python.pptprogram on python what is python where it was started by whom started

Nov 25, 2024 Advanced Programming
Spring 2002
Python
Henning Schulzrinne
Department of Computer
Science
Columbia University
(based on tutorial by Guido van Rossum)

Spring 2002
Introduction
 Most recent popular
(scripting/extension) language
 although origin ~1991
 heritage: teaching language (ABC)
 Tcl: shell
 perl: string (regex) processing
 object-oriented
 rather than add-on (OOTcl)

Spring 2002
Python philosophy
 Coherence
 not hard to read, write and maintain
 power
 scope
 rapid development + large systems
 objects
 integration
 hybrid systems

Spring 2002
Python features
no compiling or linking rapid development cycle
no type declarations simpler, shorter, more flexible
automatic memory management garbage collection
high-level data types and
operations
fast development
object-oriented programming code structuring and reuse, C++
embedding and extending in C mixed language systems
classes, modules, exceptions "programming-in-the-large"
support
dynamic loading of C modules simplified extensions, smaller
binaries
dynamic reloading of C modules programs can be modified
without stopping
Lutz, Programming Python

Spring 2002
Python features
universal "first-class" object
model
fewer restrictions and rules
run-time program construction handles unforeseen needs, end-
user coding
interactive, dynamic nature incremental development and
testing
access to interpreter information metaprogramming, introspective
objects
wide portability cross-platform programming
without ports
compilation to portable byte-
code
execution speed, protecting
source code
built-in interfaces to external
services
system tools, GUIs, persistence,
databases, etc.
Lutz, Programming Python

Spring 2002
Python
 elements from C++, Modula-3
(modules), ABC, Icon (slicing)
 same family as Perl, Tcl, Scheme,
REXX, BASIC dialects

Spring 2002
Uses of Python
 shell tools
 system admin tools, command line programs
 extension-language work
 rapid prototyping and development
 language-based modules
 instead of special-purpose parsers
 graphical user interfaces
 database access
 distributed programming
 Internet scripting

Spring 2002
What not to use Python (and
kin) for
 most scripting languages share these
 not as efficient as C
 but sometimes better built-in algorithms
(e.g., hashing and sorting)
 delayed error notification
 lack of profiling tools

Spring 2002
Using python
 /usr/local/bin/python
 #! /usr/bin/env python
 interactive use
Python 1.6 (#1, Sep 24 2000, 20:40:45) [GCC 2.95.1 19990816 (release)] on sunos5
Copyright (c) 1995-2000 Corporation for National Research Initiatives.
All Rights Reserved.
Copyright (c) 1991-1995 Stichting Mathematisch Centrum, Amsterdam.
All Rights Reserved.
>>>
 python –c command [arg] ...
 python –i script
 read script first, then interactive

Spring 2002
Python structure
 modules: Python source files or C extensions
 import, top-level via from, reload
 statements
 control flow
 create objects
 indentation matters – instead of {}
 objects
 everything is an object
 automatically reclaimed when no longer needed

Spring 2002
First example
#!/usr/local/bin/python
# import systems module
import sys
marker = '::::::'
for name in sys.argv[1:]:
input = open(name, 'r')
print marker + name
print input.read()

Spring 2002
Basic operations
 Assignment:
 size = 40
 a = b = c = 3
 Numbers
 integer, float
 complex numbers: 1j+3, abs(z)
 Strings
 'hello world', 'it's hot'
 "bye world"
 continuation via or use """ long text """"

Spring 2002
String operations
 concatenate with + or neighbors
 word = 'Help' + x
 word = 'Help' 'a'
 subscripting of strings
 'Hello'[2]  'l'
 slice: 'Hello'[1:2]  'el'
 word[-1]  last character
 len(word)  5
 immutable: cannot assign to subscript

Spring 2002
Lists
 lists can be heterogeneous
 a = ['spam', 'eggs', 100, 1234, 2*2]
 Lists can be indexed and sliced:
 a[0]  spam
 a[:2]  ['spam', 'eggs']
 Lists can be manipulated
 a[2] = a[2] + 23
 a[0:2] = [1,12]
 a[0:0] = []
 len(a)  5

Spring 2002
Basic programming
a,b = 0, 1
# non-zero = true
while b < 10:
# formatted output, without n
print b,
# multiple assignment
a,b = b, a+b

Spring 2002
Control flow: if
x = int(raw_input("Please enter #:"))
if x < 0:
x = 0
print 'Negative changed to zero'
elif x == 0:
print 'Zero'
elif x == 1:
print 'Single'
else:
print 'More'
 no case statement

Spring 2002
Control flow: for
a = ['cat', 'window', 'defenestrate']
for x in a:
print x, len(x)
 no arithmetic progression, but
 range(10)  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
 for i in range(len(a)):
print i, a[i]
 do not modify the sequence being iterated
over

Spring 2002
Loops: break, continue, else
 break and continue like C
 else after loop exhaustion
for n in range(2,10):
for x in range(2,n):
if n % x == 0:
print n, 'equals', x, '*', n/x
break
else:
# loop fell through without finding a factor
print n, 'is prime'

Spring 2002
Do nothing
 pass does nothing
 syntactic filler
while 1:
pass

Spring 2002
Defining functions
def fib(n):
"""Print a Fibonacci series up to n."""
a, b = 0, 1
while b < n:
print b,
a, b = b, a+b
>>> fib(2000)
 First line is docstring
 first look for variables in local, then global
 need global to assign global variables

Spring 2002
Functions: default argument
values
def ask_ok(prompt, retries=4,
complaint='Yes or no, please!'):
while 1:
ok = raw_input(prompt)
if ok in ('y', 'ye', 'yes'): return 1
if ok in ('n', 'no'): return 0
retries = retries - 1
if retries < 0: raise IOError,
'refusenik error'
print complaint
>>> ask_ok('Really?')

Spring 2002
Keyword arguments
 last arguments can be given as
keywords
def parrot(voltage, state='a stiff', action='voom',
type='Norwegian blue'):
print "-- This parrot wouldn't", action,
print "if you put", voltage, "Volts through it."
print "Lovely plumage, the ", type
print "-- It's", state, "!"
parrot(1000)
parrot(action='VOOOM', voltage=100000)

Spring 2002
Lambda forms
 anonymous functions
 may not work in older versions
def make_incrementor(n):
return lambda x: x + n
f = make_incrementor(42)
f(0)
f(1)

Spring 2002
List methods
 append(x)
 extend(L)
 append all items in list (like Tcl lappend)
 insert(i,x)
 remove(x)
 pop([i]), pop()
 create stack (FIFO), or queue (LIFO)  pop(0)
 index(x)
 return the index for value x

Spring 2002
List methods
 count(x)
 how many times x appears in list
 sort()
 sort items in place
 reverse()
 reverse list

Spring 2002
Functional programming
tools
 filter(function, sequence)
def f(x): return x%2 != 0 and x%3 0
filter(f, range(2,25))
 map(function, sequence)
 call function for each item
 return list of return values
 reduce(function, sequence)
 return a single value
 call binary function on the first two items
 then on the result and next item
 iterate

Spring 2002
List comprehensions (2.0)
 Create lists without map(),
filter(), lambda
 = expression followed by for clause +
zero or more for or of clauses
>>> vec = [2,4,6]
>>> [3*x for x in vec]
[6, 12, 18]
>>> [{x: x**2} for x in vec}
[{2: 4}, {4: 16}, {6: 36}]

Spring 2002
List comprehensions
 cross products:
>>> vec1 = [2,4,6]
>>> vec2 = [4,3,-9]
>>> [x*y for x in vec1 for y in vec2]
[8,6,-18, 16,12,-36, 24,18,-54]
>>> [x+y for x in vec1 and y in vec2]
[6,5,-7,8,7,-5,10,9,-3]
>>> [vec1[i]*vec2[i] for i in
range(len(vec1))]
[8,12,-54]

Spring 2002
List comprehensions
 can also use if:
>>> [3*x for x in vec if x > 3]
[12, 18]
>>> [3*x for x in vec if x < 2]
[]

Spring 2002
del – removing list items
 remove by index, not value
 remove slices from list (rather than by
assigning an empty list)
>>> a = [-1,1,66.6,333,333,1234.5]
>>> del a[0]
>>> a
[1,66.6,333,333,1234.5]
>>> del a[2:4]
>>> a
[1,66.6,1234.5]

Spring 2002
Tuples and sequences
 lists, strings, tuples: examples of
sequence type
 tuple = values separated by commas
>>> t = 123, 543, 'bar'
>>> t[0]
123
>>> t
(123, 543, 'bar')

Spring 2002
Tuples
 Tuples may be nested
>>> u = t, (1,2)
>>> u
((123, 542, 'bar'), (1,2))
 kind of like structs, but no element names:
 (x,y) coordinates
 database records
 like strings, immutable  can't assign to
individual items

Spring 2002
Tuples
 Empty tuples: ()
>>> empty = ()
>>> len(empty)
0
 one item  trailing comma
>>> singleton = 'foo',

Spring 2002
Tuples
 sequence unpacking  distribute
elements across variables
>>> t = 123, 543, 'bar'
>>> x, y, z = t
>>> x
123
 packing always creates tuple
 unpacking works for any sequence

Spring 2002
Dictionaries
 like Tcl or awk associative arrays
 indexed by keys
 keys are any immutable type: e.g., tuples
 but not lists (mutable!)
 uses 'key: value' notation
>>> tel = {'hgs' : 7042, 'lennox': 7018}
>>> tel['cs'] = 7000
>>> tel

Spring 2002
Dictionaries
 no particular order
 delete elements with del
>>> del tel['foo']
 keys() method  unsorted list of keys
>>> tel.keys()
['cs', 'lennox', 'hgs']
 use has_key() to check for existence
>>> tel.has_key('foo')
0

Spring 2002
Conditions
 can check for sequence membership with
is and is not:
>>> if (4 in vec):
... print '4 is'
 chained comparisons: a less than b AND b
equals c:
a < b == c
 and and or are short-circuit operators:
 evaluated from left to right
 stop evaluation as soon as outcome clear

Spring 2002
Conditions
 Can assign comparison to variable:
>>> s1,s2,s3='', 'foo', 'bar'
>>> non_null = s1 or s2 or s3
>>> non_null
foo
 Unlike C, no assignment within
expression

Spring 2002
Comparing sequences
 unlike C, can compare sequences
(lists, tuples, ...)
 lexicographical comparison:
 compare first; if different  outcome
 continue recursively
 subsequences are smaller
 strings use ASCII comparison
 can compare objects of different type,
but by type name (list < string < tuple)

Spring 2002
Comparing sequences
(1,2,3) < (1,2,4)
[1,2,3] < [1,2,4]
'ABC' < 'C' < 'Pascal' < 'Python'
(1,2,3) == (1.0,2.0,3.0)
(1,2) < (1,2,-1)

Spring 2002
Modules
 collection of functions and variables,
typically in scripts
 definitions can be imported
 file name is module name + .py
 e.g., create module fibo.py
def fib(n): # write Fib. series up to n
...
def fib2(n): # return Fib. series up to n

Spring 2002
Modules
 import module:
import fibo
 Use modules via "name space":
>>> fibo.fib(1000)
>>> fibo.__name__
'fibo'
 can give it a local name:
>>> fib = fibo.fib
>>> fib(500)

Spring 2002
Modules
 function definition + executable statements
 executed only when module is imported
 modules have private symbol tables
 avoids name clash for global variables
 accessible as module.globalname
 can import into name space:
>>> from fibo import fib, fib2
>>> fib(500)
 can import all names defined by module:
>>> from fibo import *

Spring 2002
Module search path
 current directory
 list of directories specified in PYTHONPATH
environment variable
 uses installation-default if not defined,
e.g., .:/usr/local/lib/python
 uses sys.path
>>> import sys
>>> sys.path
['', 'C:PROGRA~1Python2.2', 'C:Program Files
Python2.2DLLs', 'C:Program FilesPython2.2lib', 'C:
Program FilesPython2.2liblib-tk', 'C:Program Files
Python2.2', 'C:Program FilesPython2.2libsite-
packages']

Spring 2002
Compiled Python files
 include byte-compiled version of module if
there exists fibo.pyc in same directory as
fibo.py
 only if creation time of fibo.pyc matches
fibo.py
 automatically write compiled file, if possible
 platform independent
 doesn't run any faster, but loads faster
 can have only .pyc file  hide source

Spring 2002
Standard modules
 system-dependent list
 always sys module
>>> import sys
>>> sys.p1
'>>> '
>>> sys.p2
'... '
>>> sys.path.append('/some/directory')

Spring 2002
Module listing
 use dir() for each module
>>> dir(fibo)
['___name___', 'fib', 'fib2']
>>> dir(sys)
['__displayhook__', '__doc__', '__excepthook__', '__name__', '__stderr__', '__st
din__', '__stdout__', '_getframe', 'argv', 'builtin_module_names', 'byteorder',
'copyright', 'displayhook', 'dllhandle', 'exc_info', 'exc_type', 'excepthook', '
exec_prefix', 'executable', 'exit', 'getdefaultencoding', 'getrecursionlimit', '
getrefcount', 'hexversion', 'last_type', 'last_value', 'maxint', 'maxunicode', '
modules', 'path', 'platform', 'prefix', 'ps1', 'ps2', 'setcheckinterval', 'setpr
ofile', 'setrecursionlimit', 'settrace', 'stderr', 'stdin', 'stdout', 'version',
'version_info', 'warnoptions', 'winver']

Spring 2002
Classes
 mixture of C++ and Modula-3
 multiple base classes
 derived class can override any methods of its
base class(es)
 method can call the method of a base class
with the same name
 objects have private data
 C++ terms:
 all class members are public
 all member functions are virtual
 no constructors or destructors (not needed)

Spring 2002
Classes
 classes (and data types) are objects
 built-in types cannot be used as base
classes by user
 arithmetic operators, subscripting
can be redefined for class instances
(like C++, unlike Java)

Spring 2002
Class definitions
Class ClassName:
<statement-1>
...
<statement-N>
 must be executed
 can be executed conditionally (see
Tcl)
 creates new namespace

Spring 2002
Namespaces
 mapping from name to object:
 built-in names (abs())
 global names in module
 local names in function invocation
 attributes = any following a dot
 z.real, z.imag
 attributes read-only or writable
 module attributes are writeable

Spring 2002
Namespaces
 scope = textual region of Python program
where a namespace is directly accessible
(without dot)
 innermost scope (first) = local names
 middle scope = current module's global names
 outermost scope (last) = built-in names
 assignments always affect innermost scope
 don't copy, just create name bindings to objects
 global indicates name is in global scope

Spring 2002
Class objects
 obj.name references (plus module!):
class MyClass:
"A simple example class"
i = 123
def f(self):
return 'hello world'
>>> MyClass.i
123
 MyClass.f is method object

Spring 2002
Class objects
 class instantiation:
>>> x = MyClass()
>>> x.f()
'hello world'
 creates new instance of class
 note x = MyClass vs. x = MyClass()
 ___init__() special method for
initialization of object
def __init__(self,realpart,imagpart):
self.r = realpart
self.i = imagpart

Spring 2002
Instance objects
 attribute references
 data attributes (C++/Java data
members)
 created dynamically
x.counter = 1
while x.counter < 10:
x.counter = x.counter * 2
print x.counter
del x.counter

Spring 2002
Method objects
 Called immediately:
x.f()
 can be referenced:
xf = x.f
while 1:
print xf()
 object is passed as first argument of
function  'self'
 x.f() is equivalent to MyClass.f(x)

Spring 2002
Notes on classes
 Data attributes override method
attributes with the same name
 no real hiding  not usable to
implement pure abstract data types
 clients (users) of an object can add
data attributes
 first argument of method usually
called self
 'self' has no special meaning (cf. Java)

Spring 2002
Another example
 bag.py
class Bag:
def __init__(self):
self.data = []
def add(self, x):
self.data.append(x)
def addtwice(self,x):
self.add(x)
self.add(x)

Spring 2002
Another example, cont'd.
 invoke:
>>> from bag import *
>>> l = Bag()
>>> l.add('first')
>>> l.add('second')
>>> l.data
['first', 'second']

Spring 2002
Inheritance
class DerivedClassName(BaseClassName)
<statement-1>
...
<statement-N>
 search class attribute, descending
chain of base classes
 may override methods in the base
class
 call directly via BaseClassName.method

Spring 2002
Multiple inheritance
class DerivedClass(Base1,Base2,Base3):
<statement>
 depth-first, left-to-right
 problem: class derived from two
classes with a common base class

Spring 2002
Private variables
 No real support, but textual
replacement (name mangling)
 __var is replaced by
_classname_var
 prevents only accidental
modification, not true protection

Spring 2002
~ C structs
 Empty class definition:
class Employee:
pass
john = Employee()
john.name = 'John Doe'
john.dept = 'CS'
john.salary = 1000

Spring 2002
Exceptions
 syntax (parsing) errors
while 1 print 'Hello World'
File "<stdin>", line 1
while 1 print 'Hello World'
^
SyntaxError: invalid syntax
 exceptions
 run-time errors
 e.g., ZeroDivisionError,
NameError, TypeError

Spring 2002
Handling exceptions
while 1:
try:
x = int(raw_input("Please enter a number: "))
break
except ValueError:
print "Not a valid number"
 First, execute try clause
 if no exception, skip except clause
 if exception, skip rest of try clause and use
except clause
 if no matching exception, attempt outer try
statement

Spring 2002
Handling exceptions
 try.py
import sys
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except IOError:
print 'cannot open', arg
else:
print arg, 'lines:', len(f.readlines())
f.close
 e.g., as python try.py *.py

Spring 2002
Language comparison
Tcl Perl Python JavaScript Visual
Basic
Speed development     
regexp   
breadth extensible   
embeddable  
easy GUI   (Tk) 
net/web     
enterpris
e
cross-platform    
I18N    
thread-safe   
database access     

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