env var for extra_template_basedirs

This commit add the following three items.

- Updates the method `_default_extra_template_basedirs` in the module
  `templateexporter.py` to search for a new environment variable called
  `NBCONVERT_EXTRA_TEMPLATE_BASEDIRS`. This environment variable will be
  added to the basedirs list if it is found, and it exists.
- Updates the tests to ensure if the new environment variable exists
  then it is included in the `TemplateExporter.template_paths`.
- Adds a context manager method that modifies the `os.environ`
  dictionary.

Closes jupyter#2026

nbconvert/exporters/templateexporter.py

@@ -260,7 +260,13 @@ def _raw_template_changed(self, change):
 
     @default("extra_template_basedirs")
     def _default_extra_template_basedirs(self):
-        return [os.getcwd()]
+        basedirs = [os.getcwd()]
+        env = os.environ
+        if "NBCONVERT_EXTRA_TEMPLATE_BASEDIR" in env:
+            extra_template_path = env["NBCONVERT_EXTRA_TEMPLATE_BASEDIR"]
+            if os.path.exists(extra_template_path):
+                basedirs.append(extra_template_path)
+        return basedirs
 
     # Extension that the template files use.
     template_extension = Unicode().tag(config=True, affects_environment=True)

nbconvert/exporters/tests/test_templateexporter.py

@@ -8,6 +8,7 @@
 import json
 import os
 from concurrent.futures import ProcessPoolExecutor
+from contextlib import contextmanager
 from tempfile import TemporaryDirectory
 from unittest.mock import patch
 
@@ -32,6 +33,20 @@
 """
 
 
+@contextmanager
+def temp_environ(**env):
+    for key, value in env.items():
+        if value is None:
+            os.environ.pop(key, None)
+        else:
+            os.environ[key] = value
+    try:
+        yield
+    finally:
+        for key, _ in env.items():
+            os.environ.pop(key, None)
+
+
 class SampleExporter(TemplateExporter):
     """
     Exports a Python code file.
@@ -268,6 +283,24 @@ def test_absolute_template_dir(self):
             assert exporter.template_name == template
             assert os.path.join(td, template) in exporter.template_paths
 
+    def test_env_var_template_dir(self):
+        with TemporaryDirectory() as td:
+            template = "env-var"
+            template_file = os.path.join(td, template, "index.py.j2")
+            template_dir = os.path.dirname(template_file)
+            os.mkdir(template_dir)
+            test_output = "env-var!"
+            with open(template_file, "w") as f:
+                f.write(test_output)
+            with temp_environ(NBCONVERT_EXTRA_TEMPLATE_BASEDIRS=template_dir):
+                config = Config()
+                config.TemplateExporter.template_name = template
+                config.TemplateExporter.extra_template_basedirs = [td, template_dir]
+                exporter = self._make_exporter(config=config)
+                assert exporter.template.filename == template_file
+                assert exporter.template_name == template
+                assert template_dir in exporter.template_paths
+
     def test_local_template_dir(self):
         with TemporaryDirectory() as td, _contextlib_chdir.chdir(td):  # noqa
             with patch("os.getcwd", return_value=os.path.abspath(td)):