CSC1018F: Programming with Python James Gain Donald Cook

A Brief History of Python Created by Guido van Rossum (a Monty Python’s Flying Circus enthusiast) in the early 1990’s Intellectual property now owned by the Python Software Foundation (PSF) Not a toy language. Used by: Industrial Light and Magic on the Star Wars movies NASA for their engineering repository

Characteristics of Python Interpreted, interactive, object-oriented programming language Powerful Batteries included (300+ libraries) Well supported OpenSource but copyrighted Portable because it exploits the underlying ‘c’ installation on most systems Easy to learn Borrows proven concepts from other languages Less scaffolding required Zen of Python: “simple is better” or “principle of least astonishment”

Hello World Java: C++: Python: // Hello World in Java class HelloWorld { static public void main(String args[]) { System.out.println(“Hello World!”); } // Hello World in C++ #include Main() { cout << “Hello World!” << endl; return 0; } # Hello World in Python print ‘Hello World!’

Course Structure Lectures: 4th period Mondays (Overview of Section) and Fridays (Mini-test and Tutorial) Afternoon Tutorial 2 hours on Monday or Tuesday Textbook Mark Pilgrim, “Dive into Python” Self Study hours work per day Do not merely read the textbook

Syllabus Week 1 Introduction to Python DIP Ch 2-3 Week 2 Intermediate Python DIP Ch 4 Week 3 Objects and Object Orientation DIP Ch 5 Week 4 Advanced Python DIP Ch Week 5 Functional Programming DIP Ch 16 Week 6 Visual Python - Remain- der Software Engineering & Battleship Project DIP Ch

Diving In Declaring Functions: def keyword, no return data type defined, arguments (with no pre-defined type) which are comma separated def buildConnectionString(params): """Build a connection string from a dictionary of parameters. Returns string.""" return ";".join(["%s=%s" % (k, v) for k, v in params.items()]) if __name__ == "__main__": myParams = {"server":"mpilgrim", \ "database":"master", \ "uid":"sa", \ "pwd":"secret" \ } print buildConnectionString(myParams)

Comparing Python Datatypes Languages can be classified as: Statically typed - types are fixed by declaration at compile time Dynamically typed - types are discovered at execution time Strongly typed - types are enforced and cannot be interchanged without conversion Weakly typed - types are freely ignored, no explicit conversion is necessary Python is dynamically and strongly typed

Documenting Functions Triple quotes (" " ") signify a multi-line comment Allows you to create a string containing single and double quotes Often used to define a doc string First thing defined in a function Available at run-time as an attribute of the function function.__doc__

Indenting Code Blocks of code are defined by indentation Indenting starts a block and unindenting ends it No need for explicit braces, brackets or keywords Whitespace is significant Code blocks are started by a ‘:’ def fib(n): print 'n =', n if n > 1: return n * fib(n - 1) else: print 'end of the line' return 1

Dictionaries eng2fr = {"one":"un", "two":"deux", "three":"trois"} An unordered set consisting of key-element pairs Retrieval: You can get keys (e.g., eng2fr["one"]) by value But not values by key (e.g., eng2fr["un"]) No duplicate keys Assigning values to an existing key will overwrite it Dictionary keys are case-sensitive Power! Values can mix and match any datatypes (even other dictionaries) Keys can combine immutable datatypes

Operating on Dictionaries Modifying a dictionary >>> eng2fr["four"] = "quatre" >>> eng2fr {"one":"un", "two":"deux", "three":"trois", "four":"quatre"} Deleting Items >>> del eng2fr["one"] >>> eng2fr {"two":"deux", "three":"trois","four":"quatre"} >>> eng2fr.clear() >>> eng2fr { }

Lists Horsemen = ["war", "famine", "pestilence", "death"] The Python Workhorse Much more than a java array Can grow dynamically Can include datatypes of any element Indexed by element number Lists are zero-based (first element is [0]) >>> horsemen[2] => 'famine' Allow negative indices which wrap the list >>> horsemen[-2] => 'pestilence'

Operating on Lists Slicing a List Subset specified by two indices. From 1st up to but not including 2nd >>> horsemen[1:3] => ['famine', 'pestilence'] Leaving out an index implies first or last Adding elements.append adds a single element to the end.insert places an element at a particular position.extend concatenates list onto end Deleting elements.remove deletes the first occurrence of a value from a list Searching a List.index finds first occurrence of a value and returns the index

More on Lists Mapping List comprehension allows a compact way of applying a function to every element of a list li2 = [elem*2 for elem in li] Same as: j = 1 while j < len(li): li2[j] = li[j] * 2 Joining Lists into Strings Join combines a list of strings into a single string print ";".join(["2b", "or", "not", "2b"]) => 2b;or;not;2b Split reverses join "2b;or;not;2b".split(";") => ['2b', 'or', 'not', '2b']

Tuples sins = ("pride", "anger", "gluttony", "envy", "lust", "avarice", "sloth") An immutable list. No changes allowed Similar to lists Defined order, negative indices, zero-based, slicing But no methods available (except ‘in’) So what is it good for Faster than an equivalent list Write-protects data Can serve as an immutable dictionary key

Formatting Strings Can create strings using a formatting (% placeholder) specifier Formatting borrowed from ‘c’ language Concatenation (+) only works when combining strings Examples >>> print "%s two %s" % ("joining", "strings") joining two strings >>> print "today is %d / %d / %d" % (20, 2, 2006) today is 20 / 2 / 2006

Conclusion Some details skipped over (import modules, __name__ for debugging) Homework: make sure you understand aliasing def buildConnectionString(params): """Build a connection string from a dictionary of parameters. Returns string.""" return ";".join(["%s=%s" % (k, v) for k, v in params.items()]) if __name__ == "__main__": myParams = {"server":"mpilgrim", \ "database":"master", \ "uid":"sa", \ "pwd":"secret" \ } print buildConnectionString(myParams)