Is there a trick to break on the print builtin with pdb?

Question

Basically, the title.

I am trying to trace down where a spurious print happens in a large codebase, and I would like to break, or somehow get a stack trace whenever a print "happens." Any ideas?

Answer 1

For this particular case you can redirect stdout to a helper class that prints the output and its caller. You can also break on one of its methods.

Full example:

import sys
import inspect

class PrintSnooper:
    def __init__(self, stdout):
        self.stdout = stdout
    def caller(self):
        return inspect.stack()[2][3]
    def write(self, s):
        self.stdout.write("printed by %s: " % self.caller())
        self.stdout.write(s)
        self.stdout.write("\n")

def test():
    print 'hello from test'

def main():
    # redirect stdout to a helper class.
    sys.stdout = PrintSnooper(sys.stdout)
    print 'hello from main'
    test()

if __name__ == '__main__':
    main()

Output:

printed by main: hello from main
printed by main: 

printed by test: hello from test
printed by test: 

You can also just print inspect.stack() if you need more thorough information.