1 / 91

Getting Started with Python

Object-Oriented Programming in Python Goldwasser and Letscher Chapter 2. Getting Started with Python. The Python Interpreter. A piece of software that executes commands for the Python language Start the interpreter by typing python at a command prompt

baina
Download Presentation

Getting Started with Python

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Object-Oriented Programming in PythonGoldwasser and LetscherChapter 2 Getting Started with Python

  2. The Python Interpreter • A piece of software that executes commands for the Python language • Start the interpreter by typing python at a command prompt • Many developers use an Integrated Development Environment for Python known as IDLE Object-Oriented Programming in Python

  3. The Python Prompt >>> • This lets us know that the interpreter awaits our next command Object-Oriented Programming in Python

  4. Our First Example >>> groceries = list() >>> • We see a new Python prompt, so the command has completed. • But what did it do? Object-Oriented Programming in Python

  5. Instantiation >>> groceries = list() >>> list Constructs a new instance from the list class (notice the parentheses in the syntax) Object-Oriented Programming in Python

  6. Assignment Statement >>> groceries =list() >>> groceries list groceries serves as an identifier for the newly constructed object (like a “sticky label”) Object-Oriented Programming in Python

  7. Calling a Method >>> groceries = list() >>> groceries.append('bread') >>> groceries list 'bread' Object-Oriented Programming in Python

  8. Displaying Internals When working in the interpreter, we do not directly "see" the internal picture. But we can request a textual representation. >>> groceries = list() >>> groceries.append('bread') >>> groceries ['bread']  interpreter's response >>> Object-Oriented Programming in Python

  9. object method parameters • There may be many objects to choose from • The given object may support many methods • Use of parameters depends upon the method Method Calling Syntax groceries.append('bread') Object-Oriented Programming in Python

  10. Common Errors >>> groceries.append() What's the mistake? Traceback (most recent call last): File "<stdin>", line 1, in -toplevel- TypeError: append() takes exactly one argument (0 given) >>> Traceback (most recent call last): File "<stdin>", line 1, in -toplevel- NameError: name 'bread' is not defined groceries.append(bread) What's the mistake? Object-Oriented Programming in Python

  11. The append Method New item added to the end of the list (much like a restaurant's waitlist) >>> waitlist = list() >>> waitlist.append('Kim') >>> waitlist.append('Eric') >>> waitlist.append('Nell') >>> waitlist ['Kim', 'Eric', 'Nell'] Object-Oriented Programming in Python

  12. The insert Method Can insert an item in an arbitrary place using a numeric index to describe the position. An element's index is the number of items before it. waitlist ['Kim', 'Donald', 'Eric', 'Nell'] waitlist.insert(1, 'Donald') >>> >>> waitlist ['Kim', 'Eric', 'Nell'] >>> Object-Oriented Programming in Python

  13. Zero-Indexing • By this definition, • the first element of the list has index 0 • the second element has index 1 • the last element has index (length - 1) We call this convention zero-indexing. (this is a common point of confusion) Object-Oriented Programming in Python

  14. The remove Method What if Eric gets tired of waiting? >>> waitlist ['Kim', 'Donald', 'Eric', 'Nell'] >>> waitlist ['Kim', 'Donald', 'Nell'] >>> waitlist.remove('Eric') >>> Object-Oriented Programming in Python

  15. The remove Method • Notice that we didn't have to identify where the item is; the list will find it. • If it doesn't exist, a ValueError occurs • With duplicates, the earliest is removed >>> groceries ['milk', 'bread', 'cheese', 'bread'] >>> groceries.remove('bread') >>> groceries ['milk', 'cheese', 'bread'] Object-Oriented Programming in Python

  16. Return values • Thus far, all of the methods we have seen have an effect on the list, but none return any direct information to us. • Many other methods provide an explicit return value. • As our first example: the count method Object-Oriented Programming in Python

  17. The count method >>> groceries ['milk', 'bread', 'cheese', 'bread'] >>> groceries.count('bread') 2 response from the interpreter >>> groceries.count('milk') 1 >>> groceries.count('apple') 0 >>> Object-Oriented Programming in Python

  18. Saving a Return Value • We can assign an identifier to the returned object >>> groceries ['milk', 'bread', 'cheese', 'bread'] >>> numLoaves = groceries.count('bread') >>> numLoaves >>> 2 • Notice that it is no longer displayed by interpreter • Yet we can use it in subsequent commands Object-Oriented Programming in Python

  19. Operators • Most behaviors are invoked with the typical "method calling" syntax of object.method( ) • But Python uses shorthand syntax for many of the most common behaviors (programmers don't like extra typing) • For example, the length of a list can be queried as len(groceries)although this is really shorthand for a call groceries.__len__( ) Object-Oriented Programming in Python

  20. Accessing a list element >>> waitlist ['Kim', 'Donald', 'Eric', 'Nell'] >>> waitlist[1] 'Donald' >>> waitlist[3] 'Nell' >>> waitlist[4] Traceback (most recent call last): File "<stdin>", line 1, in ? IndexError: list index out of range Object-Oriented Programming in Python

  21. Negative Indices >>> waitlist ['Kim', 'Donald', 'Eric', 'Nell'] >>> waitlist[-1] 'Nell' >>> waitlist[-3] 'Donald' >>> waitlist[-4] 'Kim' >>> Object-Oriented Programming in Python

  22. List Literals • We originally used the syntax list( ) to create a new empty list. For convenience, there is a shorthand syntax known as a list literal. groceries = [ ] (experienced programmers like to type less!) • List literals can also be used to create non-empty lists, using a syntax similar to the one the interpreter uses when displaying a list. groceries = ['cheese', 'bread', 'milk'] Object-Oriented Programming in Python

  23. Copying Lists • The list( ) constructor is useful for making a new list modeled upon an existing sequence >>> favoriteColors = ['red', 'green', 'purple', 'blue'] >>> primaryColors = list(favoriteColors)  a copy >>> primaryColors.remove('purple') >>> primaryColors ['red', 'green', 'blue'] >>> favoriteColors ['red', 'green', 'purple', 'blue'] >>> Object-Oriented Programming in Python

  24. The range function • Lists of integers are commonly needed. Python supports a built-in function named range to easily construct such lists. • There are three basic forms: range(stop) goes from zero up to but not including stop >>> range(5) [0, 1, 2, 3, 4] Object-Oriented Programming in Python

  25. The range function range(start, stop) begins with start rather than zero >>> range(23, 28) [23, 24, 25, 26, 27] range(start, stop, step) uses the given step size >>> range(23, 35, 4) [23, 27, 31] >>> range(8, 3, -1) [8, 7, 6, 5, 4] Object-Oriented Programming in Python

  26. Many useful behaviors • groceries.pop( ) remove last element • groceries.pop(i) remove ith element • groceries.reverse( ) reverse the list • groceries.sort( ) sort the list • 'milk' in groceries does list contain? • groceries.index('cereal') find leftmost match These will become familiar with more practice. Object-Oriented Programming in Python

  27. Documentation • See Section 2.2.6 of the book for more details and a table summarizing the most commonly used list behaviors. • You may also type help(list) from within the Python interpreter for documentation, or for a specific method as help(list.insert) Object-Oriented Programming in Python

  28. Python's str class • A list can represent any sequence of objects • A very common need in computing is for a sequence of text characters. • There is a specialized class, named str, devoted to manipulating character strings. Object-Oriented Programming in Python

  29. String literals • Can enclose in single quotes: 'bread' • Can enclose in double quotes: "bread" • This choice helps when you want to use a single or double quote as a character within the string: "Who's there?" • Can embed a newline character using an escape character \n as in: "Knock Knock\nWho's there?" Object-Oriented Programming in Python

  30. Common behaviors greeting = 'How do you do?' • len(greeting) returns 14 • 'yo' in greeting returns True • greeting.count('do')returns2 • greeting.index('do')returns4 • greeting[2] returns'w' Object-Oriented Programming in Python

  31. alphabet[9:20:3] returns'jmps' Slicing Slicing is a generalization of indexing that is supported by strings (and lists too). 1111111111222222 01234567890123456789012345 alphabet = 'abcdefghijklmnopqrstuvwxyz' Object-Oriented Programming in Python

  32. alphabet[9:20:3] returns'jmps' Slicing Slicing is a generalization of indexing that is supported by strings (and lists too). 1111111111222222 01234567890123456789012345 alphabet = 'abcdefghijklmnopqrstuvwxyz' alphabet[4] returns'e' Object-Oriented Programming in Python

  33. alphabet[9:20:3] returns'jmps' Slicing Slicing is a generalization of indexing that is supported by strings (and lists too). 1111111111222222 01234567890123456789012345 alphabet = 'abcdefghijklmnopqrstuvwxyz' alphabet[4:13] returns'efghijklm' (starting at 4, going up to but not including 13) Object-Oriented Programming in Python

  34. alphabet[9:20:3] returns'jmps' Slicing Slicing is a generalization of indexing that is supported by strings (and lists too). 1111111111222222 01234567890123456789012345 alphabet = 'abcdefghijklmnopqrstuvwxyz' alphabet[ :6] returns'abcdef' (starting at beginning going up to but not including 6) Object-Oriented Programming in Python

  35. alphabet[9:20:3] returns'jmps' Slicing Slicing is a generalization of indexing that is supported by strings (and lists too). 1111111111222222 01234567890123456789012345 alphabet = 'abcdefghijklmnopqrstuvwxyz' alphabet[23:] returns'xyz' (starting at 23 going all the way to the end) Object-Oriented Programming in Python

  36. alphabet[9:20:3] returns'jmps' Slicing Slicing is a generalization of indexing that is supported by strings (and lists too). 1111111111222222 01234567890123456789012345 alphabet = 'abcdefghijklmnopqrstuvwxyz' alphabet[9:20:3] returns'jmps' (starting at 9, stopping before 20, stepping by 3) Object-Oriented Programming in Python

  37. alphabet[9:20:3] returns'jmps' Slicing Slicing is a generalization of indexing that is supported by strings (and lists too). 1111111111222222 01234567890123456789012345 alphabet = 'abcdefghijklmnopqrstuvwxyz' alphabet[17:5:-3] returns'roli' (starting at 17, toward but not with 5, stepping by -3) Object-Oriented Programming in Python

  38. alphabet[9:20:3] returns'jmps' Slicing Slicing is a generalization of indexing that is supported by strings (and lists too). 1111111111222222 01234567890123456789012345 alphabet = 'abcdefghijklmnopqrstuvwxyz' alphabet[ : :-1] 'zyxwvutsrqponmlkjihgfedcba' (everything, but in reverse order) Object-Oriented Programming in Python

  39. alphabet[9:20:3] returns'jmps' Summary of Slicing Notice that convention for slicing alphabet[start:stop:step] uses indices akin to that of range(start, stop, step) Object-Oriented Programming in Python

  40. Differences: list and str • We cannot change an existing string. • However, we can create new strings based upon existing ones. • List are mutable; strings are immutable (allows Python to optimize the internals) Object-Oriented Programming in Python

  41. Example: lower( ) >>> formal = 'Hello' >>> formal str 'Hello' Object-Oriented Programming in Python

  42. Example: lower( ) >>> formal = 'Hello' >>> informal = formal.lower() >>> formal informal str str 'Hello' 'hello' Note that formal is unchanged Object-Oriented Programming in Python

  43. Reassigning an Identifier >>> person = 'Alice' >>> person str 'Alice' Object-Oriented Programming in Python

  44. Reassigning an Identifier >>> person = 'Alice' >>> person = person.lower() >>> person str str 'Alice' 'alice' Object-Oriented Programming in Python

  45. Creating New Strings Each of the following leaves the original string unchanged, returning a new string as a result. • greeting.lower( ) • greeting.upper( ) • greeting.capitalize( ) • greeting.strip( ) • greeting.center(30) • greeting.replace('hi','hello') Object-Oriented Programming in Python

  46. Additional string methods Strings support other methods that are specific to the context of textual information • greeting.islower( ) not to be confused with lower( ) • greeting.isupper( ) • greeting.isalpha( ) • greeting.isdigit( ) • greeting.startswith(pattern) • greeting.endswith(pattern) Object-Oriented Programming in Python

  47. Converting between strings and lists • To support text processing, the str class has methods to split and rejoin strings. • split is used to divide a string into a list of pieces based upon a given separator. • join is used to assemble a list of strings and a separator into a composite string. Object-Oriented Programming in Python

  48. The split method By default, the pieces are based on dividing the original around any form of whitespace (e.g., spaces, tabs, newlines) >>> request = 'eggs and milk and apples' >>> request.split( ) ['eggs', 'and', 'milk', 'and', 'apples'] Object-Oriented Programming in Python

  49. The split method Some other separator can be specified as an optional parameter to split. That string will be used verbatim. >>> request = 'eggs and milk and apples' >>> request.split('and') ['eggs ', ' milk ', ' apples'] ^ ^ ^ ^ (note well the spaces that remain) Object-Oriented Programming in Python

  50. The split method Here is the same example, but with spaces embedded within the separator string. >>> request = 'eggs and milk and apples' >>> request.split(' and ') ['eggs', 'milk', 'apples'] Object-Oriented Programming in Python

More Related