tests/sitecustomize.py

exchange: guard against method invocation on `b2caps=None` args

I couldn't figure out how these are called, but the value is pretty obviously
set at least for the cases that have tests.

Differential Revision: https://phab.mercurial-scm.org/D7512

from __future__ import absolute_import
import os

if os.environ.get('COVERAGE_PROCESS_START'):
    try:
        import coverage
        import uuid

        covpath = os.path.join(
            os.environ['COVERAGE_DIR'], 'cov.%s' % uuid.uuid1()
        )
        cov = coverage.coverage(data_file=covpath, auto_data=True)
        cov._warn_no_data = False
        cov._warn_unimported_source = False
        cov.start()
    except ImportError:
        pass