Computational Concepts Toolbox Data type: values, literals, - PDF document

Computational Concepts Toolbox • Data type: values, literals, • Higher Order Functions operations, – Functions as Values – Functions with functions as • Expressions, Call Generators and Iterators argument expression – Assignment of function values • Variables • Higher order function patterns • Assignment Statement David E. Culler – Map, Filter, Reduce • Sequences: tuple, list CS8 – Computational Structures in Data Science • Function factories – create and • Dictionaries return functions http://inst.eecs.berkeley.edu/~cs88 • Data structures • Recursion • Tuple assignment • Abstract Data Types Lecture 11 • Function Definition • Mutation Statement November 5, 2018 • Class • Conditional Statement – Object Oriented Programming • Iteration: list comp, for, http://bit.ly/cs88-fa18-L11 – Inheritance while • Exceptions • Lambda function expr. 10/29/18 UCB CS88 Sp18 L10 2 Administrative Issues Today: • Project 2 “Wheel” is out • Review Exceptions – Part I due 11/10 • Sequences vs Iterables • There will be no Project 3 • Using iterators without generating all the data • No lecture 11/12 due to holiday • Generator concept – There will be lab Friday 11/16 – Generating an iterator from iteration with yield • Magic methods – next – Iter • Iterators – the iter protocol • Getitem protocol • Is an object iterable? • Lazy evaluation with iterators 10/29/18 UCB CS88 Sp18 L10 3 10/29/18 UCB CS88 Sp18 L10 4 Summary of last week Key concepts to take forward • Approach creation of a class as a design • Classes embody and allow enforcement of ADT problem methodology – Meaningful behavior => methods [& attributes] • Class definition – ADT methodology • Class namespace – What’s private and hidden? vs What’s public? • Design for inheritance • Methods – Clean general case as foundation for specialized subclasses • Instance attributes (fields) • Use it to streamline development • Class attributes • Inheritance • Anticipate exceptional cases and unforeseen • Superclass reference problems – try … catch – raise / assert 10/29/18 UCB CS88 Sp18 L10 5 10/29/18 UCB CS88 Sp18 L10 6 1

Handling Errors – try / except Exception (read 3.3) • Mechanism in a programming language to • Wrap your code in try – except statements declare and respond to “exceptional conditions” – enable non-local cntinuations of control try: • Often used to handle error conditions <try suite> except <exception class> as <name>: – Unhandled exceptions will cause python to halt and print a <except suite> stack trace ... # continue here if <try suite> succeeds w/o exception – You already saw a non-error exception – end of iterator • Exceptions can be handled by the program • Execution rule instead – <try suite> is executed first – assert, try, except, raise statements – If during this an exception is raised and not handled otherwise • Exceptions are objects! – And if the exception inherits from <exception class> – They have classes with constructors – Then <except suite> is executed with <name> bound to the exception • Control jumps to the except suite of the most recent try that handles the exception 10/29/18 UCB CS88 Sp18 L10 7 10/29/18 UCB CS88 Sp18 L10 8 Types of exceptions Exceptions are Classes • TypeError -- A function was passed the wrong number/type of argument class NoiseyException(Exception): def __init__(self, stuff): • NameError -- A name wasn't found print("Bad stuff happened", stuff) • KeyError -- A key wasn't found in a dictionary • RuntimeError -- Catch-all for troubles during interpretation • . . . try: return fun(x) except: raise NoiseyException((fun, x)) 10/29/18 UCB CS88 Sp18 L10 9 10/29/18 UCB CS88 Sp18 L10 10 Iterators - Notebook Iterable - an object you can iterate over • iterable : An object capable of yielding its members one at a time. • iterator : An object representing a stream of data. http://bit.ly/cs88-fa18-L11 • We have worked with many iterables as if they were sequences 10/29/18 UCB CS88 Sp18 L10 11 10/29/18 UCB CS88 Sp18 L10 12 2

Define objects that behave like Functions that return iterables sequences • map • range • zip • These objects are not sequences. • If we want to see all of the elements at once, we need to explicitly call list() or tuple() on them 10/29/18 UCB CS88 Sp18 L10 13 10/29/18 UCB CS88 Sp18 L10 14 Generators: turning iteration into an Nest iteration interable • Generator functions use iteration (for loops, while loops) and the yield keyword • Generator functions have no return statement, but def all_pairs(x): for item1 in x: they don’t return None for item2 in x: • They implicitly return a generator object yield(item1, item2) • Generator objects are just iterators def squares(n): for i in range(n): yield (i*i) 10/29/18 UCB CS88 Sp18 L10 15 10/29/18 UCB CS88 Sp18 L10 16 Next element in generator iterable Iterators – iter protocol • Iterables work because they have some "magic • In order to be iterable , a class must implement methods" on them. We saw magic methods when the iter protocol we learned about classes, • The iterator objects themselves are required to • e.g., __init__, __repr__ and __str__. support the following two methods, which together form the iterator protocol: • The first one we see for iterables is __next__ – __iter__() : Return the iterator object itself. This is required to allow both containers and iterators to be used with the for and in statements. • iter( ) – transforms a sequence into an iterator – This method returns an iterator object, Iterator can be self – __next__() : Return the next item from the container. If there are no further items, raise the StopIteration exception. • Classes get to define how they are iterated over by defining these methods 10/29/18 UCB CS88 Sp18 L10 17 10/29/18 UCB CS88 Sp18 L10 18 3

Getitem protocol Determining if an object is iterable • Another way an object can behave like a sequence • from collections.abc import Iterable is indexing : Using square brackets “[ ]” to access • isinstance([1,2,3], Iterable) specific items in an object. • Defined by special method: __ getitem__ (self, i) • This is more general than checking for any list of – Method returns the item at a given index particular type, e.g., list, tuple, string... 10/29/18 UCB CS88 Sp18 L10 19 10/29/18 UCB CS88 Sp18 L10 20 Computational Concepts Toolbox • Data type: values, literals, • Higher Order Functions operations, – Functions as Values – Functions with functions as • Expressions, Call argument expression – Assignment of function values • Variables • Higher order function patterns • Assignment Statement, – Map, Filter, Reduce Tuple assignment • Function factories – create and • Sequences: tuple, list return functions • Dictionaries • Recursion • Function Definition • Abstract Data Types Statement • Mutation • Conditional Statement • Class & Inheritance • Iteration: list comp, for, while • Exceptions • Lambda function expr. • Iterators & Generators 10/29/18 UCB CS88 Sp18 L10 21 4