U of T CS4HS 2011: Python Workshop Dan Zingaro July 13-14, 2011 - PowerPoint PPT Presentation

U of T CS4HS 2011: Python Workshop Dan Zingaro July 13-14, 2011

Python History ◮ Late 1970s: programming language called ABC ◮ High-level, intended for teaching ◮ Only five data types ◮ Programs are supposedly one-quarter the size of the equivalent BASIC or Pascal program ◮ Not a successful project ◮ More ABC information: http://homepages.cwi.nl/~steven/abc/

Python History... ◮ 1983: Guido van Rossum joined the ABC team ◮ Late 1980s: Guido started working on a new project, in which a scripting language would be helpful ◮ Based it on ABC, removed warts (e.g. ABC wasn’t extensible) ◮ Python, after Monty Python ◮ Guido: Benevolent Dictator for Life (BDFL)... but he’s retiring! ◮ http://www.artima.com/intv/ (search for Guido)

Why Python for Education? ◮ Readable, uniform code structure ◮ No compilation step — Python is interpreted ◮ Supports and integrates imperative and OOP features ◮ Batteries included: Python’s standard library comes with tools for a variety of problem domains, and thousands of additional modules are available for download ◮ Writing games, creating GUIs, web scripting, accessing databases, etc.

Dynamic Typing ◮ Biggest conceptual change compared to C, Pascal, Java etc. ◮ Variables do not have types. Objects have types >>> a = 5 >>> type (a) <type ’int’> >>> a = ’hello’ >>> type (a) <type ’str’> >>> a = [4, 1, 6] >>> type (a) <type ’list’>

Functions and Ifs def ining functions and using if statements is similar to what you expect ( . . . except for the indentation!): def bigger (a, b): print ’Hi’ if a > b: return a else: return b

While Loops There’s a standard while loop (but for loops are different; we’ll talk about that later) i = 8 while i > 0: print i i = i - 1

Built-in Types ◮ We’ll look at the core five object types that are built-in to Python ◮ Numbers ◮ Strings ◮ Lists ◮ Dictionaries ◮ Files ◮ They’re extremely powerful — we do many assignments with these alone!

Built-in Types: Numbers ◮ Create numbers by using numeric literals ◮ If you include no fractional component, it’s an integer; otherwise it’s a float ◮ We have all of the standard mathematical operators, and even ** for exponent ◮ Make integers as small or large as you like — they can’t go out of bounds

Exercise: Numbers Exercise 1 on handout

Built-in Types: Strings ◮ A string is a sequence of characters ◮ To indicate that something is a string, we place single- or double-quotes around it ◮ We can use + to concatenate strings ◮ This is an example of overloading: + is used to add numbers too; it knows what to do based on context ◮ What happens if we try to use + with a string and a number? ◮ Error: + doesn’t know what to do! ◮ e.g. is ’3’ + 4 supposed to be the string ’34’ or the number 7? ◮ Design philosophy: Python tries never to guess what you mean

Strings... ◮ The * operator is overloaded, too ◮ Applied to a string and an integer i , it duplicates the string i times ◮ If i ≤ 0, the result is the empty string ◮ Can also use relational operators such as < or > to lexically compare strings

Looping Through Strings for char in s: <do something with char> ◮ We’ll see this pattern again and again for each Python type ◮ It’s like Php’s foreach or Java’s for -with-the-colon ◮ Let’s write a function that counts the number of vowels in a string ◮ The in operator can be used for set membership

Possible Solution: How Many Vowels? (num vowels.py) def num_vowels(s): ’’’Return the number of vowels in string s. The letter "y" is not treated as a vowel.’’’ count = 0 for char in s: if char in "aAeEiIoOuU": count += 1 return count

String Methods ◮ Like in Java, strings are objects and have tons of methods ◮ Use dot-notation (i.e. object.method ) to access methods ◮ Use dir (str) to get a list of them, and help (str.methodname) for help on any method ◮ Useful ones: find , lower , count , replace ... ◮ Like Java again, strings are immutable — all we can do is create new strings

Indexing and Slicing Strings ◮ Assume s is a string ◮ Then, s[i] for i ≥ 0 extracts character i from the left ( 0 is the leftmost character) ◮ We can also use a negative index i to extract a character beginning from the right ( -1 is the rightmost character) ◮ Slice notation : s[i:j] extracts characters beginning at s[i] and ending at the character one to the left of s[j] ◮ If we leave out the first index, Python defaults to using index 0 to begin the slice ◮ Similarly, if we leave out the second index, Python defaults to using index len(s) to end the slice

Exercise: Strings Exercises 2 and 3 on handout

Built-in Types: Lists Lists are the closest thing we have to arrays. . . . but they’re not that close! Strings Lists Sequences of? Characters Any object types Immutable? Yes No Can be heterogeneous? No Yes Can index and slice? Yes Yes Can use for-loop? Yes Yes Created like? ’hi’ [4, 1, 6]

List Methods ◮ As with strings, there are lots of methods; use dir (list) or help (list.method) for help ◮ append is used to add an object to the end of a list ◮ extend is used to append the objects of another list ◮ insert (index, object) inserts object before index ◮ sort() sorts a list ◮ remove (value) removes the first occurrence of value from the list

Exercise: Length of Strings ◮ Write a function that takes a list of strings, and prints out the length of each string in the list ◮ e.g. if the list is [’abc’, ’q’, ’’] , the output would be as follows 3 1 0

Exercise: Lists Exercise 4 on handout

Built-in Types: Dictionaries Dictionaries are like Php associative arrays, or Java maps. Lists Dictionaries Stores? Sequences of objects Key-value pairs Immutable? No No Can be heterogeneous? Yes Yes Can index and slice? Yes No Can use for-loop? Yes Yes Created like? [4, 1] {’a’:1, ’b’:2}

Dictionaries vs. Lists ◮ Compared to using “parallel lists”, dictionaries make an explicit connection between a key and a value ◮ But unlike lists, dictionaries do not guarantee any ordering of the elements ◮ If you use for k in d , for a dictionary d , you get the keys back in arbitrary order bird_dict = { ’peregrine falcon’:1, ’harrier falcon’:5, ’red-tailed hawk’: 2, ’osprey’ : 11}

Adding to Dictionaries ◮ Dictionary keys must be of immutable types (no lists!), but values can be anything ◮ We can use d[k] = v to add key k with value v to dictionary d ◮ We can use the update method to dump another dictionary’s key-value pairs into our dictionary ◮ We can use d[k] to obtain the value associated with key k of dictionary d ◮ If k does not exist, we get an error ◮ The get method is similar, except it returns None instead of giving an error when the key does not exist

Exercise: Dictionaries Exercises 5 and 6 on handout

Built-in Types: Files ◮ To open a file in Python, we use the open function ◮ Syntax: open (filename, mode) ◮ mode is the string ’r’ to open the file for reading, ’w’ to open the file for writing, or ’a’ to open the file for appending. No mode defaults to ’r’ ◮ open gives us a file object, with lots of methods we can use to operate on the file ◮ readline gives us the next line (or the empty string at EOF) ◮ read gives us the remainder of the file as one string ◮ readlines gives us the remainder of the file as a list of strings (one string per line)

Exercise: Files Exercise 7 on handout

Messing with Sounds ◮ Some of my favourite assignments have students play with sounds and music ◮ If we import sound_media (my module, not built-in to Python), we can use load_sound to load a wav file ◮ The resultant sound object has methods just like a built-in Python object

Methods in sound media ◮ create_sound (num_samples) : create a silent sound with num_samples samples ◮ get_sample (index) : get the sample at a given index ◮ get_value (sample) : get the value of a sample ◮ We’ll start with a function that changes the volume of a sound

Changing Volume import sound_media def change_volume(snd, factor): ’’’Return a copy of snd, but whose volume is scaled by factor’’’ target = sound_media.create_sound (len(snd)) i = 0 while i < len(snd): source_sample = sound_media.get_sample(snd, i) target_sample = sound_media.get_sample(target, i) value = sound_media.get_value(source_sample) * factor sound_media.set_value(target_sample, int(value)) i = i + 1 return target

Exercise: Sounds Exercise 8 on handout

Further Python Resources ◮ http://rmi.net/~lutz/ ◮ Mark Lutz’ Python books ◮ Constantly-updated to keep up with Python releases ◮ docs.python.org/tutorial ◮ Free online Python tutorial ◮ nifty.stanford.edu ◮ SIGCSE Nifty Assignments (some Python-related) ◮ I’ve modified a couple of the Java/C assignments to fit Python — please get in touch!