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

问题

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?

回答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.

回答2:

The only thin I can think of would be to replace sys.stdout, for example with a streamwriter as returned by codecs.getwriter('utf8'). Then you can set a breakpoint on it's write method in pdb. Or replace it's write method with debugging code.

import codecs
import sys

writer = codecs.getwriter('utf-8')(sys.stdout) # sys.stdout.detach() in python3
old_write = writer.write

def write(data):
    print >>sys.stderr, 'debug:', repr(data)
    # or check data + pdb.set_trace()
    old_write(data)

writer.write = write
sys.stdout = writer

print 'spam', 'eggs'

来源：https://stackoverflow.com/questions/10742501/is-there-a-trick-to-break-on-the-print-builtin-with-pdb