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Debugging-friendly tracebacks for Python
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Better crash logs

This prints tracebacks / call stacks with code context and the values of nearby variables. It answers most of the questions I'd ask an interactive debugger: Where in the code did it happen, what's in the relevant local variables, and why was that function called with those arguments.

Basically, it's a more helpful version of Python's built-in crash message. It will either print to the console or give you a string for logging.

pip install stackprinter

Before

Traceback (most recent call last):
  File "demo.py", line 10, in <module>
    dangerous_function(somelist + anotherlist)
  File "demo.py", line 4, in dangerous_function
    return sorted(blub, key=lambda xs: sum(xs))
  File "demo.py", line 4, in <lambda>
    return sorted(blub, key=lambda xs: sum(xs))
TypeError: unsupported operand type(s) for +: 'int' and 'str'

After

File demo.py, line 10, in <module>
    8         somelist = [[1,2], [3,4]]
    9         anotherlist = [['5', 6]]
--> 10        dangerous_function(somelist + anotherlist)
    11    except:
    ..................................................
     somelist = [[1, 2], [3, 4]]
     anotherlist = [['5', 6]]
    ..................................................

File demo.py, line 4, in dangerous_function
    3     def dangerous_function(blub):
--> 4         return sorted(blub, key=lambda xs: sum(xs))
    ..................................................
     blub = [[1, 2], [3, 4], ['5', 6]]
    ..................................................

File demo.py, line 4, in <lambda>
    2
    3     def dangerous_function(blub):
--> 4         return sorted(blub, key=lambda xs: sum(xs))
    5
    ..................................................
     xs = ['5', 6]
    ..................................................

TypeError: unsupported operand type(s) for +: 'int' and 'str'

By default, it tries to be somewhat polite about screen space (showing only a few source lines & the function header, and only the variables in those lines, and only (?) 500 characters per variable). You can configure exactly how verbose things should be.

It outputs plain text by default, which is good for logging to text files. There's also a color mode for some reason 🌈, which you can activate by a style keyword in any of the functions below. (The colors track different variables instead of the language syntax.)

I sometimes use this locally instead of a debugger, but mostly it helps me sleep when my code runs somewhere where the only debug tool is a log file (though it's not a fully-grown error monitoring system).

Usage

Exception logging

To replace the default python crash printout, call set_excepthook() somewhere. This will print detailed stacktraces for any uncaught exception (to stderr, by default). You could also make this permanent for your python installation.

import stackprinter
stackprinter.set_excepthook(style='darkbg2')

For more control, call show() or format() inside an except block. show() prints to stderr by default, format() returns a string, for custom logging.

try:
    something()
except:
    # print the current exception to stderr:
    stackprinter.show()

    # ...or instead, get a string for logging:
    logger.error(stackprinter.format())

Or pass specific exceptions explicitly:

try:
    something()
except RuntimeError as exc:
    tb = stackprinter.format(exc)
    logger.error('The front fell off.\n' + tb)

For all the config options, for now, see the docstring of format().

It's also possible to integrate this neatly with standard logging calls through a bit of extra plumbing.

configure_logging() # adds a custom log formatter, see link above
# (...)
try:
    something()
except RuntimeError as e:
    logger.exception('The front fell off.')

Printing the current call stack

To see your own thread's current call stack, call show or format anywhere outside of exception handling.

stackprinter.show() # or format()

Printing the stack of another thread

To inspect the call stack of any other running thread:

thread = threading.Thread(target=something)
thread.start()
# (...)
stackprinter.show(thread) # or format(thread)

Tracing a piece of code

More for curiosity than anything else, you can watch a piece of code execute step-by-step, printing a trace of all calls & returns 'live' as they are happening. Slows everything down though, of course.

with stackprinter.TracePrinter(style='darkbg2'):
    dosomething()

or

tp = stackprinter.TracePrinter(style='darkbg2')
tp.enable()
dosomething()
# (...) +1 million lines
tp.disable()

Making it stick

To permanently replace the crash message for your python installation, you could put a file sitecustomize.py into the site-packages directory under one of the paths revealed by python -c "import site; print(site.PREFIXES)", with contents like this:

    # in e.g. some_virtualenv/lib/python3.x/site-packages/sitecustomize.py:
    import stackprinter
    stackprinter.set_excepthook(style='darkbg2')

That would give you colorful tracebacks automatically every time, even in the REPL.

(You could do a similar thing for IPython, but they have their own method, where the file goes into ~/.ipython/profile_default/startup instead, and also I don't want to talk about what this module does to set an excepthook under IPython.)

How it works

Basically, this is a frame formatter. For each frame on the call stack, it grabs the source code to find out which source lines reference which variables. Then it displays code and variables in the neighbourhood of the last executed line.

Since this already requires a map of where each variable occurs in the code, it was difficult not to also implement the whole semantic highlighting color thing seen in the screenshots. The colors are ANSI escape codes now, but it should be fairly straightforward™ to render the underlying data without any 1980ies terminal technology. Say, a foldable and clickable HTML page with downloadable pickled variables. For now you'll have to pipe the ANSI strings through ansi2html or something.

The format and everything is inspired by the excellent ultratb in IPython. One day I'd like to contribute the whole "find out which variables in locals and globals are nearby in the source and print only those" machine over there, after trimming its complexity a bit.

Caveats

This displays variable values as they are at the time of formatting. In multi-threaded programs, variables can change while we're busy walking the stack & printing them. So, if nothing seems to make sense, consider that your exception and the traceback messages are from slightly different times. Sadly, there is no responsible way to freeze all other threads as soon as we want to inspect some thread's call stack (...or is there?)

Docs

*coughs*

For now, just look at all the doc strings, e.g. those of format()

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