PEP: 553 Title: Built-in breakpoint() Author: Barry Warsaw 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 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 debugger pluggable. 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 [1]_) complain about this line because it contains two statements. Breaking the idiom up into two lines further complicates the use of the debugger, These problems can be solved by modeling a solution based on prior art in other languages, and utilizing a convention that already exists in Python. Proposal ======== The JavaScript language provides a ``debugger`` statement [2]_ 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__`` then stashes the default value of ``sys.breakpointhook()`` to make it easy to reset. This exactly models the existing ``sys.displayhook()`` / ``sys.__displayhook__`` and ``sys.excepthook()`` / ``sys.__excepthook__`` hooks [3]_. 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()``. Since ``sys.breakpointhook()`` by default calls ``pdb.set_trace()`` by default it accepts no arguments. Open issues =========== Confirmation from other debugger vendors ---------------------------------------- We want to get confirmation from at least one alternative debugger implementation (e.g. PyCharm) that the hooks provided in this PEP will be useful to them. Breakpoint bytecode ------------------- Related, there has been an idea to add a bytecode that calls ``sys.breakpointhook()``. Whether built-in ``breakpoint()`` emits this bytecode (or gets peephole optimized to the bytecode) is an open issue. The bytecode is useful for debuggers that actively modify bytecode streams to trampoline into their own debugger. Having a "breakpoint" bytecode might allow them to avoid bytecode modification in order to invoke this trampoline. *NOTE*: It probably makes sense to split this idea into a separate PEP. Environment variable -------------------- Should we add an environment variable so that ``sys.breakpointhook()`` can be set outside of the Python invocation? E.g.:: $ export PYTHONBREAKPOINTHOOK=my.debugger:Debugger This would provide execution environments such as IDEs which run Python code inside them, to set an internal breakpoint hook before any Python code executes. If we did add that hook, what should be the default value and should it be easy to disable ``breakpoint()``? The rationale here is calls to ``breakpoint()`` that might accidentally sneak into production. The author's opinion is that if we support this environment variable it should be enabled --and point to ``pdb.set_trace()``-- by default. This way it Just Works for the average developer. The maintainers of CI, build, or production systems should be able to easily configure their environments to disable ``breakpoint()``. We probably want to support a shorthand for "disable built-in breakpoint()", e.g.:: $ export PYTHONBREAKPOINTHOOK=0 Call a fancier object by default -------------------------------- Some folks want to be able to use other ``pdb`` interfaces such as ``pdb.pm()``. Although this is a less commonly used API, it could be supported by binding ``sys.breakpointhook`` to an object that implements ``__call__()``. Calling this object would call ``pdb.set_trace()``, but the object could expose other methods, such as ``pdb.pm()``, making invocation of it as handy as ``breakpoint.pm()``. Implementation ============== A pull request exists with the proposed implementation [4]_. 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 =============== * 2017-09-07 * ``debug()`` renamed to ``breakpoint()`` * Signature changed to ``breakpoint(*args, **kws)`` which is passed straight through to ``sys.breakpointhook()``. References ========== .. [1] http://flake8.readthedocs.io/en/latest/ .. [2] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/debugger .. [3] https://docs.python.org/3/library/sys.html#sys.displayhook .. [4] https://github.com/python/cpython/pull/3355 Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: