python-peps/pep-0553.rst

PEP: 553
Title: Built-in breakpoint()
Author: Barry Warsaw <barry@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 2017-09-05
Python-Version: 3.7
Post-History: 2017-09-05, 2017-09-07, 2017-09-13


Abstract
========

This PEP proposes adding a new built-in function called ``breakpoint()`` which
enters a Python debugger at the point of the call.  Additionally, two new
names are added to the ``sys`` module to make the choice of which debugger is
entered configurable.


Rationale
=========

Python has long had a great debugger in its standard library called ``pdb``.
Setting a break point is commonly written like this::

    foo()
    import pdb; pdb.set_trace()
    bar()

Thus after executing ``foo()`` and before executing ``bar()``, Python will
enter the debugger.  However this idiom has several disadvantages.

* It's a lot to type (27 characters).

* It's easy to typo.  The PEP author often mistypes this line, e.g. omitting
  the semicolon, or typing a dot instead of an underscore.

* It ties debugging directly to the choice of pdb.  There might be other
  debugging options, say if you're using an IDE or some other development
  environment.

* Python linters (e.g. flake8 [linters]_) complain about this line because it
  contains two statements.  Breaking the idiom up into two lines complicates
  its use because there are more opportunities for mistakes at clean up time.
  I.e. you might forget to delete one of those lines when you no longer need
  to debug the code.

Python developers also have many other debuggers to choose from, but
remembering how to invoke them can be problematic.  For example, even when
IDEs have user interface for setting breakpoints, it may still be more
convenient to just edit the code.  The APIs for entering the debugger
programmatically are inconsistent, so it can be difficult to remember exactly
what to type.

We can solve all these problems by providing a universal API for entering the
debugger, as proposed in this PEP.


Proposal
========

The JavaScript language provides a ``debugger`` statement [java]_ which enters
the debugger at the point where the statement appears.

This PEP proposes a new built-in function called ``breakpoint()``
which enters a Python debugger at the call site.  Thus the example
above would be written like so::

    foo()
    breakpoint()
    bar()

Further, this PEP proposes two new name bindings for the ``sys``
module, called ``sys.breakpointhook()`` and
``sys.__breakpointhook__``.  By default, ``sys.breakpointhook()``
implements the actual importing and entry into ``pdb.set_trace()``,
and it can be set to a different function to change the debugger that
``breakpoint()`` enters.

``sys.__breakpointhook__`` is initialized to the same function as
``sys.breakpointhook()`` so that you can always easily reset
``sys.breakpointhook()`` to the default value (e.g. by doing
``sys.breakpointhook = sys.__breakpointhook__``).  This is exactly the same as
how the the existing ``sys.displayhook()`` / ``sys.__displayhook__`` and
``sys.excepthook()`` / ``sys.__excepthook__`` work [hooks]_.

The signature of the built-in is ``breakpoint(*args, **kws)``.  The positional
and keyword arguments are passed straight through to ``sys.breakpointhook()``
and the signatures must match or a ``TypeError`` will be raised.  The return
from ``sys.breakpointhook()`` is passed back up to, and returned from
``breakpoint()``.

The rationale for this is based on the observation that the underlying
debuggers may accept additional optional arguments.  For example, IPython
allows you to specify a string that gets printed when the break point is
entered [ipython-embed]_.  As of Python 3.7, the pdb module also supports an
optional ``header`` argument [pdb-header]_.


Environment variable
====================

The default implementation of ``sys.breakpointhook()`` consults a new
environment variable called ``PYTHONBREAKPOINT``.  This environment variable
can have various values:

* ``PYTHONBREAKPOINT=0`` disables debugging.  Specifically, with this value
  ``sys.breakpointhook()`` returns ``None`` immediately.

* ``PYTHONBREAKPOINT=`` (i.e. the empty string).  This is the same as not
  setting the environment variable at all, in which case ``pdb.set_trace()``
  is run as usual.

* ``PYTHONBREAKPOINT=some.importable.callable``.  In this case,
  ``sys.breakpointhook()`` imports the ``some.importable`` module and gets the
  ``callable`` object from the resulting module, which it then calls.  The
  value may be a string with no dots, in which case it names a built-in
  callable, e.g. ``PYTHONBREAKPOINT=int``.  (Guido has expressed the
  preference for normal Python dotted-paths, not setuptools-style entry point
  syntax [syntax]_.)

This environment variable allows external processes to control how breakpoints
are handled.  Some uses cases include:

* Completely disabling all accidental ``breakpoint()`` calls pushed to
  production.  This could be accomplished by setting ``PYTHONBREAKPOINT=0`` in
  the execution environment.  Another suggestion by reviewers of the PEP was
  to set ``PYTHONBREAKPOINT=sys.exit`` in this case.

* IDE integration with specialized debuggers for embedded execution.  The IDE
  would run the program in its debugging environment with ``PYTHONBREAKPOINT``
  set to their internal debugging hook.

``PYTHONBREAKPOINT`` is re-interpreted every time ``sys.breakpointhook()`` is
reached.  This allows processes to change its value during the execution of a
program and have ``breakpoint()`` respond to those changes.  It is not
considered a performance critical section since entering a debugger by
definition stops execution.  Thus, programs can do the following::

    os.environ['PYTHONBREAKPOINT'] = 'foo.bar.baz'
    breakpoint()    # Imports foo.bar and calls foo.bar.baz()

Overriding ``sys.breakpointhook`` defeats the default consultation of
``PYTHONBREAKPOINT``.  It is up to the overriding code to consult
``PYTHONBREAKPOINT`` if they want.

If access to the ``PYTHONBREAKPOINT`` callable fails in any way (e.g. the
import fails, or the resulting module does not contain the callable), a
``RuntimeWarning`` is issued, and no breakpoint function is called.




Implementation
==============

A pull request exists with the proposed implementation [impl]_.

While the actual implementation is in C, the Python pseudo-code for this
feature looks roughly like the following::

    # In builtins.
    def breakpoint(*args, **kws):
        import sys
        missing = object()
        hook = getattr(sys, 'breakpointhook', missing)
        if hook is missing:
            raise RuntimeError('lost sys.breakpointhook')
        return hook(*args, **kws)

    # In sys.
    def breakpointhook(*args, **kws):
        import importlib, os, warnings
        hookname = os.getenv('PYTHONBREAKPOINT')
        if hookname is None or len(hookname) == 0:
            hookname = 'pdb.set_trace'
        elif hookname == '0':
            return None
        modname, dot, funcname = hookname.rpartition('.')
        if dot == '':
            modname = 'builtins'
        try:
            module = importlib.import_module(modname)
            hook = getattr(module, funcname)
        except:
            warnings.warn(
                'Ignoring unimportable $PYTHONBREAKPOINT: {}'.format(
                    hookname),
                RuntimeWarning)
        return hook(*args, **kws)

    __breakpointhook__ = breakpointhook


Rejected alternatives
=====================

A new keyword
-------------

Originally, the author considered a new keyword, or an extension to an
existing keyword such as ``break here``.  This is rejected on several fronts.

* A brand new keyword would require a ``__future__`` to enable it since almost
  any new keyword could conflict with existing code.  This negates the ease
  with which you can enter the debugger.

* An extended keyword such as ``break here``, while more readable and not
  requiring a ``__future__`` would tie the keyword extension to this new
  feature, preventing more useful extensions such as those proposed in
  PEP 548.

* A new keyword would require a modified grammar and likely a new bytecode.
  Each of these makes the implementation more complex.  A new built-in breaks
  no existing code (since any existing module global would just shadow the
  built-in) and is quite easy to implement.


sys.breakpoint()
----------------

Why not ``sys.breakpoint()``?  Requiring an import to invoke the debugger is
explicitly rejected because ``sys`` is not imported in every module.  That
just requires more typing and would lead to::

    import sys; sys.breakpoint()

which inherits several of the problems this PEP aims to solve.


Version History
===============

* 2017-09-13

  * The ``PYTHONBREAKPOINT`` environment variable is made a first class
    feature.

* 2017-09-07

  * ``debug()`` renamed to ``breakpoint()``
  * Signature changed to ``breakpoint(*args, **kws)`` which is passed straight
    through to ``sys.breakpointhook()``.


References
==========

.. [ipython-embed]
   http://ipython.readthedocs.io/en/stable/api/generated/IPython.terminal.embed.html

.. [pdb-header]
   https://docs.python.org/3.7/library/pdb.html#pdb.set_trace

.. [linters]
   http://flake8.readthedocs.io/en/latest/

.. [java]
   https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/debugger

.. [hooks]
   https://docs.python.org/3/library/sys.html#sys.displayhook

.. [syntax]
    http://setuptools.readthedocs.io/en/latest/setuptools.html?highlight=console#automatic-script-creation

.. [impl]
   https://github.com/python/cpython/pull/3355

.. [envar]
   https://mail.python.org/pipermail/python-dev/2017-September/149447.html


Copyright
=========

This document has been placed in the public domain.



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