Prevent notebook tests from changing the pytest Python environment (#…

…6032)

`test_notebooks_against_released_cirq` should not change the installed
packages in the Python pytest environment.  Adjust OS environment
so that pip invoked from tested notebooks installs to a temporary directory.
Also add test for changed packages that catches downgrade to ply==3.4
caused by a test of interop.ipynb.

dev_tools/notebooks/notebook_test.py

@@ -20,6 +20,8 @@
 # tests ensure that notebooks are still working with the latest version of cirq.
 
 import os
+import sys
+import tempfile
 
 import pytest
 
@@ -50,9 +52,41 @@
 ]
 
 
+@pytest.fixture
+def require_packages_not_changed():
+    """Verify notebook test does not change packages in the Python test environment.
+
+    Raise AssertionError if the pre-existing set of Python packages changes in any way.
+    """
+    # TODO: remove this after deprecation of Python 3.7
+    if sys.version_info < (3, 8, 0):
+        return
+    import importlib.metadata
+
+    packages_before = set((d.name, d.version) for d in importlib.metadata.distributions())
+    yield
+    packages_after = set((d.name, d.version) for d in importlib.metadata.distributions())
+    assert packages_after == packages_before
+
+
+@pytest.fixture(scope='module')
+def env_with_temporary_pip_target():
+    """Setup system environment that tells pip to install packages to a temporary directory."""
+    with tempfile.TemporaryDirectory(suffix='-notebook-site-packages') as tmpdirname:
+        pythonpath = (
+            f'{tmpdirname}{os.pathsep}{os.environ["PYTHONPATH"]}'
+            if 'PYTHONPATH' in os.environ
+            else tmpdirname
+        )
+        env = {**os.environ, 'PYTHONPATH': pythonpath, 'PIP_TARGET': tmpdirname}
+        yield env
+
+
 @pytest.mark.slow
 @pytest.mark.parametrize("notebook_path", filter_notebooks(list_all_notebooks(), SKIP_NOTEBOOKS))
-def test_notebooks_against_released_cirq(notebook_path):
+def test_notebooks_against_released_cirq(
+    notebook_path, require_packages_not_changed, env_with_temporary_pip_target
+):
     """Test that jupyter notebooks execute.
 
     In order to speed up the execution of these tests an auxiliary file may be supplied which
@@ -73,7 +107,12 @@ def test_notebooks_against_released_cirq(notebook_path):
 papermill {rewritten_notebook_path} {out_path} {papermill_flags}"""
 
     result = shell_tools.run(
-        cmd, log_run_to_stderr=False, shell=True, check=False, capture_output=True
+        cmd,
+        log_run_to_stderr=False,
+        shell=True,
+        check=False,
+        capture_output=True,
+        env=env_with_temporary_pip_target,
     )
 
     if result.returncode != 0: