[Python-ideas] Function arguments in tracebacks

[Ammar Askar]

Consider the following similar C and Python code and their
tracebacks:

C
-------
  int divide(int x, int y, char* some_string) {
      return x / y;
  }
  int main(...) {
      divide(2, 0, "Hello World");
  }
-------
  Program received signal SIGFPE, Arithmetic exception.
  (gdb) bt
  #0  0x00000000004004c4 in divide (x=2, y=0, some_string=0x4005a8
"Hello World") at test.c:2
  #1  0x00000000004004e7 in main (argc=1, argv=0x7fffffffe328) at test.c:6

Python
-------
  def divide(x, y, some_string):
    return x / y

  divide(2, 0, "Hello World")
-------
  Traceback (most recent call last):
    File "test.py", line 4, in <module>
    File "test.py", line 2, in divide
  ZeroDivisionError: division by zero


By including the function arguments within the traceback, we
can get more information at a glance than we could with just
the names of methods.

This would be pretty cool and stop the occasional "printf"
debugging without cluttering up the traceback too much.

There will definitely need to be some reasonable line length
limit  because the repr() of parameters could be really long.
In similar situations gdb replaces the value in the traceback
with elipsis, and I believe that's a good solution for python
as well.

Obviously this isn't a great example since the error is immediately
obvious but I think this could be potentially useful in a bunch
of situations.

I've made a a quick toy implementation in traceback.c, this is what
it looks like for the script above.

  Traceback (most recent call last):
    File "test.py", line 4, in <module>
      divide(2, 0, "Hello World")
    File "test.py", line 2, in divide (x=2, y=0, some_string='Hello World')
      return x / y
  ZeroDivisionError: division by zero


== Potential Downsides ==

There's probably a lot more than these, but I could only think of
these so far.

* Private data might be leaked, imagine a

    def login(username, password):
        ...

  method. While function names/source files/source code are also
  private, variables can potentially contain all kinds of sensitive data.

* A variable that takes a long time to return a string representation may
  significantly slow down the time it takes to generate a traceback.

* We can really only return the state of the variables when the
  traceback is printed, this might result in some slightly un-intuitive
  behavior. (Easier to explain with an example)

    def f(x):
       x = 2
       raise Exception()

    f(1)

  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "<stdin>", line 3, in f(x=2)

  The fact that x is mutated within the function body means that the
value printed in
  the traceback is the changed value which might be slightly misleading.


I'd love to hear your guy's thoughts on the idea.