Improve error message if using RenderLaTeX without jinja2 installed.

lena/context/__init__.py

@@ -5,6 +5,7 @@
     intersection, iterate_update, make_context,
     str_to_dict, str_to_list, update_nested, update_recursively,
 )
+# will import, but can't be used if jinja2 is missing
 from .update_context import UpdateContext
 
 

lena/context/update_context.py

@@ -1,9 +1,7 @@
-from __future__ import print_function
-
+# requires jinja2
 import copy
 import re
 
-import jinja2
 import lena.core
 import lena.context
 import lena.flow
@@ -88,6 +86,7 @@ def __init__(self, subcontext, update, value=False, default=_sentinel,
         and the last two symbols of *update*,
         otherwise :exc:`.LenaValueError` is raised.
         """
+        import jinja2
         # The "Context" in the class name means any general context
         # (not only :class:`.Context`).
 
@@ -173,6 +172,7 @@ def __call__(self, value):
         *raise_on_missing* is ``True`` and
         the update argument is missing in *value*'s context.
         """
+        import jinja2
         data, context = lena.flow.get_data_context(value)
         if isinstance(self._update, (str, jinja2.Template)):
             if self._context_value:

lena/output/__init__.py

@@ -4,15 +4,21 @@
 from .write import Writer, Write
 from .to_csv import ToCSV, hist1d_to_csv, hist2d_to_csv
 
-imported = []
+
+def raise_on_usage(clsname, modname):
+    def stub(*args, **kwargs):
+        raise ImportError("{} can't be used because {} is not found".
+                          format(clsname, modname))
+    return stub
+
+
+# we do it here, because jinja2 is used at top level in render_latex.py
+# (in classes' parents)
 try:
-    # won't work if jinja2 is not installed
-    from lena.output.render_latex import RenderLaTeX # , Template, Environment
+    # will raise if jinja2 is not installed
+    from lena.output.render_latex import RenderLaTeX  #, Template, Environment
 except ImportError:
-    pass
-else:
-    imported.extend(['RenderLaTeX'])
-    # imported.extend(['RenderLaTeX', 'Template', 'Environment'])
+    RenderLaTeX = raise_on_usage("RenderLaTeX", "jinja2")
 
 
 __all__ = [
@@ -22,5 +28,6 @@
     'RenderLaTeX',
     'Write',
     'Writer',
-    'ToCSV', 'hist1d_to_csv', 'hist2d_to_csv'
-] + imported
+    'ToCSV', 'hist1d_to_csv', 'hist2d_to_csv',
+    'RenderLaTeX'
+]

tests/output/test_missing_jinja2.py

@@ -0,0 +1,24 @@
+import sys
+
+import pytest
+
+
+# this function works when used solely,
+# but not with other modules tests.
+@pytest.mark.skip(reason="can only test this separately from other modules")
+def test_missing_jinja2():
+    # we check that a proper and clear exception is raised 
+    # if jinja2 is missing when using RenderLaTeX
+    # from https://stackoverflow.com/a/1350574/952234
+    import jinja2
+    # doesn't help.
+    # jinja2_tmp = jinja2
+    sys.modules["jinja2"] = None
+    from lena.output import RenderLaTeX
+    with pytest.raises(ImportError) as err:
+        r = RenderLaTeX("hello")
+    assert str(err.value) == "RenderLaTeX can't be used because jinja2 is not found"
+    # seems tests are run non-atomically, so need to return this
+    # del sys.modules["jinja2"]
+    # doesn't work:
+    # sys.modules["jinja2"] = jinja2_tmp

tests/output/test_render_latex.py

@@ -1,12 +1,10 @@
-from __future__ import print_function
-
+import copy
+from copy import deepcopy
 import inspect
 import jinja2
 import os
 import pytest
 import sys
-import copy
-from copy import deepcopy
 
 import lena
 from lena.output.render_latex import (