Find source code line which prints some debugging stuff on stdout

Question

I have a huge code base before me, and can't find the place where this strings gets printed to stdout:

[{}]

I can reproduce it, but searching the relevant code line was not successful up to now.

I guess this was added accidentally by a developer to aid debugging.

Any clue how to find the matching code line which emits this string to stdout?

Edward Z. Yang · Accepted Answer · 2024-10-29 16:45:54Z

2

You can find the relevant source code line be raising an exception, if this line gets emitted like this:

class WrapperOfStdout():
    def __init__(self, old_stdout):
        self.old_stdout=old_stdout

    def write(self, data):
        assert not '{}' in data, data
        return self.old_stdout.write(data)

    def __getattr__(self, name):
        return getattr(self.old_stdout, name)

Add this wrapper into your script, and wrap stdout as early as possible (soon after main):

sys.stdout=WrapperOfStdout(sys.stdout)

You will get an Exception like this:

Traceback (most recent call last):
  File "/home/u/src/foo-bar/foo_bar/utils/common.py", line 94, in issue_to_tar
    meta = issuemeta(request, issue).encode('utf8')
  File "/home/u/src/foo/foo/views/issue/view.py", line 198, in issue_meta
    div = index_model.view(issue, request)
  File "/home/u/src/foo/foo/utils/indexutils.py", line 96, in view
    form = form_cls(request, issue, None, prefix=prefix)
  File "/home/u/src/foo-x/foo_x/logic/forms.py", line 115, in __init__
    print(fake_initial)
  File "/home/u/tmp/issue_to_tar.py", line 13, in write
    assert not '{}' in data, data
AssertionError: [{}]

Now you see where the unwanted output happens. In this case it is logic/forms.py line 115.

edited Oct 29, 2024 at 16:45

Edward Z. Yang

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answered Apr 4, 2017 at 15:01

guettli

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1 Comment

Edward Z. Yang Over a year ago

One thing to note is that the writes to WrapperOfStdout are not necessarily line buffered. So you may not be able to search for the exact full line without buffering yourself.

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