CSC1018F: Intermediate Python Diving into Python Ch. 4 Think Like a Comp. Scientist Ch. 1-6

Lecture Outline Recap of Intro to Python [week 1] Basics: Conditionals Iteration Introspection: Optional and Named Arguments Built-in Functions Filtering Lists and & or Lambda Functions

Recap of Python Intro Defining and documenting functions Block structure by indenting Native Datatypes: Dictionary List Tuple String Formatting List Comprehension (Mapping)

Recap Exercise: Functions Design a python function Dictize that will take a tuple and return a dictionary with: Values corresponding to the tuple elements and Keys that take the form “key0”, “key1”, etc. (where the number matches the tuple index) Create a doc string for your function and use the tuple (“Cleese”, “Gilliam”, “Chapman”, “Jones”, “Idle”, “Palin”) as a test within the module Dictize

Bits and Pieces Strings: Input: Pseudo-mathematical operators + (concatenation) and * (repetition) are valid E.g., >>> "spam" * 3 => "spamspamspam" Input: There are built in functions to get keyboard input raw_input(prompt) Accepts user input terminated by pressing return Returns the typed input as a string Prompt is a string displayed before accepting input

Conditionals Conditionals: Chained Conditionals: Comparison operators (==, >, <, >=, <=, !=) return True (1) or False (0). Logical operators (and, or, not) can combine boolean expressions Conditionals: if condition: block1 With optional else clause Chained Conditionals: if condition1: elif condition2: block2 else: block3

Iteration The while statement: The for loop: while condition: block Repeats block until condition is False The for loop: for value in string: for char in fruit: print char Traverses every element of a list, tuple or string A compact equivalent of certain forms of the while loop Java-style ‘for’ loops can still be done

Optional and Named Arguments Function arguments can: Have default values Be called in any order Syntax def info(object, spacing = 10, collapse = 1) Parameters called out of order must be named (e.g., object = newtuple) Usage info(li) # spacing = 10, collapse = 1 info(li, 12) # spacing = 12, collapse = 1 info(li, collapse = 0) # spacing = 10 info(spacing = 15, object = li) # collapse = 1

Built-in Functions Python has built-in functions that are part of the core language (don’t need to be imported) Full list available in the Python Library Reference type Returns the datatype of an arbitrary object >>> type(obdchelper) <type 'module'> str Very general conversion of any object of any type into a string >>> str(1) '1' >>> str(obdchelper) "module 'obdchelper' from …" Other conversion functions also available = int, float Include dir, getattr, callable, id

Filtering Lists An extension of list comprehension Apply the mapping function to selected elements Syntax: [mapping_expression for element in source_list if filter_expression] Example: [elem for elem in li if elem != 'b'] [elem for elem in li if li.count(elem) == 1]

The weirdness of and & or The logical operators can be applied to more than just boolean expressions They return one of the values being compared 0, '', [], (), {}, None are considered False, everything else is True and Evaluated left to right. Returns the first False value, or the last value >>> 'a' and 'b' => 'b' >>> '' and 'b' => '' or Evaluated left to right. Returns the first True value, or the last value >>> 'a' or 'b' => 'a' >>> '' or 'b' => 'b'

The and-or trick Statements of the form: Can be converted into: if x: val = a else val = b Can be converted into: val = x and a or b >>> 1 and "first" or "second" => "first" >>> 0 and "first" or "second" => "second" But watch out for ‘a’ being false >>> 1 and "" or "second" => "second" Solution: (1 and [a] or [b])[0]

Lambda Functions One-line mini functions Function of the form: Become: Body of function can only be a single expression Drop parentheses around arguments Return keyword is implied Function of the form: def f(x): return x*2 Become: >>> g = lambda x: x*2 >>> g(3) => 6