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GDB-like Python Debugger in the Trepan family

Project description

|buildstatus| |license| |Supported Python Versions|

.. contents:: :local:


This is a gdb-like debugger for Python. It is a rewrite of *pdb* from
the ground up.

A command-line interface (CLI) is provided as well as an remote access
interface over TCP/IP.

See the Tutorial_ for how to use. See ipython-trepan_ for using this
in *ipython* or an *ipython notebook*.

This package is for Python 2.6 and 2.7. See trepan3k_ for the same code modified to work with Python 3.
For Python before 2.4, use pydb_ .


Since this debugger is similar to other_ trepanning_ debuggers_ and *gdb*
in general, knowledge gained by learning this is transferable to those
debuggers and vice versa.

There's a lot of cool stuff here that's not in the stock
Python debugger *pdb*.

Exact location information

Python reports line information on the granularity of a line. To get
more precise information, we can (de)parse into Python the byte code
around a bytecode offset such as the place you are stopped at.

So far as I know, there is no other debugger that can do this.

Debugging Python bytecode (no source available)

You can pass the debugger the name of Python bytecode and many times,
the debugger will merrily proceed. This debugger tries very hard find
the source code. Either by using the current executable search path
(e.g. `PATH`) or for some by looking inside the bytecode for a
filename in the main code object (`co_filename`) and applying that
with a search path which takes into account directory where the
bytecode lives.

Failing to find source code this way, and in other situations where
source code can't be found, the debugger will decompile the bytecode
and use that for showing source test. *This allows us to debug `eval`'d
or `exec''d code.*

But if you happen to know where the source code is located, you can
associate a file source code with the current name listed in the
bytecode. See the set_substitute_ command for details here.

Source-code Syntax Colorization

Starting with release 0.2.0, terminal source code is colorized via
pygments_ . And with that you can set the pygments color style,
e.g. "colorful", "paraiso-dark". See set_style_ . Furthermore, we make use
of terminal bold and emphasized text in debugger output and help
text. Of course, you can also turn this off. Starting with release
0.6.0, you can use your own pygments_style_, provided you have a
terminal that supports 256 colors. If your terminal supports the basic
ANSI color sequences only, we support that too in both dark and light

Command Completion

Starting with release 2.8, readline command completion has been
added. Command completion is not just a simple static list, but varies
depending on the context. For example, for frame-changing commands
which take optional numbers, on the list of *valid numbers* is

Terminal Handling

We can adjust debugger output depending on the line width of your
terminal. If it changes, or you want to adjust it, see set_width_ .

Smart Eval

Starting with release 0.2.0, if you want to evaluate the current
source line before it is run in the code, use ``eval`` or
``deval``. To evaluate text of a common fragment of line, such as the
expression part of an *if* statement, you can do that with
``eval?`` or ``deval?``. See eval_ for more information.

More Stepping Control

Sometimes you want small steps, and sometimes large stepping.

This fundamental issue is handled in a couple ways:

Step Granularity

There are now ``step`` *event* and ``next`` *event* commands with
aliases to ``s+``, ``s>`` and so on. The plus-suffixed commands force
a different line on a subsequent stop, the dash-suffixed commands
don't. Suffixes ``>``, ``<``, and ``!`` specify ``call``, ``return``
and ``exception`` events respectively. And without a suffix you get
the default; this is set by the `set different` command.



Event Filtering and Tracing

By default the debugger stops at every event: ``call``, ``return``,
``line``, ``exception``, ``c-call``, ``c-exception``. If you just want
to stop at ``line`` events (which is largely what you happens in
*pdb*) you can. If however you just want to stop at calls and returns,
that's possible too. Or pick some combination.

In conjunction with handling *all* events by default, the event status is shown when stopped. The reason for stopping is also available via `info program`.

Event Tracing of Calls and Returns

I'm not sure why this was not done before. Probably because of the
lack of the ability to set and move by different granularities,
tracing calls and returns lead to too many uninteresting stops (such
as at the same place you just were at). Also, stopping on function
definitions probably also added to this tedium.

Because we're really handling return events, we can show you the return value. (*pdb* has an "undocumented" *retval* command that doesn't seem to work.)

Debugger Macros via Python Lambda expressions

Starting with release 0.2.3, there are debugger macros. In *gdb*,
there is a *macro* debugger command to extend debugger commands.

However Python has its own rich programming language so it seems silly
to recreate the macro language that is in *gdb*. Simpler and more
powerful is just to use Python here. A debugger macro here is just a
lambda expression which returns a string or a list of strings. Each
string returned should be a debugger command.

We also have *aliases* for the extremely simple situation where you
want to give an alias to an existing debugger command. But beware:
some commands, like step_ inspect command suffixes and change their
behavior accordingly.

We also envision a number of other ways to allow extension of this
debugger either through additional modules, or user-supplied debugger
command directories.

If what you were looking for in macros was more front-end control over
the debugger, then consider using the experimental (and not finished)
Bullwinkle protocol.

Byte-code Instruction Introspection

We do more in the way of looking at the byte codes to give better information. Through this we can provide:

* a *skip* command. It is like the *jump* command, but you don't have to deal with line numbers.
* disassembly of code fragments. You can now disassemble relative to the stack frames you are currently stopped at.
* Better interpretation of where you are when inside *execfile* or *exec*. (But really though this is probably a Python compiler misfeature.)
* Check that breakpoints are set only where they make sense.
* A more accurate determination of if you are at a function-defining *def* statement (because the caller instruction contains ``MAKE_FUNCTION``.)

Even without "deparsing" mentioned above, the abilty to disassemble by line number range or byte-offset range lets you tell exactly where you are and code is getting run.

Some Debugger Command Arguments can be Variables and Expressions

Commands that take integer arguments like frame-moving commands like
*up*, allow you to use a Python expression which may include local or
global variables that evaluates to an integer. This eliminates the
need in *gdb* for special "dollar" debugger variables. (Note however
because of *shlex* parsing, expressions can't have embedded blanks.)

Out-of-Process Debugging

You can now debug your program in a different process or even a different computer on a different network!

Egg, Wheel, and Tarballs

Can be installed via the usual *pip* or *easy_install*. There is a
source tarball. `How To Install
<>`_ has
full instructions and installing from git and by other means.


The Debugger plays nice with other trace hooks. You can have several debugger objects.

Many of the things listed below doesn't directly effect end-users, but
it does eventually by way of more robust and featureful code. And
keeping developers happy is a good thing.(TM)

* Commands and subcommands are individual classes now, not methods in a class. This means they now have properties like the context in which they can be run, minimum abbreviation name or alias names. To add a new command you basically add a file in a directory.
* I/O is it's own layer. This simplifies interactive readline behavior from reading commands over a TCP socket.
* An interface is it's own layer. Local debugging, remote debugging, running debugger commands from a file (`source`) are different interfaces. This means, for example, that we are able to give better error reporting if a debugger command file has an error.
* There is an experimental Python-friendly interface for front-ends
* more testable. Much more unit and functional tests. More of *pydb*'s integration test will eventually be added.

See Also

* trepan3k_ : trepan debugger for Python 3
* pydbgr_ : previous incarnation of debugger
* pydb_ : even older incarnation of debugger (for very old Python 2)
* Tutorial_: Tutorial for how to use
* : Python decompiler
* : cross-platform disassembler

.. _pygments:
.. _pygments_style:
.. _pydb:
.. _trepan3k:
.. _pydbgr:
.. _other:
.. _trepanning:
.. _debuggers:
.. _this:
.. _Tutorial:
.. |downloads| image::
:alt: Daily PyPI downloads
.. |buildstatus| image::
:alt: CircleCI Build status
.. _ipython-trepan:
.. |license| image::
:alt: License
.. _set_substitute:
.. _set_style:
.. _set_width:
.. _eval:
.. _step:
.. _install:
.. |Supported Python Versions| image::

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