55>>> a.get_first()52

constructor inherited, just like any other method

Single inheritance (example)

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class Pair(): def __init__(self, i, j): self.fst = i self.snd = j def get_first(self): return self.fst

class SillyPair(Pair): def get_first(self): return Pair.get_first(self) + 50 def get_second(self): return self.get_first() + self.snd

>>> a = SillyPair(2, 3)>>> a.get_second()55>>> a.get_first()52

call the superclass's version

Single inheritance (example)

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class Pair(): def __init__(self, i, j): self.fst = i self.snd = j def get_first(self): return self.fst

class SillyPair(Pair): def get_first(self): return Pair.get_first(self) + 50 def get_second(self): return self.get_first() + self.snd

>>> a = SillyPair(2, 3)>>> a.get_second()55>>> a.get_first()52

call a method on the current instance

Modules: The basics

What is a module?

• Basic problems in programming Python:• Typing things into the interpreter repeatedly gets old• You want to reuse code that someone else has written

• Modules provide a basic way of organizing code

• A module mod is simply a Python file mod.py,i.e., "module" is just another name for a Python file

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Loading modules

• import foo will import the module foo, which should be in the file foo.py

• What import foo does:• If foo has already been imported, do nothing• Otherwise, search for the file foo.py in the search path

• By default, the search path includes the current working directory and the standard library directories

• For now, put your modules in the current directory

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Loading modules

• import foo will import the module foo, which should be in the file foo.py

• What import foo does:• If foo has already been imported, do nothing• Otherwise, search for the file foo.py in the search path• Parse foo.py, raise an exception if there is a syntax error• Otherwise, add foo to the list of imported modules

• The interpreter keeps track of all imported modules

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Loading modules

• import foo will import the module foo, which should be in the file foo.py

• What import foo does:• If foo has already been imported, do nothing• Otherwise, search for the file foo.py in the search path• Parse foo.py, raise an exception if there is a syntax error• Otherwise, add foo to the list of imported modules• Execute the code in foo.py

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"Executing" a module

• About that last step: what does it mean?

• Each loaded module has a symbol table which contains entries for every name it defines

• Each statement is executed, and if necessary, an entry is added to the symbol table

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Saving some typing

• from mod import foo, bar, baz• Lets you use foo, bar, baz without typing "mod."

• from mod import *• Lets you use anything from mod without typing "mod."

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Saving some typing

• from mod import foo, bar, baz• Lets you use foo, bar, baz without typing "mod."

• from mod import *• Lets you use anything from mod without typing "mod."

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a lie, but it's good enough

for now

Saving some typing

• from mod import foo, bar, baz• Lets you use foo, bar, baz without typing "mod."

• from mod import *• Lets you use anything from mod without typing "mod."

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>>> import sys>>> sys.stdout.write("Hello!\n")Hello!>>> sys.modules{'copy_reg': <module 'copy_reg' from ...

Saving some typing

• from mod import foo, bar, baz• Lets you use foo, bar, baz without typing "mod."

• from mod import *• Lets you use anything from mod without typing "mod."

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>>> from sys import stdout>>> stdout.write("Hello!\n")Hello!>>> modules(... error messages ...)NameError: name 'modules' is not defined

Saving some typing

• from mod import foo, bar, baz• Lets you use foo, bar, baz without typing "mod."

• from mod import *• Lets you use anything from mod without typing "mod."

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>>> from sys import *>>> stdout.write("Hello!\n")Hello!>>> modules{'copy_reg': <module 'copy_reg' from ...

Saving some typing

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>>> from sys import *>>> sys.stdout.write("Hello!\n")Traceback (most recent call last): File "<stdin>", line 1, in <module>NameError: name 'sys' is not defined

The from forms of import don't define the module name!

Tips about importing modules

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• Python evaluates the contents of a module onlythe first time it's imported

• Avoid from mod import * when possible• It clutters the namespace• It becomes difficult to determine who defined what

Modules as programs

• Many tools for Python import modules so that they can analyze them

• Therefore, it is a bad idea if non-trivial code gets executed when a module is imported

• For example:

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def foo(): return 42

print "Hello!"

not a good idea

• If you're "running" a file bar.py as a program, then in that case, bar.__name__ will be '__main__'.

Modules as programs

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>>> import bar>>> bar.__name__'bar'

def foo(): return 42

if __name__ == "__main__": print "Hello!"

bar.py

• If you're "running" a file bar.py as a program, then in that case, bar.__name__ will be '__main__'.

def foo(): return 42

if __name__ == "__main__": print "Hello!"

bar.py

Modules as programs

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prompt$ python bar.pyHello!

Next time

• More on classes• Scoping and namespaces in Python

• Times after that:• Review some basic datatypes and constructs• Exceptions• Generators (?)• Iterators (?)

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