python/helpers/coverage/fullcoverage/encodings.py

"""Imposter encodings module that installs a coverage-style tracer.

This is NOT the encodings module; it is an imposter that sets up tracing
instrumentation and then replaces itself with the real encodings module.

If the directory that holds this file is placed first in the PYTHONPATH when
using "coverage" to run Python's tests, then this file will become the very
first module imported by the internals of Python 3.  It installs a
coverage-compatible trace function that can watch Standard Library modules
execute from the very earliest stages of Python's own boot process.  This fixes
a problem with coverage - that it starts too late to trace the coverage of many
of the most fundamental modules in the Standard Library.

"""

import sys

class FullCoverageTracer(object):
    def __init__(self):
        # `traces` is a list of trace events.  Frames are tricky: the same
        # frame object is used for a whole scope, with new line numbers
        # written into it.  So in one scope, all the frame objects are the
        # same object, and will eventually all will point to the last line
        # executed.  So we keep the line numbers alongside the frames.
        # The list looks like:
        #
        #   traces = [
        #       ((frame, event, arg), lineno), ...
        #       ]
        #
        self.traces = []

    def fullcoverage_trace(self, *args):
        frame, event, arg = args
        self.traces.append((args, frame.f_lineno))
        return self.fullcoverage_trace

sys.settrace(FullCoverageTracer().fullcoverage_trace)

# In coverage/files.py is actual_filename(), which uses glob.glob.  I don't
# understand why, but that use of glob borks everything if fullcoverage is in
# effect.  So here we make an ugly hail-mary pass to switch off glob.glob over
# there.  This means when using fullcoverage, Windows path names will not be
# their actual case.

#sys.fullcoverage = True

# Finally, remove our own directory from sys.path; remove ourselves from
# sys.modules; and re-import "encodings", which will be the real package
# this time.  Note that the delete from sys.modules dictionary has to
# happen last, since all of the symbols in this module will become None
# at that exact moment, including "sys".

parentdir = max(filter(__file__.startswith, sys.path), key=len)
sys.path.remove(parentdir)
del sys.modules['encodings']
import encodings