Python multi-line expressions and stack trace

Question

We have a simple AssertTrue function used in our python project and I wanted to modify the output it provides to print the code statement from which it was called. The code looks something like this:

1 import traceback
2
3 def AssertTrue(expr, reason=None):
4     print traceback.format_stack()[-2]
5
6 AssertTrue(1 == 2,
7         reason='One is not equal to two')

The output:

File "/tmp/fisken.py", line 7, in <module>
  reason='One is not equal to two')

I'm wondering why traceback.format_stack only gives me the code on line 7. The statement starts on line 6 and the expression I would like to see in the output is also on that same line. Doesn't traceback handle multi-line function calls?

(Never mind that there are better ways to do AssertTrue(...). I'm just wondering why traceback.format_stack (and .extract_stack) does not behave as I expected it to)

score 2 · Accepted Answer · edited May 23 '17 at 12:20

Doesn't traceback handle multi-line function calls?

Many functions are tens or even (horrors) hundreds of lines long. If traceback did print the whole function, then stack traces would become incomprehensibly long. So I guess what you are seeing is an attempt to keep things clean and minimal.

I have pulled together some answers to similar questions:

With the consideration that it inspect can only obtain the source for the whole function (if the source is available on the path) I can offer you this:

import traceback
import inspect
import gc

def giveupthefunc(frame):
    code  = frame.f_code
    globs = frame.f_globals
    functype = type(lambda: 0)
    funcs = []
    for func in gc.get_referrers(code):
        if type(func) is functype:
            if getattr(func, "func_code", None) is code:
                if getattr(func, "func_globals", None) is globs:
                    funcs.append(func)
                    if len(funcs) > 1:
                        return None
    return funcs[0] if funcs else None


def AssertTrue(expr, reason=None):
    print traceback.format_stack()[-2]
    frame = inspect.currentframe().f_back
    func = giveupthefunc(frame)
    if func:
        source = inspect.getsourcelines(func)
        i = source[1]
        for line in source[0]:
            print i, ":", line,
            i += 1



def my_fun():
    AssertTrue(1 == 2,
             reason='One is not equal to two')

my_fun()

Which produces:

/Library/Frameworks/Python.framework/Versions/2.7/bin/python /Users/xxxx/Documents/PycharmProjects/scratchpad/test.py
  File "/Users/xxxx/Documents/PycharmProjects/scratchpad/test.py", line 35, in my_fun
    reason='One is not equal to two')

33 : def my_fun():
34 :     AssertTrue(1 == 2,
35 :              reason='One is not equal to two')

I guess you are right about the assumption of length. The traceback function will only print the one line to limit the output. The code you provided looks like a beautiful hack. Thanks... :-) — Vegar Westerlund, Jul 17 '12 at 12:52
I would of course like to get only the assert statement and not the entire function as a string. — Vegar Westerlund, Jul 17 '12 at 13:00
That's beyond my hacking skills at the moment. If I think of something, I'll get back to you :) — Andrew Alcock, Jul 18 '12 at 01:12

Python multi-line expressions and stack trace

1 Answers1