1 An Introduction to python, Harrison B. Prosper, 1997 An Introduction to Python Harrison B. Prosper...

An Introduction to python, Harrison

B. Prosper, 1997

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An Introduction toAn Introduction to

PythonPythonAn Introduction toAn Introduction to

PythonPythonHarrison B. ProsperFlorida State University


B. Prosper, 1997

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AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements

I learnt a great deal by browsing the python web site and its linked pages. http://www.python.org http://www.python.org/doc/tut/tut.html

Guido van Rossum (python author) David Beazley (SWIG author, University of Utah) Mark Lutz’s excellent book.

Programming Python, O’Reilly Associates Inc. Frederick Lundh’s Tkinter documentation is very helpful.

http://www.pythonware.com/library.htm


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OutlineOutlineOutlineOutline

Part I What is python? Getting started Namespaces Sequences Functions Classes Odds and ends

Part 2 Graphical User Interfaces A Simple GIF/JPEG/BMP Browser


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What is python?What is python?What is python?What is python?

Python is a scripting language, created by Guido van Rossum ([email protected]), that is: interpreted object-oriented dynamically typed simple powerful and free

Python website: http://www.python.org


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Getting startedGetting startedGetting startedGetting started

The python interpreter has been ported to all the popular platforms, including Windows 95/NT.

Running python interactively: setup python python >>> print ‘Thou lump of foul deformity’ Thou lump of foul deformity >>> x = 2; y = 5 >>> z = x*y >>> z 10


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Example 1 (from, import, for, in, if, else)Example 1 (from, import, for, in, if, else)Example 1 (from, import, for, in, if, else)Example 1 (from, import, for, in, if, else)

from string import atoi, split # Add atoi, split to global namespace.import os # Add os to global namespace.f = os.popen(‘cd; ls -l’) # Pipe command output to file.s = f.read() # Read into a single string. l = split(s,’\n’) # Split into a list of strings.n = len(l) # Get length of list.for j in xrange(n): # Iterate over a range object.

a = split(l[j]) # Split into a list of strings.if (len(a) > 4): # If length of list > 4 convert

size = atoi(a[4]) # string value into an integer.print size, l[j]

else: LOOK MA, NO BRACES BECAUSEprint l[j] PYTHON USES INDENTATION!


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NamespacesNamespacesNamespacesNamespaces

__builtins__

Global (import)

osos __builtins____builtins__

f, s, atoi, splitl, n, j, a, size

len, xrange, dir, open,...

popen, system,listdir, environ,path,...


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Looking at namespacesLooking at namespacesLooking at namespacesLooking at namespaces

l = dir() Create a list of the namesfor j in xrange(len(l)): in the global namespace

print l[j]

l = dir(__builtins__) Create a list of the namesin the namespace of the module__builtins__

l = dir(os) Do likewise for the module os


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Example 2 (map, lambda)Example 2 (map, lambda)Example 2 (map, lambda)Example 2 (map, lambda)

l = dir(__builtins__) # List names in module __builtins__o= map(lambda x: x+`\n’,l) # For every element x of list apply the

# one-line function f(x) = x+`\n’ and# return the result in the list o.

f = open(‘builtins.txt’,’w’) # Open a file object f for output.f.writelines(o) # Write out list of newline-terminated

# strings. f.close() # Close file object.

from string import strip # Add strip to global namespace.g = open(‘builtins.txt’,’r’) # Open file for input.t = g.readlines() # Read entire file into list t.t = map(strip,t) # Strip off whitespace (here newline).


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Example 3 (filter)Example 3 (filter)Example 3 (filter)Example 3 (filter)

from string import strip, find # Import strip and findt = open(‘builtins.txt’).readlines() # Read entire file into list t.t = map(strip,t) # Strip off whitespace.

o = filter(lambda x: find(x,`Error’) > 0, t) # For every x in list t# apply the one-line function # f(x) = find(x,’Error’) > 0# If f(x) is true (i.e., f(x)=1) # copy x to output list o.

# Example of python error code: AttributeError


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Sequence typesSequence typesSequence typesSequence types

Immutable types (can’t be changed) Strings aString = ‘Go boil your head’ Tuples aTuple = (‘Bozo’,42,3.14,aString)

Mutable types (can be changed) Lists aList = [aString, aTuple,1997] Dictionaries aMap = {‘string’:aString,(1,2):aList}

General operators on sequence s len(s), min(s), max(s), s[i], s[i:j] (= s[i]..s[j-1]) x [not] in s 1 if item x [not] in s, else 0 s + t concatenate sequences s and t n*s n copies, concatenated


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Tuples (immutable; can’t be changed)Tuples (immutable; can’t be changed)Tuples (immutable; can’t be changed)Tuples (immutable; can’t be changed)

0 1 2 3 4 5>>> IamAtuple = (‘The’,’time’,3.141,’has’,42,’come’)>>> IamAtuple[0]; IamAtuple[5]‘The’‘come’

>>> IamAtupleSlice = IamAtuple[3:6]>>> IamAtupleSlice(‘has’,42,’come’)

>>> IamAtupleCopy = IamAtuple[:]>>> len(IamAtupleCopy)6


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Lists (mutable; can be changed)Lists (mutable; can be changed)Lists (mutable; can be changed)Lists (mutable; can be changed)

>>> IamAlist = [‘The’,’time’,3.141,’has’,42,’come’]>>> IamAlist[0]‘The’

>>> IamAlist[2] = (1,2,3); IamAlist[‘The’,’time’,(1,2,3),’has’,42,’come’]

>>> IamAlist.sort()[42,’The’,’come’,’has’,’time’,(1,2,3)]

>>> IamAlist.append(1997); IamAlist[42,’The’,’come’,’has’,’time’,(1,2,3),1997]


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Dictionaries, or mappings (mutable)Dictionaries, or mappings (mutable)Dictionaries, or mappings (mutable)Dictionaries, or mappings (mutable)

>>> IamAmap = {} # An empty dictionary>>> IamAmapAlso = {1:’one’,(2,’boo’):’two’}

>>> IamAmap[‘language’] = (‘python’,1997) # Bind keys to>>> IamAmap[‘comment’] = ‘Superb!’ # values.>>> IamAmap[‘cost’] = 0>>> IamAmap[‘map’] = IamAmapAlso

>>> IamAmap.keys() # Get list of keys[‘cost’,’language’,’comment’,’map’]

>>> IamAmap.values() # Get list of values[0,(‘python’,1997),’Superb!’,{1:’one’,(2,’boo’):’two’}]


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Functions (def, None, return)Functions (def, None, return)Functions (def, None, return)Functions (def, None, return)

def goofy(a, b=‘yahoo’,*t, **k): # a is a required arg.# b is a req. arg. with a default.# *t, **k are optional args.

r1 = r2 = None # None is a python object.if ( len(t) > 0 ): # *t is a tuple of args.

r1 = t[0] # **k is a set of keyword args.if ( k.has_key(‘title’) ): # Check if key ‘title’ exists.

r2 = k[‘title’] # Get value for given key.return (r1,r2) # Return as a tuple.

>>> a, b = goofy(1,’sucks’,1,2, title=‘2001’,author=’Clarke’)>>> print a, b1 2001


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ClassesClassesClassesClasses

class FourVector: # Name of class

def __init__(self, px=0,py=0,pz=0,E=0): # Constructorself.ok = 1 # Creation okif E >= 0:

self.px, self.py, self.pz, self.e = px,py,pz,Eelse:

self.ok = 0 # Creation failed

def __del__(self): # Destructorpass

>>> p = FourVector(12,-14,13,80)


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Classes, __str__ methodClasses, __str__ methodClasses, __str__ methodClasses, __str__ method

def __str__(self): # Used by print and str(*)s = ‘px %d\n’ % self.px # Uses the C printfs = s + ‘py %d\n’ % self.py # format stringss = s + ‘pz %d\n’ % self.pzs = s = ‘E %d’ % self.ereturn s # Return string

# Note: We could also have written the above # as s = ‘%s %d\n’ % (‘px’,self.px), etc..>>> print ppx 12py -14pz 13E 80


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Classes, __add__, __sub__ methodsClasses, __add__, __sub__ methodsClasses, __add__, __sub__ methodsClasses, __add__, __sub__ methods

def __add__(self, v): # Implement +u = FourVector()u.px = self.px + v.pxu.py = self.py + v.pyu.pz = self.pz + v.pzu.e = self.e + v.ereturn u

def __sub__(self, v): # Implement -: :


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Classes, __mul__ methodClasses, __mul__ methodClasses, __mul__ methodClasses, __mul__ method

def __mul__(self, v): # Implement *u = self.px*v.px+self.py*v.py+self.pz*v.pzreturn self.e*v.e - u

>>> p = FourVector(1,2,3,10)>>> q = FourVector(0,0,-1,20)>>> pq = p*q


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Classes, InheritanceClasses, InheritanceClasses, InheritanceClasses, Inheritance

class Particle(FourVector):def __init__(self, label=‘e-’,px=0,py=0,pz=0,E=0):

# Call FourVector constructor to initialize px, etc.FourVector.__init__(self, px,py,pz,E)

if self.ok: # Check for successself.label = label # Create another attribute

else: return# NB: Constructor return value ignored

def __del__(self):if self.ok: print ‘Goodbye cruel world!’else: print ‘I never got properly made anyway!’


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Classes, dynamically created attributesClasses, dynamically created attributesClasses, dynamically created attributesClasses, dynamically created attributes

from math import * # Import all math functions

class Particle(FourVector):: :def __getattr__(self, attribute): # Method to get values

# of dynamic attributesif attribute == ‘mass’:

x = sqrt(self.e**2-(self.px**2+self.py**2+self.pz**2))

return xelse: # Okay; have a tantrum!

raise AttributeError, attribute


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Setting dynamically created attributesSetting dynamically created attributesSetting dynamically created attributesSetting dynamically created attributes

class Particle(FourVector):: :def __setattr__(self, attribute, value): # Method to set

# values of dynamic attributesif attribute == ‘version’:

#NB: Do not write self.version = value here because this# would create a statically defined attribute! Set using the # object’s dictionary attribute:

self.__dict__[attribute] = valueelse:

raise AttributeError, attribute


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A simple Internet serverA simple Internet serverA simple Internet serverA simple Internet server

from socket import *from os import *def server(PORT=50000): # Choose any non-privileged port

HOST = ‘’ # Local hosts = socket(AF_INET, SOCK_STREAM) # Create sockets.bind(HOST, PORT) # Bind socket to local hosts.listen(1) # Allow only a single queued connectionclient, address = s.accept() # Wait for a connectionwhile 1: # Wait “forever”

data = client.recv(4096) # Get up to 4k bytesif data == ‘quit’: break else: print data

print “Goodbye from server!”


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A simple internet clientA simple internet clientA simple internet clientA simple internet client

from socket import *from os import *def client(HOST=gethostname(), # Default is local server

PORT=50000): # Server ports = socket(AF_INET, SOCK_STREAM) # Create sockets.connect(HOST, PORT) # Connect to serverwhile 1:

data = raw_input(‘Client> ’)# Get keyboard inputif data == ‘quit’: break else: s.send(data) # Send to server

s.send(‘quit’)s.shutdown(2) # Terminate all pending I/O s.close() # Close connectionprint “Goodbye from client!”


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And finally, a client/server sessionAnd finally, a client/server sessionAnd finally, a client/server sessionAnd finally, a client/server session

Server:>>> from server import server>>> server()Go Boil your head!Goodbye from server!>>>

Client:>>> from client import client>>> client(‘d02ka.fnal.gov’)Client>> Go boil your head!Client>> quitGoodbye from client!>>>


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Odds and endsOdds and endsOdds and endsOdds and ends

Changing the python search path on-the-flyimport syssys.path.append(‘~harry/python/tutorial’)sys.path.append(‘~harry/python/examples’)

Spawning commands to the operating systemimport osos.system(‘cd; ls -la’)

Debugging python scripts (type h for help)from pdb import runrun(‘p = FourVector(1,1,2,90)’)


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Odds and ends (eval, exec, try, except)Odds and ends (eval, exec, try, except)Odds and ends (eval, exec, try, except)Odds and ends (eval, exec, try, except)

Evaluating strings and updating current namespacep = eval(‘FourVector(1,1,2,90)’)

Executing statements and updating current namespace)exec(‘p = FourVector(1,1,2,90)’)

Handling errors>>> s = open(‘FourVector.txt’,’r’).read()>>> try: # Try following statements and. . . p = eval(s) # if an error (i.e., an exception) occurs. . . except: # execute these statements. . . . exec(s) # Note: Can also catch specified>>> print p # exceptions with the except keyword.


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Graphical User InterfacesGraphical User InterfacesGraphical User InterfacesGraphical User Interfaces

Tk is a widely supported toolkit for building portable Graphical User Interfaces (GUIs). Its purpose is to hide the unbelievable horrors of X windows.

Tkinter is a python module that provides an object-oriented wrapping of Tk. It is available with the standard distribution of python on many different platforms, including Windows NT/95. http://www.pythonware.com/downloads.htm download self-extracting archive (py15-980706.exe) double-click on file to install python

To load all of the Tkinter attributes and methods into the global namespace do: from Tkinter import *


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Event-driven programsEvent-driven programsEvent-driven programsEvent-driven programs

A GUI application is an example of an event-driven program. An application goes through two steps: Construct GUI layout and bind screen objects

(widgets) to functions (callbacks). Enter a wait loop and bind events to callbacks

Construct GUI layout

WaitForEventDecide which callbacks to callDispatch callbacks

Construct GUI layout

WaitForEventDecide which callbacks to callDispatch callbacks


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Anatomy of a simple GUIAnatomy of a simple GUIAnatomy of a simple GUIAnatomy of a simple GUI

Frames are containers for Tk objects (widgets).Gui layouts are created by using nested Frames intowhich useful widgets are placed. The root widget provides(platform-dependent) window management.

Entry

Scale

Root


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GUI Example 1GUI Example 1GUI Example 1GUI Example 1

from Tkinter import * # Add all names to global namespaceimport os

class Gui(Frame): # Gui is an extended Framedef __init__(self, parent=None): # Default parent is root

Frame.__init__(self, parent) # Initialize Frameself.b = Button(self, text=‘List Directory’,

command=self.list, fg=‘red’, bg=‘green’)

self.pack(side=TOP); self.b.pack(side=LEFT)def list(self):

os.system(‘cd; ls -la’)if __name__ == ‘__main__’: Gui.mainloop() # Run as script


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GUI Example 2GUI Example 2GUI Example 2GUI Example 2

from Tkinter import * #Add all names to global namespacefrom ScrolledText import ScrolledText

class Gui(Frame):def __init__(self, parent=None):

Frame.__init__(self, parent)self.pack()self.config(relief=GROOVE, bd=2)self.master.title(‘Example 2’)self.text = ScrolledText(self)self.text.pack()

if __name__ == ‘__main__’: Gui.mainloop()


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A couple of other widgetsA couple of other widgetsA couple of other widgetsA couple of other widgets

o is object, p is parent widget, v is value

o = Scale(p, orient=HORIZONTAL, from_= 0, to =1000)v = o.get(), o.set(v)

o = Entry(p), x = StringVar(); o[‘textvariable’] = xo.bind(‘<Key-Return>‘, callback); x.get(), x.set()


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Python Imaging LibraryPython Imaging LibraryPython Imaging LibraryPython Imaging Library

We shall use this extension (by Frederick Lundh) to build a simple graphics browser (gbrowser.py)

Import modules class Gui(Frame):

Constructor• Get a list of filenames• Loop over filenames

– Create and store full graphics image in a dictionary– Create a button, attach a thumbnail image to it and

bind button to the display method

Display• Paste full image to a label in a separate window


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A Simple Graphics BrowserA Simple Graphics BrowserA Simple Graphics BrowserA Simple Graphics Browser

from Tkinter import *import Image # Load a couple of PIL import ImageTk # classesimport os, sysfrom string import find, rfind, lowerFILETYPES = (‘.gif’,’.jpg’,’.bmp’) BIG = (768,576); SMALL=(80,60)# Handle different path delimiters between # windows and UNIXif sys.platform == 'win32':

DELIM = "\\" # ‘\’ is the python escape characterelse:

DELIM = "/" # To make UNIX happy :-(


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Graphics Browser, part 1Graphics Browser, part 1Graphics Browser, part 1Graphics Browser, part 1

class Gui(Frame): # Gui is an extended framedef __init__(self, parent=None, path=('.',)):

# Try to get a listing of given path ( (‘.’,) -- syntax for 1-tuple)try:

files = [] # Create an empty listfor d in path: # path is a tuple of directories

l = os.listdir(d)f = map(lambda x,y=d:y+delim+x,l)files = files + f # Concatenate lists

except:# Come here if listing fails

print "**Errror**, bad directory: ", dsys.exit(1)


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# Get list of graphics files. # Extract the last 4 characters of each string x in the list files.# Make lower case, then test if the 4-character string matches a# string in the FILETYPES tuple.

gifs = filter(lambda x: lower(x[len(x)-4]) in FILETYPES, files)

# Initialize inherited frameFrame.__init__(self, parent, relief=SUNKEN, bd=2)self.pack()

self.image = {} # Dictionary for imagesrow = col = 0 # For use with grid method

# Next we shall loop over the GIF files and try to read each one


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for giffile in gifs: # Loop over graphics files# Extract file name

gif = giffile[rfind(giffile,DELIM)+1:] # Search from right

# Exclude duplicate imagesif not self.image.has_key(gif):

print "Reading ", gif, # Note comma to suppress NL

# Read GIF file and create imagetry:

ig = Image.open(giffile)ig.thumbnail(BIG)self.image[gif] = ImageTk.PhotoImage(ig)


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# Paste button b with a thumbnail version of image.# Note the use of lambda to pass context information to # the single callback self.display

: :ig.thumbnail(SMALL) # 80 x 60 pixelsb = Button(self, image=ImageTk.PhotoImage(ig),

command=lambda s=self, x=gif: s.display(x)): :


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# Pack using grid method, which requires that we give the row# and column numbers of a matrix. Make buttons expand in all # directions (N+S+E+W) to fill available space.

b.grid(row=row, column=col, sticky=N+S+E+W)col = col + 1 # Update column numberif col == 5:

row = row + 1 # Update row numbercol = 0

print "" # Complete print statementexcept: # associated with Try (see part 3)

print "*** FAILED *** " # Here if we fail to read file


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Graphics Browser, part6Graphics Browser, part6Graphics Browser, part6Graphics Browser, part6

# Method to display full image

def display(self, gif):top = Toplevel() # Create a separate windowtop.title(gif)top.iconname(gif)Label(top,image=self.image[gif]).pack()

# Note: Because we do not plan to do anything with the# label other than display it, we don’t bother to create a# reference (a name) for the label.


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Graphics Browser, running itGraphics Browser, running itGraphics Browser, running itGraphics Browser, running it

# Code added at the end to execute file if it is run as a script,# that is: python gbrowser.py [dir1] [dir2] ...

if __name__ == '__main__':

if not sys.argv[1:]: # Check for NO arguments and, ifpath = '.' # none, default to working directory

else: path = sys.argv[1:] # Get arguments

root = Tk() # Create Tk root widget root.title('Graphics Browser')

Gui(root, path).mainloop() # Enter event loop


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The EndThe EndThe EndThe End

Python in action!Python in action!

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