In the natural evolution of programming languages it is sometimes
necessary to make changes that modify the behavior of older
programs. This PEP proposes a policy for implementing these
changes in a manner respectful of the installed base of Python
Implementation of this PEP requires the addition of a formal
warning and deprecation facility that will be described in another
These guidelines apply to future versions of Python that introduce
backward-incompatible behavior. Backward incompatible behavior is
a major deviation in Python interpretation from an earlier
behavior described in the standard Python documentation. Removal
of a feature also constitutes a change of behavior.
This PEP does not replace or preclude other compatibility
strategies such as dynamic loading of backwards-compatible
parsers. On the other hand, if execution of "old code" requires a
special switch or pragma then that is indeed a change of behavior
from the point of view of the user and that change should be
implemented according to these guidelines.
In general, common sense must prevail in the implementation of
these guidelines. For instance changing "sys.copyright" does not
constitute a backwards-incompatible change of behavior!
Steps For Introducing Backwards-Incompatible Features
1. Propose backwards-incompatible behavior in a PEP. The PEP must
include a section on backwards compatibility that describes in
detail a plan to complete the remainder of these steps.
2. Once the PEP is accepted as a productive direction, implement
an alternate way to accomplish the task previously provided by
the feature that is being removed or changed. For instance if
the addition operator were scheduled for removal, a new version
of Python could implement an "add()" built-in function.
3. Formally deprecate the obsolete construct in the Python
4. Add an an optional warning mode to the parser that will inform
users when the deprecated construct is used. In other words,
all programs that will behave differently in the future must
trigger warnings in this mode. Compile-time warnings are
preferable to runtime warnings. The warning messages should
steer people from the deprecated construct to the alternative
5. There must be at least a one-year transition period between the
release of the transitional version of Python and the release
of the backwards incompatible version. Users will have at
least a year to test their programs and migrate them from use
of the deprecated construct to the alternative one.