Choose Your Own Pyventure/Midclass Review

REVIEW Okay, it's not all review. Some of it is new. The rest of it is review. These examples and questions should be challenging! Get in, and get dirty! The only way to learn to code is to try to make code. When it breaks: take a deep breath read the error message. They are often quite informative. check your indentation matching parentheses / braces / brackets / quotes ego. We all make mistakes! If you're confused about anything here, seek out a more complete Python reference, or bring questions to class. Dictionaries dicts associate keys with values are accessed through their keys have no inherent order Exercise: Create the dictionary D, which we'll use throughout the exercises. Note: we can use the dict() function to make a dictionary as well. D = {'i':1, 'l': [1,2,3], 's': "my favorite string", d= dict(a=1,b=2)} What is broken here? Now try this. D = {'i':1, 'l': [1,2,3], 's': "my favorite string", 'd': dict(a=1,b=2)} what are the keys of D Answer get all keys: d.keys() # this won't work since d isn't defined! D.keys() what are the values? Answer get values: D.values() try to access the values of D Answer Accessing: D['i'] D['d'] D['c'] # what happens when it doesn't exist? try deleting a key-value pair from D Answer delete using del del D['i'] del D['c'] try adding a key-value pair to D: 'newkey' -> True Answer adding: D['newkey'] = True Examine the get method of dictionaries help(D.get) try: D.get('i',"notfound") D.get('c',"notfound") Answer Help on built-in function get: get(...) D.get(k[,d]) -> D[k] if k in D, else d. d defaults to None. Bascially, return D[k] if k is in D, otherwise, return d. This saves our bacon from having a KeyError when a key isn't in the dictionary. extra credit: try dict(1='a'). Predict what will happen, and explain it. dict(a=1) dict('a'=1) dict(class=1) help(dict) Answer 1 isn't a valid python variable name. The dict() function expects valid python identifiers, as described in the help. extra credit: What is the first element of D? Answer Trick question! Dictionaries don't have inherent order! It may vary from machine to machine or from python version to python version. In any case, it's not something to rely on. Lists, Tuples, Strings List, tuples and strings are all python sequence types they are ordered they are accessed by their index [0,..,n], unlike dictionaries, which are accessed by key some are mutable (meaning the elements can change) strings have additional methods like upper, lower, split etc. sequences support slicing (see below) construct L,t,s like this: L = [8,'a',[1,2,3],None,'b'] t = ('hallway', "a creepy hallway!") s = "a few of my favoirite strings" try accessing different elements. L[0] s[1] t[2] L['a'] Answer L['a'] fails because sequences are accessed by index not by name. which ones are immutable? L[0] = 1 s[0] = 1 t[0] = 1 Answer Tuples and Strings are immutable. Tuples imply that the positions mean something, where lists can be appended to, deleted from, and the like. extra credit experiments in slicing L[:2] L[2:] L[-3] L[10] L[::-1] Functions make this function: yell_month(name,monthint) -> string that works like this: >>> yell_month('gregg', 1) 'January GREGG' >>> yell_month('mIRANda', 7) 'July MIRANDA' try it using a dictionary, and if/elif/else statements Answer two methods # here's one method, using a dict. def yell_month(name,monthint): months = {1: 'January', 2: 'February', } # etc. name = name.strip().upper() return months.get(monthint, "MONTHUARY") + " " + name # another, using if/else def yell_month(name,monthint): month = "SMARCH" if monthint == 1: month = "January" elif monthint == 2: month = "February" ... # lots of omitted code else: month = "SMARCH" name = name.strip().upper() return month + " " + name # which do you think is easier? Loops Sometimes we want to do things more than once. Computers don't get bored by repetitive work, and have infinite patience. Exploit this! Iterators Python has a magic appearing construct called for...in, that iterates over data structures. for element in thing: print element, type(element) Try iterating over our sequences, substituting our friends D,s,t,L for 'thing'. Answer iterator example for thing in (D,s,t,L): print thing for element in thing: print element, type(element) While Loops The while constructor runs the body of the loop for as long as the condition is True. Here is an example: def countdown(n): while n >= 1: print n n = n -1 print "liftoff" try this with different values for n, like 0, 1, -1, 'a', None write a function that counts up to a number. final_countdown, which despite its name, uses a for loop def final_countdown(): ''' print a version of Europe's 'The Final Countdown' ''' phrases = [ 'do de do dooo', 'do de do do dooo', 'de do de do', 'de do de do do de dooo' 'do de doo', 'do de do', 'do de do do deee dooooo', 'WEEEEE WOOOOOOOO', ] for phrase in phrases: print phrase extra credit: make countdown wait What if we actually want the function to wait one second between loops? There is a python function called [time.sleep] that we can use. Example showing time.sleep import time def countdown(n, wait=1): while n >= 1: print n time.sleep(wait) n = n -1 print "liftoff" countdown(5,.5) extra credit: break and continue break breaks out of a loop continue goes on to the repeat in the loop examples example with continue and break: def cases_spaces_breaks(sentence): for letter in sentence: if letter == " ": break elif letter.islower(): continue elif letter.isupper(): print letter, "is uppity!" else: print letter, "is middle cased?" try this out with various input sentences, to discover what it does The Standard Library Explore the standard library. Choose a function or module, and experiment with it. Tell the class which one you chose! Show some code using it. Describe its inputs and outputs. Get Dirty! Look at the standard exceptions (errors), and see which ones you recognize. Try to write code that triggers these errors: SyntaxError KeyError NameError TypeError (hint: int() is a good candidate!) IndexError ImportError Answer Generate some errors: a = '1 # SyntaxError {}['a'] # KeyError print fake_var # NameError int('a') # TypeError [1,2,3][8] # IndexError import fake # ImportError