code: def default(session):
      
      # Install all test dependencies, then install local packages in-place.
      session.install("mock", "pytest", "pytest-cov")
      for local_dep in LOCAL_DEPS:
          session.install("-e", local_dep)
  
      # Pyarrow does not support Python 3.7
      dev_install = ".[all]"
      session.install("-e", dev_install)
  
      # IPython does not support Python 2 after version 5.x
      if session.python == "2.7":
          session.install("ipython==5.5")
      else:
          session.install("ipython")
  
      # Run py.test against the unit tests.
      session.run(
          "py.test",
          "--quiet",
          "--cov=google.cloud.bigquery",
          "--cov=tests.unit",
          "--cov-append",
          "--cov-config=.coveragerc",
          "--cov-report=",
          "--cov-fail-under=97",
          os.path.join("tests", "unit"),
          *session.posargs
      )
docs: Default unit test session.
  
      This is intended to be run **without** an interpreter set, so
      that the current ``python`` (on the ``PATH``) or the version of
      Python corresponding to the ``nox`` binary the ``PATH`` can
      run the tests.