MI-RUB Exceptions Lecture 7

Transcription

1 MI-RUB Exceptions Lecture 7 Pavel Strnad pavel.strnad@fel.cvut.cz Dept. of Computer Science, FEE CTU Prague, Karlovo nám. 13, Praha, Czech Republic MI-RUB, WS 2011/12 Evropský sociální fond Praha & EU: Investujeme do vaší budoucnosti Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

2 Contents 1 Exceptions 2 Exception Hierarchy 3 Handling Exceptions 4 Raising Exception 5 Catch and Throw 6 Literature Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

3 Exceptions Exceptions mechanism when something goes wrong, specific reaction to an error situation, and a programming style! Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

4 Exceptions Exceptions mechanism when something goes wrong, specific reaction to an error situation, and a programming style! Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

5 Exceptions Exceptions mechanism when something goes wrong, specific reaction to an error situation, and a programming style! Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

6 Exception Class Hierarchy Figure: The above figure is from the Programming Ruby 1.9 book. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

7 Exception Class Iheritance You can raise one of the built-in exception classes. You can subclass StandardError. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

8 Exception Class Iheritance You can raise one of the built-in exception classes. You can subclass StandardError. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

9 Unhandled Exception r e q u i r e openuri web_page = open ( " h t t p : / / pragprog. com / podcasts " ) o u tput = F i l e. open ( " podcasts. html ", "w" ) while l i n e = web_page. gets o u tput. puts o u tput. close l i n e Question Where does an error occur? Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

10 Handled Exception r e q u i r e openuri page = " podcasts " file_name = " #{ page }. html " web_page = open ( " h t t p : / / pragprog. com / # { page } " ) o u tput = F i l e. open ( file_name, "w" ) begin while l i n e = web_page. gets o u tput. puts l i n e o u tput. close rescue Exception STDERR. puts " F a i l e d to download #{page } : #{$! } " o u tput. close F i l e. d e l e te ( file_name ) r a i s e Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

11 Handling Exceptions Handling Exceptions Exceptions are handled in a rescue section of a begin block. There can be more rescue blocks. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

12 Handling Exceptions Handling Exceptions Exceptions are handled in a rescue section of a begin block. There can be more rescue blocks. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

13 Handling Exceptions class MyError < StandardError begin r a i s e MyError rescue StandardError=>e puts " StandardError " rescue MyError=>e puts " MyError " Question What will be on output? Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

14 Handling Exceptions The match is made using parameter===$!. The parameter match if exception type specified in rescue is the same class or a superclass! (Same in Java) Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

15 Handling Exceptions The match is made using parameter===$!. The parameter match if exception type specified in rescue is the same class or a superclass! (Same in Java) Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

16 Tidying Up Sometimes you need to guarantee that some processing is done at the of a block of code, regardless of whether an exception was raised. For example, you may have a file open on entry to the block, and you need to make sure it gets closed as the block exits. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

17 Ensure Clause The ensure clause does just this. ensure goes after the last rescue clause and contains a chunk of code that will always be executed as the block terminates. (In Java it is finally) f = F i l e. open ( " t e s t f i l e " ) begin #.. process rescue #.. handle e r r o r ensure f. close Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

18 Raising Exception r a i s e raise Reraises the current exception (or a RuntimeError if there is no current exception). Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

19 Raising Exception r a i s e " bad mp3 encoding " raise Creates a new RuntimeError exception, setting its message to the given string. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

20 Raising Exception r a i s e InterfaceException, " Keyboard f a i l u r e ", c a l l e r raise Uses the first argument to create an exception and then sets the associated message to the second argument and the stack trace to the third argument. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

21 Play it = tru e begin i f rescue i ehlo ( helodom ) helo ( helodom ) P r o t o c o l E r r o = false r e t r y else r a i s e Retry is not recommed to use. You can easily create infinite loop. Retry will execute begin block again. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

22 Catch! w o r d _ l i s t = F i l e. open ( " w o r d l i s t " ) catch ( : done ) do r e s u l t = [ ] while l i n e = w o r d _ l i s t. gets word = l i n e. chomp puts throw : done unless word =~ / ^ \ w+$ / r e s u l t << word r e s u l t. reverse catch defines a block that is labeled with the given name (which may be a Symbol or a String). The block is executed normally until a throw is encountered. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

23 Catch! w o r d _ l i s t = F i l e. open ( " w o r d l i s t " ) word_in_error = catch ( : done ) do r e s u l t = [ ] while l i n e = w o r d _ l i s t. gets puts word = l i n e. chomp throw ( : done, word ) unless word =~ / ^ \ w+$ / r e s u l t << word r e s u l t. reverse i f word_in_erro r puts " F a i l e d : # { word_in_erro r } found, but a word was expected " produces : F a i l e d : wow found, but a word was expected catch returns a value if a second argument to throw is given. Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19

24 All examples are from Pavel Strnad (Czech Technical University in Prague) MI-RUB Exceptions Lecture 7 MI-RUB, / 19