PEP: 553 Title: Built-in debug() Author: Barry Warsaw Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 2017-09-05 Python-Version: 3.7 Post-History: Abstract ======== This PEP proposes adding a new built-in function called ``debug()`` 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 ``debug()`` which enters a Python debugger at the call site. Thus the example above would be written like so:: foo() debug() bar() Built-in ``debug()`` takes no arguments. Further, this PEP proposes two new name bindings for the ``sys`` module, called ``debughook()`` and ``__debughook__``. By default, ``sys.debughook()`` implements the actual importing and entry into ``pdb.set_trace()``, and it can be set to a different function to change the debugger that ``debug()`` enters. ``sys.__debughook__`` then stashes the default value of ``sys.debughook()`` to make it easy to reset. This exactly models the existing ``sys.displayhook()`` / ``sys.__displayhook__`` and ``sys.excepthook()`` / ``sys.__excepthook__`` hooks [3]_. ``sys.displayhook()`` would be called with no arguments. It returns whatever is returned from the underlying debugger entry point. ``debug()`` returns whatever ``sys.displayhook()`` returns. Open issues =========== 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. Related, there has been an idea to add a bytecode that calls ``sys.debughook()``. Whether built-in ``debug()`` 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 "debug" bytecode might allow them to avoid bytecode modification in order to invoke this trampoline. Does it make sense to define the built-in function's signature as ``debug(*args, **kws)`` which would just be passed along to the ``sys.debughook()``? One argument for doing this is that it would allow users to pass useful arguments to their actual debugger. This isn't useful for ``pdb`` but might be useful for alternatives. 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.debug() ----------- Why not ``sys.debug()``? 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.debug() which inherits several of the problems this PEP aims to solve. 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] XXX 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: