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Discover source of stdout output

asked 一月 10 at 20:58 by Ingo Kegel

I'm using a third party lib that is printing \n a couple of hundred times to stdout - it appears to be a bug in their logging.

What is the best approach to discover what line of code is producing this output (I'm hoping it's python and not an external library)?

Some ideas:

  • How can I change the display of the \n character to something recognizable like A so that if I step through with a debugger, I can see when the output occurs?

  • Can I monkey-patch a low level function to cause some output or a debug-break everytime characters are sent to std-out?

Your answer

1 Answers

answered 一月 11 at 16:01 by C. A. McCann

You could try to overtake stdout:

class StdoutFilter:
    def __init__(self, realStdout):
        self.realStdout = realStdout

    def write(self, text):
        if text == '\n':   # or some more complicated condition
            raise Exception("Newline alert!")

        self.realStdout.write(text)            

import sys
sys.stdout = StdoutFilter(sys.stdout)

# import the 3rd party library and use it

In this filter class you can add a counter and raise the exception only after more than N subsequent newlines were printed. The stack trace of the exception will reveal the source of the malicious print.

Or you can put a break point there, or change the \n to something else.

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