Python a modern hybrid A language for scripting and prototyping Balance between extensibility and powerful built in data structures genealogy Setl NYU J Schwartz et al 1969 1980 ABC Amsterdam Meertens et al 1980 Python Van Rossum et all 1996 Very active open source community Prototyping Emphasis on experimental programming interactive like LISP ML etc minimal translation to bytecode like Java dynamic typing like LISP SETL APL higher order functions LISP garbage collected no pointers LISP etc Uniform treatment of indexable structures like SETL Built in associative structures like SETL Light syntax indentation is significant from ABC No one will look at it without OOP Simple model of modules and classes inheritance of implementation multiple inheritance No information hiding simple visibility model clumsy mechanism for operator overloading limited nesting built in scope global scope local scope What is looks like rulers france chirac 1995 7 general mapping us clinton 1996 4 peru fujimori 1998 0 romania illiescu 2000 5 for country in rulers keys built in iterators pres elected term rulers country assignment if 2000 elected term print country pres has I years to go term 2000 elected else print country pres is out of office Simple interactive model python pres py load and execute france chirac has 2 years to go us clinton is out of office romania illiescu has 6 years to go peru fujimori is out of office can also write python import pres load execute continue Uniform treatment of indexable data Strings lists and arrays have common operations characters are strings of length 1 name Python courses languages compilers databases basketry coordinates 0 0 1 5 4 5 2 0 indexing from 0 negative index indexing from end name 2 is o courses 3 is compilers if ix is negative lis ix is lis len lis ix Slicing every which way and iterating slicing s m n from mth component up to but excluding nth s m to end s n from beginning s all components s 4 repetition built in iterators for c in name for course in courses c bound to each char Dictionaries General purpose associative maps domain keys of arbitrary types retrieval by key rulers peru yields fujimori 1998 0 assignment modification rulers peru 2 10 coup another 8 years to go rulers mexico fox 2000 6 rulers israel no type consistency required Set theory as a model of computation Alternative to lists recursion sets membership iterators set constructors in SETL S2 f x x in S P x in Python S2 for x in S if P x S2 append f x Loops Iterators over collections for x in L Iterators over dictionaries for k in mydict keys Explicit iterators for x in 1 1 2 3 5 8 13 Numeric iterators for x in range 1 100 Functions def intersect seq1 seq2 res for x in seq1 if x in seq2 res append x return res no type info initialize list iterate over list built in membership in place modification assigned names are local unless declared global no possible hiding Modules Modules are namespaces unit of encapsulation Modules are objects components can be accessed Modules can be inspected dynamically dict provides dictionary for module keys are strings for entities for attr in module dict keys print attr if attr 0 2 print atrr built in name look at all entities comma prevents LF naming convention Classes and inheritance Standard notions superclasses derived classes self for this dynamic dispatching Each class and each object is a namespace with a dictionary To locate an operation lookup in dictionary of object dispatch table If not found examine superclasses Operator overloading through predefined names init del add repr constructor destructor operator printing external representation Data members are created implicitly Class Number def init self start self data start data is defined def add self other number number return Number self data other data def repr self return self data convert to string note no way to overload Number integer etc Any class can be a collection class collection def getitem self i return self data I X collection X data 1 2 3 for item in X print item item attribute data is indexable member exists assignment ok for calls getitem equivalent to item 2 item Exceptions etc internally iterator implemented as i 0 try while 1 item getitem self i i i 1 except IndexError pass except print unexpected chaos no boolean type body of loop eventually i too large null statement all other exceptions