|Python Language Reference Manual|
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
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6.12.1 Future statements
A future statement is a directive to the compiler that a particular module should be compiled using syntax or semantics that will be available in a specified future release of Python. The future statement is intended to ease migration to future versions of Python that introduce incompatible changes to the language. It allows use of the new features on a per-module basis before the release in which the feature becomes standard.
future_statement "from" "__future__"
- "import" feature ["as" name]( "," feature ["as" name] )* | "from" "__future__" "import""(" feature ["as" name]( "," feature ["as" name] )*[","] ")"
A future statement must appear near the top of the module. The only lines that can appear before a future statement are:
- the module docstring (if any),
- blank lines, and
- other future statements.
The features recognized by Python 2.3 are ‘generators’, ‘division’ and ‘nested_scopes’. ‘generators’ and ‘nested_scopes’ are redundant in 2.3 because they are always enabled.
A future statement is recognized and treated specially at compile time: Changes to the semantics of core constructs are often implemented by generating different code. It may even be the case that a new feature introduces new incompatible syntax (such as a new reserved word), in which case the compiler may need to parse the module differently. Such decisions cannot be pushed off until runtime.
For any given release, the compiler knows which feature names have been defined, and raises a compile-time error if a future statement contains a feature not known to it.
The direct runtime semantics are the same as for any import statement: there is a standard module ‘__future__’, described later, and it will be imported in the usual way at the time the future statement is executed.
The interesting runtime semantics depend on the specific feature enabled by the future statement.
Note that there is nothing special about the statement:
import __future__ [as name]
That is not a future statement; it's an ordinary import statement with no special semantics or syntax restrictions.
Code compiled by an
exec statement or calls to the builtin functions
execfile() that occur in a module
‘M’ containing a future statement will, by default, use the new
syntax or semantics associated with the future statement. This can,
starting with Python 2.2 be controlled by optional arguments to
compile()---see the documentation of that function in the
Python Library Reference Manual for
A future statement typed at an interactive interpreter prompt will
take effect for the rest of the interpreter session. If an
interpreter is started with the
-i option, is passed a
script name to execute, and the script includes a future statement, it
will be in effect in the interactive session started after the script
|ISBN 0954161785||Python Language Reference Manual||See the print edition|