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Python ProgrammingD. Sheefa Ruby GraceDepartment of Computer ScienceSarah Tucker College

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

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 """"

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

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

Basic programminga,b = 0, 1# non-zero = truewhile b < 10:# formatted output, without nprint b,# multiple assignmenta,b = b, a+b

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

Control flow: fora = ['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 beingiterated over

Loops: break, continue, else break and continue like C else after loop exhaustionfor n in range(2,10):for x in range(2,n):if n % x == 0:print n, 'equals', x, '*', n/xbreakelse:# loop fell through without finding afactorprint n, 'is prime'

Defining functionsdef fib(n):"""Print a Fibonacci series up to n."""a, b = 0, 1while 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

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

Keyword arguments last arguments can be given as keywordsdef 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 ", typeprint "-- It's", state, "!"parrot(1000)parrot(action='VOOOM', voltage=100000)

Lambda forms anonymous functions may not work in older versionsdef make_incrementor(n):return lambda x: x + nf = make_incrementor(42)f(0)f(1)

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

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

Functional programming tools filter(function, sequence)def f(x): return x%2 != 0 and x%3 0filter(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

List comprehensions (2.0) Create lists without map(), filter(), lambda = expression followed by for clause + zero or more for orof 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}]

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 inrange(len(vec1))][8,12,-54]

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][]

del – removing list items remove by index, not value remove slices from list (rather than byassigning 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]

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')

Tuples Tuples may be nested>>> u = t, (1,2)>>> u((123, 542, 'bar'), (1,2)) kind of like structs, but no elementnames: (x,y) coordinates database records like strings, immutable  can't assign toindividual items

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

Tuples sequence unpacking  distribute elements acrossvariables>>> t = 123, 543, 'bar'>>> x, y, z = t>>> x123 packing always creates tuple unpacking works for any sequence

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

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

Conditions can check for sequence membershipwith is and is not:>>> if (4 in vec):... print '4 is' chained comparisons: a less than bAND 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

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

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)

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

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)

Modules function definition + executablestatements 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 *

Module search path current directory list of directories specified in PYTHONPATHenvironment 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 FilesPython2.2DLLs', 'C:Program FilesPython2.2lib', 'C:Program FilesPython2.2liblib-tk', 'C:Program FilesPython2.2', 'C:Program FilesPython2.2libsite-packages']

Classes mixture of C++ and Modula-3 multiple base classes derived class can override any methods ofits base class(es) method can call the method of a baseclass 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)

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

Class definitionsClass ClassName:<statement-1>...<statement-N> must be executed can be executed conditionally (see Tcl) creates new namespace

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

Namespaces scope = textual region of Python programwhere 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 innermostscope don't copy, just create name bindings toobjects global indicates name is in global scope

Class objects obj.name references (plus module!):class MyClass:"A simple example class"i = 123def f(self):return 'hello world'>>> MyClass.i123 MyClass.f is method object

Class objects class instantiation:>>> x = MyClass()>>> x.f()'hello world' creates new instance of class note x = MyClass vs. x = MyClass() ___init__() special method forinitialization of objectdef __init__(self,realpart,imagpart):self.r = realpartself.i = imagpart

Instance objects attribute references data attributes (C++/Java data members) created dynamicallyx.counter = 1while x.counter < 10:x.counter = x.counter * 2print x.counterdel x.counter

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

InheritanceclassDerivedClassName(BaseClassName)<statement-1>...<statement-N> search class attribute, descending chain of base classes may override methods in the base class call directly viaBaseClassName.method

Multiple inheritanceclassDerivedClass(Base1,Base2,Base3):<statement> depth-first, left-to-right problem: class derived from two classes with a commonbase class

Private variables No real support, but textual replacement (namemangling) __var is replaced by _classname_var prevents only accidental modification, not trueprotection

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

Handling exceptionswhile 1:try:x = int(raw_input("Please enter a number: "))breakexcept ValueError:print "Not a valid number" First, execute try clause if no exception, skip except clause if exception, skip rest of try clause and useexcept clause if no matching exception, attempt outer trystatement

Handling exceptions try.pyimport sysfor arg in sys.argv[1:]:try:f = open(arg, 'r')except IOError:print 'cannot open', argelse:print arg, 'lines:',len(f.readlines())f.close e.g., as python try.py *.py

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