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How to trace Python source code with settrace()

Imagine you have a test which works in one environment, and fails in a different environment.

If it is reproducible, then you can add this snippet, and compare the files created in /tmp/out*log.

You can modify skip_filename() and truncate_filename() to you needs.

This is copy+paste code, use it only temporarily.

The code uses sys.settrace() from the standard library and PySnooper which you can install via pip install pysnooper.

You can copy+paste this snippet into the code which you want to trace.

In my case I wanted to see how my code behaves different under a differen Python version.

It will create a file /tmp/out-... with every line which the interpreter executed.

You can compare two created files with vimdiff on the shell, with the GUI meld or any other diff-tool.

from pysnooper.tracer import get_path_and_source_from_frame

stream = io.open('/tmp/out-%s.log' % sys.version.split()[0], 'wt', encoding='utf8')


def tracer(frame, event, arg):
    filename, source = get_path_and_source_from_frame(frame)
    if skip_filename(filename):
        return
    filename = truncate_filename(filename)
    source_line = source[frame.f_lineno - 1]
    stream.write('{event}, {filename}:{line_number} {source_line}\n'.format(
        event=event, filename=filename, source_line=source_line, line_number=frame.f_lineno))
    return tracer


def truncate_filename(filename):
    for remove in ['/home/me/foo/projects/']:  # modify this to fit your liking
        filename = filename.replace(remove, '')
    return filename


def skip_filename(filename):
    if filename.startswith('<'):
        return True
    if 'lib/python' in filename:
        # Skip /usr/lib/ and lib/python/site-packages
        # I only want to trace my code.
        return True


sys.settrace(tracer)

I don't think it makes sense that I create a new tracing library for Python. There are already several.

See my Python Tips "Tracing"

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How to trace Python source code with settrace()

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