diff options
| -rw-r--r-- | Doc/whatsnew/whatsnew21.tex | 39 |
1 files changed, 37 insertions, 2 deletions
diff --git a/Doc/whatsnew/whatsnew21.tex b/Doc/whatsnew/whatsnew21.tex index ef2315c..4215358 100644 --- a/Doc/whatsnew/whatsnew21.tex +++ b/Doc/whatsnew/whatsnew21.tex @@ -90,7 +90,7 @@ This change may cause some compatibility problems for code where the same variable name is used both at the module level and as a local variable within a function that contains further function definitions. This seems rather unlikely though, since such code would have been -pretty confusing to read in the first place. +pretty confusing to read in the first place. One side effect of the change is that the \code{from \var{module} import *} and \keyword{exec} statements have been made illegal inside @@ -126,6 +126,14 @@ This shouldn't be much of a limitation, since \keyword{exec} is rarely used in most Python code (and when it is used, it's often a sign of a poor design anyway). +Compatibility concerns have led to nested scopes being introduced +gradually; in Python 2.1, they aren't enabled by default, but can be +turned on within a module by using a future statement as described in +PEP 236. (See the following section for further discussion of PEP +236.) In Python 2.2, nested scopes will become the default and there +will be no way to turn them off, but users will have had all of 2.1's +lifetime to fix any breakage resulting from their introduction. + \begin{seealso} \seepep{227}{Statically Nested Scopes}{Written and implemented by @@ -137,7 +145,34 @@ Jeremy Hylton.} %====================================================================== \section{PEP 236: \module{__future__} Directives} -XXX +The reaction to nested scopes was widespread concern about the dangers +of breaking code with the 2.1 release, and it was strong enough to +make the Pythoneers take a more conservative approach. This approach +consists of introducing a convention for enabling optional +functionality in release N that will become compulsory in release N+1. + +The syntax uses a \code{from...import} statement using the reserved +module name \module{__future__}. Nested scopes can be enabled by the +following statement: + +\begin{verbatim} +from __future__ import nested_scopes +\end{verbatim} + +While it looks like a normal \keyword{import} statement, it's not; +there are strict rules on where such a future statement can be put. +They can only be at the top of a module, and must precede any Python +code or regular \keyword{import} statements. This is because such +statements can affect how the Python bytecode compiler parses code and +generates bytecode, so they must precede any statement that will +result in bytecodes being produced. + +\begin{seealso} + +\seepep{236}{Back to the \module{__future__}}{Written by Tim Peters, +and primarily implemented by Jeremy Hylton.} + +\end{seealso} %====================================================================== \section{PEP 207: Rich Comparisons} |