stackprinter 0.1.5

Debug-friendly call stacks & tracebacks with variable values and semantic highlighting

More talkative tracebacks

If my code runs somewhere where the only debugging tool is a printout or a log file, I tend to sleep better if I at least have good crash logging.

This prints tracebacks / call stacks with more code context and the values of nearby variables. It answers 80% of the questions I'd ask an interactive debugger: Where in the code were we, what's in the relevant local variables, and why was that function called with those arguments.

It can add colorful highlighting if you want, and some other things.

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'

For each frame on the stack, it shows a couple lines of code and the variables in those lines, as well as the call arguments of the current function scope. You can configure exactly how verbose things should be.

Installation

pip install stackprinter

Exception logging

To globally replace the default python crash message, call set_excepthook() somewhere. This will print to stderr by default.

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

To log the current exception, call show() or format() inside an except block. show() prints to stderr by default. format() just returns a string, for custom logging. You can also explicitely pass previously caught exception objects into these methods.

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

    # ...or only get a string, e.g. for logging:
    message = stackprinter.format()
    logging.log(message)

# or explicitely grab a particular exception
try:
    something()
except ValueError as e:
    stackprinter.show(e)

# or collect exceptions in a little jar somewhere, to log them later
try:
    something()
except ValueError as e:
    errors.append(e)

# later:
for err in errors:
    message = stackprinter.format(err)
    logging.log(message)

By default, all these calls will generate plain text. Pass style='color' to get funky terminal colors like this. (It's a kind of semantic highlighting, i.e. it highlights the different variables, not the language syntax.).

You can blacklist certain file paths, to make the stack less verbose whenever it runs through those files. For example, if you call format(exc, suppressed_paths=[r"lib/python.*/site-packages"]), calls within installed libraries are shrunk to one line each.

For more options & details, see the docs of format().

Printing the current stack of another thread

Pass a thread object to show or format:

thread = threading.Thread(target=something)
thread.start()
while True:
    stackprinter.show(thread) # or format(thread)
    time.sleep(0.1)

Printing the stack of the current thread

Call show or format outside of exception handling.

stackprinter.show() # or format()

There's also show_current_stack(), which does the same thing everywhere, even inside except blocks.

Tracing a piece of code as it is executed

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='color'):
    a = np.ones(111)
    dosomething(a)

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 to not 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.

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()