Escape environment variable when using Python string formatting

repo2docker/buildpacks/python/__init__.py

@@ -53,7 +53,9 @@ class PythonBuildPack(CondaBuildPack):
             if os.path.exists(nb_requirements_file):
                 assemble_scripts.append((
                     '${NB_USER}',
-                    '${NB_PYTHON_PREFIX}/bin/pip install --no-cache-dir -r "{}"'.format(nb_requirements_file)
+                    # want the $NB_PYHTON_PREFIX environment variable, not for
+                    # Python's string formatting to try and replace this
+                    '${{NB_PYTHON_PREFIX}}/bin/pip install --no-cache-dir -r "{}"'.format(nb_requirements_file)
                 ))
 
         # install requirements.txt in the kernel env

tests/venv/py2-with-kernel-requirements/requirements.txt

@@ -0,0 +1,2 @@
+# install this in the kernel's environment, the user wants Python 2 there
+numpy

tests/venv/py2-with-kernel-requirements/requirements3.txt

@@ -0,0 +1,3 @@
+# install this in the environment in which the notebook server
+# is running nbgitpuller does not work with Python 2
+nbgitpuller==0.6.1

tests/venv/py2-with-kernel-requirements/runtime.txt

@@ -0,0 +1 @@
+python-2.7

tests/venv/py2-with-kernel-requirements/verify

@@ -0,0 +1,7 @@
+#!/usr/bin/env python2
+import sys
+
+print(sys.version_info)
+assert sys.version_info[:2] == (2, 7)
+
+import numpy