test postBuild behavior with legacy Python 3.6

tests/conda/py36-postBuild/environment.yml

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+dependencies:
+  - python=3.6
+  - numpy

tests/conda/py36-postBuild/postBuild

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+#!/bin/bash
+set -ex
+
+# mamba/conda installs in kernel env
+mamba install -y make
+
+# note `pip` on path is _not_ the kernel env!
+# is this what we (or users) want?
+pip install pytest

tests/conda/py36-postBuild/verify

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+#!/bin/bash
+set -ex
+pytest -vs ./verify.py

tests/conda/py36-postBuild/verify.py

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+"""
+tests to be run with pytest inside the container
+
+can't be called test_whatever.py because then _host_ pytest will try to run it!
+"""
+
+import json
+import os
+import shutil
+from subprocess import check_output
+
+from pytest import fixture
+
+kernel_prefix = os.environ.get("KERNEL_PYTHON_PREFIX")
+server_prefix = os.environ.get("NB_PYTHON_PREFIX")
+
+
+def json_cmd(cmd):
+    """Run a command and decode its JSON output"""
+    out = check_output(cmd)
+    return json.loads(out.decode("utf8", "replace"))
+
+
+def conda_pkgs(prefix):
+    """Conda package list as a dict"""
+    conda_json = json_cmd(["conda", "list", "--json", "-p", prefix])
+    return {pkg["name"]: pkg for pkg in conda_json}
+
+
+def pip_pkgs(prefix):
+    """Pip package list as a dict"""
+    pip_json = json_cmd([f"{prefix}/bin/pip", "list", "--format=json"])
+    return {pkg["name"]: pkg for pkg in pip_json}
+
+
+@fixture(scope="session")
+def kernel_conda():
+    return conda_pkgs(kernel_prefix)
+
+
+@fixture(scope="session")
+def server_conda():
+    return conda_pkgs(server_prefix)
+
+
+@fixture(scope="session")
+def kernel_pip():
+    return pip_pkgs(kernel_prefix)
+
+
+@fixture(scope="session")
+def server_pip():
+    return pip_pkgs(server_prefix)
+
+
+def test_which_python():
+    # server python comes first. Is this expected?
+    assert shutil.which("python3") == f"{server_prefix}/bin/python3"
+
+
+def test_kernel_env(kernel_conda):
+    assert kernel_prefix != server_prefix
+    kernel_python = kernel_conda["python"]["version"]
+    assert kernel_python[:3] == "3.6"
+    # test environment.yml packages
+    assert "numpy" in kernel_conda
+
+
+def test_server_env(server_conda):
+    # this should be the default version
+    # it will need updating when the default changes
+    assert server_conda["python"]["version"][:3] == "3.7"
+
+
+def test_conda_install(kernel_conda, server_conda):
+    # test that postBuild conda install went in the kernel env
+    assert "make" in kernel_conda
+    assert "make" not in server_conda
+
+
+def test_pip_install(kernel_pip, server_pip):
+    # server env comes first for pip
+    # is this expected?
+    assert "pytest" not in kernel_pip
+    assert "pytest" in server_pip