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Cosc 4750

Python. Cosc 4750. 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.

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Cosc 4750

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  1. Python Cosc 4750

  2. 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.

  3. 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.

  4. 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

  5. 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

  6. 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()

  7. Operators • lamba args: expr (anonymous function generation) • or, and, not • <, <=, >, >=, ==, <>, !=, • |, ^, &, <<, >>, ~ (bitwise operations) • -, +, *, %, /, // (int division), ** (power)

  8. 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

  9. 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’

  10. 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.

  11. 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’

  12. 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’

  13. 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 % example x = 1234 y = jim z = 1.234 str = ‘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 formatting

  14. 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()

  15. 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’]

  16. 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

  17. 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.

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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.

  24. 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

  25. 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!

  26. 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 file log = open (‘log.txt’,’a’) #append mode print >> log, “whatever”, variables, etc… log.close()

  27. 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

  28. Hello World script #!/usr/bin/python print “hello World!” • In unix • python hello.py or make it executable

  29. 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

  30. if example x = ‘Elmer fud’ if x == ‘roger’: print “How’s Jessica?” elif x == ‘bugs’: print “What’s up doc?” else: print “Run away!

  31. block delimiters • example of indentation and blocks x =1 if 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’

  32. 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

  33. 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

  34. 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++

  35. loops • General loop format while <test>: #header <statements> [if <test>: ]break #exit loop now [if <test>: ]continue # jumps to loop header else: <statements> #only run if loop doesn’t exit on a break statements. IE exits loop “normally”

  36. 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.

  37. loops (2) • General format for the for loop for <target> in <object>: #assign object to target <statements> [if <test>: ]break #exit loop now [if <test>: ]continue # jumps to loop header else: <statements> #only run if loop doesn’t exit on a break statements. IE exits loop “normally”

  38. 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

  39. 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.

  40. functions • General format of a function def <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()

  41. 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”

  42. 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.

  43. functions and scope (2) • example: x = 99; z = 100 def func(y): z = x +y #local z, gobal x, and parameter y return z def func2(y): global x z = (++x) +y return z a = func(1) #a = 100, x =99 b = func2(1) #a =101, x=100

  44. 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.

  45. 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]

  46. 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”

  47. 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)

  48. 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.

  49. 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.

  50. 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

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