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61A Lecture 24 Friday, November 1 Announcements 2 Announcements - PowerPoint PPT Presentation

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61A Lecture 24 Friday, November 1 Announcements 2 Announcements Homework 7 due Tuesday 11/5 @ 11:59pm. 2 Announcements Homework 7 due Tuesday 11/5 @ 11:59pm. Project 1 composition revisions due Thursday 11/7 @ 11:59pm. 2 Heard on

  1. Raise Statements Exceptions are raised with a raise statement. raise <expression> <expression> must evaluate to a subclass of BaseException or an instance of one. Exceptions are constructed like any other object. E.g., TypeError('Bad argument!') TypeError -- A function was passed the wrong number/type of argument NameError -- A name wasn't found KeyError -- A key wasn't found in a dictionary 9

  2. Raise Statements Exceptions are raised with a raise statement. raise <expression> <expression> must evaluate to a subclass of BaseException or an instance of one. Exceptions are constructed like any other object. E.g., TypeError('Bad argument!') TypeError -- A function was passed the wrong number/type of argument NameError -- A name wasn't found KeyError -- A key wasn't found in a dictionary RuntimeError -- Catch-all for troubles during interpretation 9

  3. Raise Statements Exceptions are raised with a raise statement. raise <expression> <expression> must evaluate to a subclass of BaseException or an instance of one. Exceptions are constructed like any other object. E.g., TypeError('Bad argument!') TypeError -- A function was passed the wrong number/type of argument NameError -- A name wasn't found KeyError -- A key wasn't found in a dictionary RuntimeError -- Catch-all for troubles during interpretation (Demo) 9

  4. Try Statements

  5. Try Statements 11

  6. Try Statements Try statements handle exceptions 11

  7. Try Statements Try statements handle exceptions try: <try suite> except <exception class> as <name>: <except suite> ... 11

  8. Try Statements Try statements handle exceptions try: <try suite> except <exception class> as <name>: <except suite> ... Execution rule: 11

  9. Try Statements Try statements handle exceptions try: <try suite> except <exception class> as <name>: <except suite> ... Execution rule: The <try suite> is executed first. 11

  10. Try Statements Try statements handle exceptions try: <try suite> except <exception class> as <name>: <except suite> ... Execution rule: The <try suite> is executed first. If, during the course of executing the <try suite> , an exception is raised that is not handled otherwise, and 11

  11. Try Statements Try statements handle exceptions try: <try suite> except <exception class> as <name>: <except suite> ... Execution rule: The <try suite> is executed first. If, during the course of executing the <try suite> , an exception is raised that is not handled otherwise, and If the class of the exception inherits from <exception class> , then 11

  12. Try Statements Try statements handle exceptions try: <try suite> except <exception class> as <name>: <except suite> ... Execution rule: The <try suite> is executed first. If, during the course of executing the <try suite> , an exception is raised that is not handled otherwise, and If the class of the exception inherits from <exception class> , then The <except suite> is executed, with <name> bound to the exception. 11

  13. Handling Exceptions 12

  14. Handling Exceptions Exception handling can prevent a program from terminating 12

  15. Handling Exceptions Exception handling can prevent a program from terminating >>> try: 12

  16. Handling Exceptions Exception handling can prevent a program from terminating >>> try: x = 1/0 12

  17. Handling Exceptions Exception handling can prevent a program from terminating >>> try: x = 1/0 except ZeroDivisionError as e: 12

  18. Handling Exceptions Exception handling can prevent a program from terminating >>> try: x = 1/0 except ZeroDivisionError as e: print('handling a', type(e)) 12

  19. Handling Exceptions Exception handling can prevent a program from terminating >>> try: x = 1/0 except ZeroDivisionError as e: print('handling a', type(e)) x = 0 12

  20. Handling Exceptions Exception handling can prevent a program from terminating >>> try: x = 1/0 except ZeroDivisionError as e: print('handling a', type(e)) x = 0 handling a <class 'ZeroDivisionError'> 12

  21. Handling Exceptions Exception handling can prevent a program from terminating >>> try: x = 1/0 except ZeroDivisionError as e: print('handling a', type(e)) x = 0 handling a <class 'ZeroDivisionError'> >>> x 12

  22. Handling Exceptions Exception handling can prevent a program from terminating >>> try: x = 1/0 except ZeroDivisionError as e: print('handling a', type(e)) x = 0 handling a <class 'ZeroDivisionError'> >>> x 0 12

  23. Handling Exceptions Exception handling can prevent a program from terminating >>> try: x = 1/0 except ZeroDivisionError as e: print('handling a', type(e)) x = 0 handling a <class 'ZeroDivisionError'> >>> x 0 Multiple try statements : Control jumps to the except suite of the most recent try statement that handles that type of exception. 12

  24. Handling Exceptions Exception handling can prevent a program from terminating >>> try: x = 1/0 except ZeroDivisionError as e: print('handling a', type(e)) x = 0 handling a <class 'ZeroDivisionError'> >>> x 0 Multiple try statements : Control jumps to the except suite of the most recent try statement that handles that type of exception. (Demo) 12

  25. WWPD: What Would Python Do? How will the Python interpreter respond? 13

  26. WWPD: What Would Python Do? How will the Python interpreter respond? 13

  27. WWPD: What Would Python Do? How will the Python interpreter respond? def invert(x): result = 1/x # Raises a ZeroDivisionError if x is 0 print('Never printed if x is 0') return result def invert_safe(x): try : return invert(x) except ZeroDivisionError as e: return str(e) 13

  28. WWPD: What Would Python Do? How will the Python interpreter respond? def invert(x): result = 1/x # Raises a ZeroDivisionError if x is 0 print('Never printed if x is 0') return result def invert_safe(x): try : return invert(x) except ZeroDivisionError as e: return str(e) >>> invert_safe(1/0) 13

  29. WWPD: What Would Python Do? How will the Python interpreter respond? def invert(x): result = 1/x # Raises a ZeroDivisionError if x is 0 print('Never printed if x is 0') return result def invert_safe(x): try : return invert(x) except ZeroDivisionError as e: return str(e) >>> invert_safe(1/0) >>> try : 13

  30. WWPD: What Would Python Do? How will the Python interpreter respond? def invert(x): result = 1/x # Raises a ZeroDivisionError if x is 0 print('Never printed if x is 0') return result def invert_safe(x): try : return invert(x) except ZeroDivisionError as e: return str(e) >>> invert_safe(1/0) >>> try : ... invert_safe(0) 13

  31. WWPD: What Would Python Do? How will the Python interpreter respond? def invert(x): result = 1/x # Raises a ZeroDivisionError if x is 0 print('Never printed if x is 0') return result def invert_safe(x): try : return invert(x) except ZeroDivisionError as e: return str(e) >>> invert_safe(1/0) >>> try : ... invert_safe(0) ... except ZeroDivisionError as e: 13

  32. WWPD: What Would Python Do? How will the Python interpreter respond? def invert(x): result = 1/x # Raises a ZeroDivisionError if x is 0 print('Never printed if x is 0') return result def invert_safe(x): try : return invert(x) except ZeroDivisionError as e: return str(e) >>> invert_safe(1/0) >>> try : ... invert_safe(0) ... except ZeroDivisionError as e: ... print ('Handled!') 13

  33. WWPD: What Would Python Do? How will the Python interpreter respond? def invert(x): result = 1/x # Raises a ZeroDivisionError if x is 0 print('Never printed if x is 0') return result def invert_safe(x): try : return invert(x) except ZeroDivisionError as e: return str(e) >>> invert_safe(1/0) >>> try : ... invert_safe(0) ... except ZeroDivisionError as e: ... print ('Handled!') >>> inverrrrt_safe(1/0) 13

  34. Interpreters

  35. Reading Scheme Lists 15

  36. Reading Scheme Lists A Scheme list is written as elements in parentheses: 15

  37. Reading Scheme Lists A Scheme list is written as elements in parentheses: (<element_0> <element_1> ... <element_n>) 15

  38. Reading Scheme Lists A Scheme list is written as elements in parentheses: (<element_0> <element_1> ... <element_n>) Each <element> can be a combination or primitive. 15

  39. Reading Scheme Lists A Scheme list is written as elements in parentheses: (<element_0> <element_1> ... <element_n>) Each <element> can be a combination or primitive. (+ (* 3 (+ (* 2 4) (+ 3 5))) (+ (- 10 7) 6)) 15

  40. Reading Scheme Lists A Scheme list is written as elements in parentheses: (<element_0> <element_1> ... <element_n>) Each <element> can be a combination or primitive. (+ (* 3 (+ (* 2 4) (+ 3 5))) (+ (- 10 7) 6)) The task of parsing a language involves coercing a string representation of an expression to the expression itself. 15

  41. Reading Scheme Lists A Scheme list is written as elements in parentheses: (<element_0> <element_1> ... <element_n>) Each <element> can be a combination or primitive. (+ (* 3 (+ (* 2 4) (+ 3 5))) (+ (- 10 7) 6)) The task of parsing a language involves coercing a string representation of an expression to the expression itself. Parsers must validate that expressions are well-formed. 15

  42. Reading Scheme Lists A Scheme list is written as elements in parentheses: (<element_0> <element_1> ... <element_n>) Each <element> can be a combination or primitive. (+ (* 3 (+ (* 2 4) (+ 3 5))) (+ (- 10 7) 6)) The task of parsing a language involves coercing a string representation of an expression to the expression itself. Parsers must validate that expressions are well-formed. (Demo) http://composingprograms.com/projects/scalc/scheme_reader.py.html 15

  43. Reading Scheme Lists A Scheme list is written as elements in parentheses: A recursive (<element_0> <element_1> ... <element_n>) Scheme list Each <element> can be a combination or primitive. (+ (* 3 (+ (* 2 4) (+ 3 5))) (+ (- 10 7) 6)) The task of parsing a language involves coercing a string representation of an expression to the expression itself. Parsers must validate that expressions are well-formed. (Demo) http://composingprograms.com/projects/scalc/scheme_reader.py.html 15

  44. Reading Scheme Lists A Scheme list is written as elements in parentheses: A recursive (<element_0> <element_1> ... <element_n>) Scheme list Each <element> can be a combination or primitive. (+ (* 3 (+ (* 2 4) (+ 3 5))) (+ (- 10 7) 6)) The task of parsing a language involves coercing a string representation of an expression to the expression itself. Parsers must validate that expressions are well-formed. (Demo) http://composingprograms.com/projects/scalc/scheme_reader.py.html 15

  45. Reading Scheme Lists A Scheme list is written as elements in parentheses: A recursive (<element_0> <element_1> ... <element_n>) Scheme list Each <element> can be a combination or primitive. (+ (* 3 (+ (* 2 4) (+ 3 5))) (+ (- 10 7) 6)) The task of parsing a language involves coercing a string representation of an expression to the expression itself. Parsers must validate that expressions are well-formed. (Demo) http://composingprograms.com/projects/scalc/scheme_reader.py.html 15

  46. Parsing

  47. Parsing 17

  48. Parsing A Parser takes text and returns an expression. 17

  49. Parsing A Parser takes text and returns an expression. Text Expression 17

  50. Parsing A Parser takes text and returns an expression. Lexical Text Expression analysis 17

  51. Parsing A Parser takes text and returns an expression. Lexical Text Tokens Expression analysis 17

  52. Parsing A Parser takes text and returns an expression. Lexical Syntactic Text Tokens Expression analysis analysis 17

  53. Parsing A Parser takes text and returns an expression. Lexical Syntactic Text Tokens Expression analysis analysis '(+ 1' ' (- 23)' ' (* 4 5.6))' 17

  54. Parsing A Parser takes text and returns an expression. Lexical Syntactic Text Tokens Expression analysis analysis '(+ 1' ' (- 23)' ' (* 4 5.6))' 17

  55. Parsing A Parser takes text and returns an expression. Lexical Syntactic Text Tokens Expression analysis analysis '(+ 1' '(', '+', 1 ' (- 23)' ' (* 4 5.6))' 17

  56. Parsing A Parser takes text and returns an expression. Lexical Syntactic Text Tokens Expression analysis analysis '(+ 1' '(', '+', 1 ' (- 23)' '(', '-', 23, ')' ' (* 4 5.6))' 17

  57. Parsing A Parser takes text and returns an expression. Lexical Syntactic Text Tokens Expression analysis analysis '(+ 1' '(', '+', 1 ' (- 23)' '(', '-', 23, ')' ' (* 4 5.6))' 17

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