Motivation

From time to time, Python makes an incompatible change to the advertised semantics of core language constructs, or changes their accidental (implementation-dependent) behavior in some way. While this is never done capriciously, and is always done with the aim of improving the language over the long term, over the short term it’s contentious and disrupting.

PEP 5, Guidelines for Language Evolution suggests ways to ease the pain, and this PEP introduces some machinery in support of that.

PEP 227, Statically Nested Scopes is the first application, and will be used as an example here.

Intent

[Note: This is policy, and so should eventually move into PEP 5]

When an incompatible change to core language syntax or semantics is being made:

1. The release C that introduces the change does not change the syntax or semantics by default.
2. A future release R is identified in which the new syntax or semantics will be enforced.
3. The mechanisms described in PEP 230, Warning Framework are used to generate warnings, whenever possible, about constructs or operations whose meaning may [1] change in release R.
4. The new future_statement (see below) can be explicitly included in a module M to request that the code in module M use the new syntax or semantics in the current release C.

So old code continues to work by default, for at least one release, although it may start to generate new warning messages. Migration to the new syntax or semantics can proceed during that time, using the future_statement to make modules containing it act as if the new syntax or semantics were already being enforced.

Note that there is no need to involve the future_statement machinery in new features unless they can break existing code; fully backward- compatible additions can– and should –be introduced without a corresponding future_statement.

Syntax

A future_statement is simply a from/import statement using the reserved module name __future__:

future_statement: "from" "__future__" "import" feature ["as" name]
                  (","feature ["as" name])*

feature: identifier
name: identifier

In addition, all future_statements must appear near the top of the module. The only lines that can appear before a future_statement are:

• The module docstring (if any).
• Comments.
• Blank lines.
• Other future_statements.

Example:

"""This is a module docstring."""

# This is a comment, preceded by a blank line and followed by
# a future_statement.
from __future__ import nested_scopes

from math import sin
from __future__ import alabaster_weenoblobs  # compile-time error!
# That was an error because preceded by a non-future_statement.

Semantics

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 [2].

The direct runtime semantics are the same as for any import statement: there is a standard module __future__.py, 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(s) “imported” by the future_statement(s) appearing in the module.

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.

Example

Consider this code, in file scope.py:

x = 42
def f():
    x = 666
    def g():
        print "x is", x
    g()
f()

Under 2.0, it prints:

x is 42

Nested scopes (PEP 227) are being introduced in 2.1. But under 2.1, it still prints:

x is 42

and also generates a warning.

In 2.2, and also in 2.1 if from __future__ import nested_scopes is included at the top of scope.py, it prints:

x is 666

Standard Module __future__.py

Lib/__future__.py is a real module, and serves three purposes:

1. To avoid confusing existing tools that analyze import statements and expect to find the modules they’re importing.
2. To ensure that future_statements run under releases prior to 2.1 at least yield runtime exceptions (the import of __future__ will fail, because there was no module of that name prior to 2.1).
3. To document when incompatible changes were introduced, and when they will be– or were –made mandatory. This is a form of executable documentation, and can be inspected programmatically via importing __future__ and examining its contents.

Each statement in __future__.py is of the form:

FeatureName = "_Feature(" OptionalRelease "," MandatoryRelease ")"

where, normally, OptionalRelease < MandatoryRelease, and both are 5-tuples of the same form as sys.version_info:

(PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
 PY_MINOR_VERSION, # the 1; an int
 PY_MICRO_VERSION, # the 0; an int
 PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
 PY_RELEASE_SERIAL # the 3; an int )

OptionalRelease records the first release in which:

from __future__ import FeatureName

was accepted.

In the case of MandatoryReleases that have not yet occurred, MandatoryRelease predicts the release in which the feature will become part of the language.

Else MandatoryRelease records when the feature became part of the language; in releases at or after that, modules no longer need:

from __future__ import FeatureName

to use the feature in question, but may continue to use such imports.

MandatoryRelease may also be None, meaning that a planned feature got dropped.

Instances of class _Feature have two corresponding methods, .getOptionalRelease() and .getMandatoryRelease().

No feature line will ever be deleted from __future__.py.

Example line:

nested_scopes = _Feature((2, 1, 0, "beta", 1), (2, 2, 0, "final", 0))

This means that:

from __future__ import nested_scopes

will work in all releases at or after 2.1b1, and that nested_scopes are intended to be enforced starting in release 2.2.

Resolved Problem: Runtime Compilation

Several Python features can compile code during a module’s runtime:

1. The exec statement.
2. The execfile() function.
3. The compile() function.
4. The eval() function.
5. The input() function.

Since a module M containing a future_statement naming feature F explicitly requests that the current release act like a future release with respect to F, any code compiled dynamically from text passed to one of these from within M should probably also use the new syntax or semantics associated with F. The 2.1 release does behave this way.

This isn’t always desired, though. For example, doctest.testmod(M) compiles examples taken from strings in M, and those examples should use M’s choices, not necessarily the doctest module’s choices. In the 2.1 release, this isn’t possible, and no scheme has yet been suggested for working around this. NOTE: PEP 264 later addressed this in a flexible way, by adding optional arguments to compile().

In any case, a future_statement appearing “near the top” (see Syntax above) of text compiled dynamically by an exec, execfile() or compile() applies to the code block generated, but has no further effect on the module that executes such an exec, execfile() or compile(). This can’t be used to affect eval() or input(), however, because they only allow expression input, and a future_statement is not an expression.

Resolved Problem: Native Interactive Shells

There are two ways to get an interactive shell:

1. By invoking Python from a command line without a script argument.
2. By invoking Python from a command line with the -i switch and with a script argument.

An interactive shell can be seen as an extreme case of runtime compilation (see above): in effect, each statement typed at an interactive shell prompt runs a new instance of exec, compile() or execfile(). A future_statement typed at an interactive shell applies to the rest of the shell session’s life, as if the future_statement had appeared at the top of a module.

Resolved Problem: Simulated Interactive Shells

Interactive shells “built by hand” (by tools such as IDLE and the Emacs Python-mode) should behave like native interactive shells (see above). However, the machinery used internally by native interactive shells has not been exposed, and there isn’t a clear way for tools building their own interactive shells to achieve the desired behavior.

NOTE: PEP 264 later addressed this, by adding intelligence to the standard codeop.py. Simulated shells that don’t use the standard library shell helpers can get a similar effect by exploiting the new optional arguments to compile() added by PEP 264.

Questions and Answers

Copyright

This document has been placed in the public domain.

References and Footnotes

[1]

Note that this is may and not will: better safe than sorry. Of course spurious warnings won’t be generated when avoidable with reasonable cost.

[2]

This ensures that a future_statement run under a release prior to the first one in which a given feature is known (but >= 2.1) will raise a compile-time error rather than silently do a wrong thing. If transported to a release prior to 2.1, a runtime error will be raised because of the failure to import __future__ (no such module existed in the standard distribution before the 2.1 release, and the double underscores make it a reserved name).