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Diffstat (limited to 'Doc/tut/tut.tex')
-rw-r--r-- | Doc/tut/tut.tex | 72 |
1 files changed, 55 insertions, 17 deletions
diff --git a/Doc/tut/tut.tex b/Doc/tut/tut.tex index 054985bbeb..7f9a7ee407 100644 --- a/Doc/tut/tut.tex +++ b/Doc/tut/tut.tex @@ -1012,7 +1012,7 @@ individual elements of a list: \end{verbatim} Assignment to slices is also possible, and this can even change the size -of the list: +of the list or clear it entirely: \begin{verbatim} >>> # Replace some items: @@ -1027,9 +1027,14 @@ of the list: ... a[1:1] = ['bletch', 'xyzzy'] >>> a [123, 'bletch', 'xyzzy', 1234] ->>> a[:0] = a # Insert (a copy of) itself at the beginning +>>> # Insert (a copy of) itself at the beginning +>>> a[:0] = a >>> a [123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234] +>>> # Clear the list: replace all items with an empty list +>>> a[:] = [] +>>> a +[] \end{verbatim} The built-in function \function{len()} also applies to lists: @@ -2023,9 +2028,9 @@ applied to complex expressions and nested functions: There is a way to remove an item from a list given its index instead of its value: the \keyword{del} statement. This differs from the \method{pop()}) method which returns a value. The \keyword{del} -statement can also be used to -remove slices from a list (which we did earlier by assignment of an -empty list to the slice). For example: +statement can also be used to remove slices from a list or clear the +entire list (which we did earlier by assignment of an empty list to +the slice). For example: \begin{verbatim} >>> a = [-1, 1, 66.25, 333, 333, 1234.5] @@ -2035,6 +2040,9 @@ empty list to the slice). For example: >>> del a[2:4] >>> a [1, 66.25, 1234.5] +>>> del a[:] +>>> a +[] \end{verbatim} \keyword{del} can also be used to delete entire variables: @@ -3710,19 +3718,49 @@ Traceback (most recent call last): KeyboardInterrupt \end{verbatim} -A \emph{finally clause} is executed whether or not an exception has -occurred in the try clause. When an exception has occurred, it is -re-raised after the finally clause is executed. The finally clause is -also executed ``on the way out'' when the \keyword{try} statement is -left via a \keyword{break} or \keyword{return} statement. +A \emph{finally clause} is always executed before leaving the +\keyword{try} statement, whether an exception has occurred or not. +When an exception has occurred in the \keyword{try} clause and has not +been handled by an \keyword{except} clause (or it has occurred in a +\keyword{except} or \keyword{else} clause), it is re-raised after the +\keyword{finally} clause has been executed. The \keyword{finally} clause +is also executed ``on the way out'' when any other clause of the +\keyword{try} statement is left via a \keyword{break}, \keyword{continue} +or \keyword{return} statement. A more complicated example: + +\begin{verbatim} +>>> def divide(x, y): +... try: +... result = x / y +... except ZeroDivisionError: +... print "division by zero!" +... else: +... print "result is", result +... finally: +... print "executing finally clause" +... +>>> divide(2, 1) +result is 2 +executing finally clause +>>> divide(2, 0) +division by zero! +executing finally clause +>>> divide("2", "1") +executing finally clause +Traceback (most recent call last): + File "<stdin>", line 1, in ? + File "<stdin>", line 3, in divide +TypeError: unsupported operand type(s) for /: 'str' and 'str' +\end{verbatim} -The code in the finally clause is useful for releasing external -resources (such as files or network connections), regardless of -whether the use of the resource was successful. +As you can see, the \keyword{finally} clause is executed in any +event. The \exception{TypeError} raised by dividing two strings +is not handled by the \keyword{except} clause and therefore +re-raised after the \keyword{finally} clauses has been executed. -A \keyword{try} statement must either have one or more except clauses -or one finally clause, but not both (because it would be unclear which -clause should be executed first). +In real world applications, the \keyword{finally} clause is useful +for releasing external resources (such as files or network connections), +regardless of whether the use of the resource was successful. \chapter{Classes \label{classes}} @@ -5340,7 +5378,7 @@ users. \item \citetitle[../ref/ref.html]{Language Reference}: A detailed explanation of Python's syntax and semantics. It's heavy reading, -but is useful as a +but is useful as a complete guide to the language itself. \end{itemize} |