Cosc 4750

Python

Python

• Python is a scripting language similar to Perl and PHP

• But Python takes a minimalist approach.– While Perl provides maximal approach. PHP is

somewhere in between.• “features of the language interact in consistent and

limited ways, and follow naturally from a small set of core concepts”

• Python can be defined as a object-oriented scripting language.

What can you do with Python

• System programming• GUIs• Internet Scripts• Component Integration

– Can be extended by C/C++ or embedded into C/C++. On windows can be used like VB.

• Database Programming• Numeric Programming with NumPy• Gaming, Images, AI, XML, and more.

IDE User Interface

• Python comes with an IDE, Called IDLE• Linux: idle

• Let you edit, run, browse, and debug Python programs• Works in two parts

– An editor, uses syntax-directed colorization

– A shell, where output goes when running scripts form the editor.

• There are other IDEs for python, Komodo, PythonWorks, PythonWin (comes with ActiveState’s ActivePyton), Visual Python and more. See http://www.python.org

The Language

• Since you already know a several langauges, like perl, c/c++, PHP. I use those to describe Python.

• Like Perl # is a comment– Comment starts at # and continue to the end of

line

Built-in object types

• Numbers– 3.1312, 123, 999L (long integers unlimited size), 3+4j (complex number literals),

• Strings– ‘spam’, “Jim’s”

• Lists– [1, [2, ‘three’], 4]

• Dictionaries– {‘food’: ‘spam’, ‘taste’: ‘yum’}

• Tuples– (1, ‘spam’, 4, ‘U’)

• Files – Text = open (‘eggs’, ‘r’).read()

Operators

• lamba args: expr (anonymous function generation)

• or, and, not

• <, <=, >, >=, ==, <>, !=,

• |, ^, &, <<, >>, ~ (bitwise operations)

• -, +, *, %, /, // (int division), ** (power)

Variables

• Like perl and PHP variables come into existence when they are assigned a value

• They don’t require anything special characters to denote they are variables

• a = 3

• b = ‘jim’

• c = 3 + 12 * 6

strings

• Single and double quoted strings are the same– allows user to use ‘ and “ in strings– ‘knight”s’, “knight’s”– you can also escape the them “knight\”s”– title = “meaning “ ‘of’ “ Life”

• Python automatically concatenates adjacent strings or can use +

• ‘meaning of life’

strings (2)

• Tripe quotes for multiline Block strings– saying = “””Always look on the bright side of life.”””– saying contains ‘Always look\n on the bright\n side of

life.’• Raw strings

– turns off all escape mechanism in the string– r (upper or lower) before the string– r’c:\new\test.dat’ makes it the literal string.

stings operations

• len(string variable) return the length of the string• var = ‘-’*80

– comes out with 80 dashes in the string

• all strings can be used a arrays (like char[] in c)– s = ‘spam’– print s[0] #prints s– print s[-2] #prints a

• slicing like perl range in strings or substringing– t = s[1:3] #t=‘pa’ gets from element 1 to 3 non-inclusive– t = s[1:] #t = ‘pam’– t = s[:-1] #t = ‘spa’

string concatenation

• unlike perl, python won’t auto cast to strings from other types or vise versa

• str = “42” + 1 # Error

• var = int(“42”) + 1 # var = 43

• str = “42” + str(1) #str = ‘421’

string formatting

• using similar format as printf (in perl and C)

• %s string• %c character• %d decimal• %i integer• %u unsigned int• %o octal• %x hex• %e floating point exponent

• %f floating point decimal• %g either e or f• %% literal %• examplex = 1234y = jimz = 1.234str = ‘int %d, str %s, floats %f, %e,

%g’ % (x, y, z, z,z)str =‘int 1234, str jim, floats 1.234,

1.234+e00, 1.234

String methods• This is a list of common methods

– assume S has a string value

– S.capitalize(), S.center(width), S.count(sub [, start [,end]]), S.endswith(suffix, [,start [, end]]), S. epandtabes([tabsize]), S.find(sub [,start [,end]]) , S.find(sub [,start [,end]]), S.isalnum(), S.isalpa(), S.isdigit(), S.islower(), S.isspace(), S.isupper(), S.istitle()

– S.join(seq), S.ljust(width), S.lower(), S.lstrip(), S.replace(old, new [, maxsplit]), S.rfind(sub [,start [,end]]), S.rindex(sub [,start [,end]]), S.rjust(width), S.rstrip(), S.split( [sep [, maxsplit]]), S.splitlines( [keepends]), S.startswith(prefix [,start [,end]]), S.strip(), S.swapcase(), S.title(), S.translate(table [, delchars]), S.upper

• strings are immutable, so you can’t change them in-place directly so S = S.toupper()

examples of string methods• replace can replace all, or spec number

– S = ‘spammy’

– S = S.replace(‘mm’, ‘xx’) # S=‘spaxxy’

– S = S.replace(‘m’, ‘x’, 1) # S= ‘spamxy’

• find returns offset in string– x = S.find(‘x’) #x = 4

• list return a list of the string (not a method)– L = list(S) #L = [‘s’, ‘p’, ‘a’, ‘m’, ‘x’, ‘y’]

• join method a list into a string– S = ‘’.join(L) # S = ‘spamxy’ NOTE the ‘’ to call join

– S = S.join([‘X’, ‘Y’]) #s = ‘XspamxyY’

• split method, defaults to white space– S = ‘ba ac ad’

– L = S.split() # L = [‘ba’, ‘ac’, ‘ad’]

– L = S.split(‘a’) #L = [‘b’, ‘ ‘, ‘c ‘, ‘d’]

lists

• What we normally think of as arrays, except it based on the LISP.– uses the [] operators.

– L1 = [] #empty list

– L2 = [0,1,2,3] # for items, index 0 .. 3

– L3 = [‘abc’, [‘def’, ‘ghi’]] # nested lists.

– a = L2[1] #a = 1

– a = L3[1][0] # a = ‘def’

– L = L2[1:3] # L = [1,2] Remember not inclusive

– L = L2 + L3 # concatenate the lists

– L = L2 * 3 # repeat L2 3 times

lists (2)

• for x in L2 Iteration in the list– Written in perl: foreach $x (@L2) { $x …}

• x in L2 #membership, return 1 if true– Is the value of x in the list L2

• L2.append(4) # put 4 at the end of the list• L2.extend([3,4,5]) # add 3, 4, 5 to the end of the list• L2.sort() # sorts the list ascending• L2.reverse() # reverse the elements in the list• a = L2.pop() # remove last element, put in a• del L2[i] # remove element I• del L2[1:3] #remove range, not inclusive• L2[1] = ‘hi’ #change element 1 to ‘hi’• L2[i:j] = [4,5,6] #slice assissment.

Dictionaries (2)• associate arrays in Perl. Hash variables.

• D1 = {} #empty dictionary

• D2 = {‘spam’: 2, ‘eggs’:3} #spam and eggs are the keys

• D3 = {‘food’: {‘ham’:1, ‘egg’:2}} #nested

• a = D[‘eggs’] # a = 3

• D2[‘ham’] = 2 #new key and value ‘ham’:2

• del D2[‘ham’] #removes key/value from dictionary

• a = d[‘food’][‘ham’] #a = 1

• len(D2) # returns the number of entries

• L = D2.keys() #L contains a list of the keys

• L = D2.values() #L contains a list of values

• L = D2.items() #returns a list of tuples (key, value)

• D1.update(D2) #add dictionary D2 to D1

tuples

• Work just like lists, except they can’t be changed in place.

• t1 = (0,) #one item tuple (not an expression)

• t2 = (0, ‘Ni’, 1.2, 3)

• t2 = 0, ‘Ni’, 1.2, 3 #same as above

• ERROR: t[1] = 1

• otherwise same operations as list

file type

• Python has files a open type (like streams in C++), instead of just using filehandes.

• output = open(‘/tmp/stuff’, ‘w’) #open stuff to write• input = open(‘data’, ‘r’) #open data to read• S = input.read() #read entire file into string variable• S = input.read(N) #read N bytes• S = input.readline() #read next line • L = input.readlines() #read entire file into list of line strings• output.write(S) #write string S to file• output.writelines(L) #write all lines in list L to file• output.close() #close file

Python and true/false

• Generally use this rule for true/false• Numbers are true if nonzero• Objects are true if nonempty• Examples“spam” true, “” false, [] false, () false, {}

false, 1 true, 0.0 false, None False– None is like NULL or similar to undef from

perl

Comparisons and Equality

• S1= ‘spam’, S2 = ‘spam’– S1 == S2 true, S1 is S2 true (but should be false)

• is operator test for object identity (is it the same object

• S1 = ‘a longer string’ S2=‘a longer string– S1 == S2 true, S1 is S2 false– Because python cache short strings, but not “longer”

strings

• L1 = [1, (‘a’, 3)] L2 = [1, (‘a’), 3]L1 == L2 false, L1 < L2 false, L1 > L2 true

Some problems.

• Python uses underlying pointers for objects and sometimes you may not think it should

• L = [1,2,3]• M = [‘X’, L, ‘Y’] # M has [‘X’, [1,2,3], ‘Y’]• L[1] = 0• M now has [‘X’, [1,0,3], ‘Y’]• To fix this• M = [‘X’, L[:], ‘Y’] # M has [‘X’, [1,2,3], ‘Y’]

– Now L is not linked in M.

Statements

• quick list– print if/elif/else– for/else while/else– break, continue try/except/finally– import, from def, return, yield– class global– del exec– assert

assignments

spam = ‘SPAM’

spam, ham = ‘yum’, ‘YUM’ (Tuple form)

[spam, ham] = [‘yum, ‘YUM’] (list form)

spam = ham = ‘lunch’ Multiple-target• Note spam and ham point to the same object!

print statement

• Works like perl’s print statement• print “whatever “, variables, etc…• print “whatever”,

– no end of line markers, waits for next print statement.

• To print to a filelog = open (‘log.txt’,’a’) #append modeprint >> log, “whatever”, variables, etc… log.close()

Writing a script and Syntax rules

• There are no blocks boundaries {} like most languages, instead they are detected automatically– Based on indentation of statements under a header

• You MUST write indented code for blocks!!!!

– Header, “:”, indented statements

• The statement boundaries, there is no ;– normally end of line, except in some cases

• we come back to this

Hello World script

#!/usr/bin/python

print “hello World!”

• In unix– python hello.py or make it executable

if statements

• general format

if <test>: #if test block

<statements> #block of statements

elif <test>: #optional elifs

<statements> #block of statements

else: #optional else, note the “:”

<statements>#block of statements

if example

x = ‘Elmer fud’

if x == ‘roger’:

print “How’s Jessica?”

elif x == ‘bugs’:

print “What’s up doc?”

else:

print “Run away!

block delimiters

• example of indentation and blocksx =1if x: y = 2 if y: #if y is true print ‘block 2’ #then print print ‘block 1’ #outside if y:print ‘outside of if block’

statement boundaries

• special cases where statements span lines– any statement with (), {}, []

• If the open on 1 line, then it can continue lines until the close is reached.

– A backslash (\) at the end of the line will continue it to the next line

– literals “strings” can span lines– and triple quotes are designed to span lines

statement boundaries (2)

• Maybe you don’t want to span lines with statements…– You can use semicolons to end statements

– Every doc and books says DO NOT, bad style

• x =1; y = 12; etc…

• Finally headers can have one statement– if x==1: print “hello”– Includes whiles, etc.. anything with a header

case/switch

• There is no case/switch statement in Python– Instead multiway branching can use elif’s – or Dictionary

branch = {‘spam’: 1.25, ‘ham’: 1.99, ‘eggs’: 0.99}

print branch.get(‘spam’, ‘Bad choice’)– ouput is 1.25 If spam was there, output is Bad Choice

– Neither allow for multi matching of the data• ie where you leave out the break in C/C++

loops

• General loop formatwhile <test>: #header <statements> [if <test>: ]break #exit loop now [if <test>: ]continue # jumps to loop headerelse: <statements> #only run if loop doesn’t exit on a

break statements. IE exits loop “normally”

note about <test>

• <test> is an expression that evals to true or false– There is a on things that python DO NOT allow

in a expersion: Assignments– no while line = input.readline():

– Does however remove the bug, x = 1 in expresions. But you have to return “classical” loops.

loops (2)

• General format for the for loopfor <target> in <object>: #assign object to target <statements> [if <test>: ]break #exit loop now [if <test>: ]continue # jumps to loop headerelse: <statements> #only run if loop doesn’t exit on a

break statements. IE exits loop “normally”

for loop example

sum = 0

for x in [1,2,3,4]: #or as complex as needed

sum = sum = x

print x,

print sum

• output

1 2 3 4 10

loop example with files

for line in open(‘text.txt’).readlines():

print line

• open a file and prints out each line to stdout.– Note there is a .xreadlines() method that reads

on demand, so large files, won’t file up memory.

functions

• General format of a functiondef <name> ([arg1,…,argN]): <statements> [return <value>]• Functions are declared at runtime, so you can have

the same function name for 2 or more functions, so long as only one function is declared at any given point.

• A function can also be assigned to “variable”– Othername = func; othername()

Function example

Def add (x, y):

return x + y; #no type, so x and y can be any type

var1 = 12; var2 = 1;

var3 = add(var1,var2) # var3 = 13

str1 = “hi “; str2 = “ there”

str3 = add(str1, str2) # str3= “hi there”

functions and scope

• variables/names defined inside a function are local to the function– note functions can be defined inside functions– variables with the same name are shadowed in the

function.• global variables can be accessed inside a function,

but must be declared global in order to change them. – Any global variables can be read, only declared inside

the function as global can be changed.– uses the global reserved word.

functions and scope (2)

• example:x = 99; z = 100def func(y): z = x +y #local z, gobal x, and parameter y return zdef func2(y): global x z = (++x) +y return za = func(1) #a = 100, x =99b = func2(1) #a =101, x=100

A word about parameters

• Parameters are passed by assignment– similar to pass by reference in C++

– this maybe the same way Java deals with parameters

• Since all variables are pointers to objects, so when parameter’s object is changed, it changes the calling variables object.– The issue, there is not a name aliasing, so it a new

object is created, it won’t change the calling variable. Different object.

A word about parameters (2)

• example

def changer (x, y):

x = 2

y[0] = ‘spam’

X =1; L =[1,2]

changer(X, L)

#X is unchanged, L = [‘spam’, 2]

A word about parameters (3)

• Default parameters– def func (x, a=1, str=“hi”):– <statements>– func(12)# a =1, str=“hi”

• Keyword for parameters– func(12, str=“There”) #x=12, a=1, str=“There”

A word about parameters (4)

• Arbitrary arguments• Uses * or ** depending on how you want to see it.

– * args are a list• def func( *arg):• <statements> #arg = ( )• func(1,2,3) #arg would be (1,2,3)

– ** args are in a dictionary• def func(**arg):• <statements>• Func(x=1, y=2, z=12) # arg (x:1, y:2, z:12)

Anonymous Functions: Lamba

• General form• variable = lamba arg1, …, argN: <expression>

– Note they is not a statement block

• Example:• def func(x,y,z): return x+y+z #function• f = lamaba x,y,z: x + y +z #lamba function• Calling the function just like you set a function “equal” to

another variable. – x= f(1,2,3) #x = 6

• lamba function can be nested.

modules

• use the import command and the name of the module

• example– Import os– imports the os module with a number of

function to interact with the filesystem.

regular expressions

• Can you do then just the simple match, but the expressions seem to follow their own special language instead of the standards.

• uses module re

Some useful modules

• urllib and urlpase– modules for Universal resource locators

• htmllib, mimetools, mimetypes, mail, uu– lots of modules to how internet file processing

• xml.parses.expat, xml.dom, etc…– xml processing

Not covered in this lectures

• building classes and OOP design– inheritance

• exception handling– type/except/else– similar to java’s try/caught

• creating modules

Embedding Python

• In C/C++ #include <Python.h>…Py_Initialize();PyRun_SimpleString(“x = brave + sir + robon”);/* run code snippet See the python doc’s for more,

C programs may also gain access to Python objects, process or execute them using other Python API tools.

References

• http://www.python.org/

• Learning Python, Lutz & Ascher, O’Reilly, 2rd, 2003

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