handle missing jinja

.docs/md/generate_classes.md

@@ -14,8 +14,6 @@ The FloPy classes for MODFLOW 6 are largely generated by a utility which convert
 
 **Note**: to use this functionality, the `codegen` optional dependency group must be installed.
 
-For instance (output much abbreviated):
-
 ```bash
 $ python -m flopy.mf6.utils.generate_classes
 

.github/workflows/commit.yml

@@ -101,7 +101,7 @@ jobs:
         run: |
           pip install --upgrade pip
           pip install .
-          pip install ".[test, optional]"
+          pip install ".[dev]"
       
       - name: Install Modflow executables
         uses: modflowpy/install-modflow-action@v1

flopy/mf6/utils/codegen/__init__.py

@@ -1,19 +1,25 @@
 from pathlib import Path
-
-from jinja2 import Environment, PackageLoader
+from warnings import warn
 
 from flopy.mf6.utils.codegen.context import Context
 from flopy.mf6.utils.codegen.dfn import Dfn, Dfns, Ref, Refs
+from flopy.utils import import_optional_dependency
 
 __all__ = ["make_targets", "make_all"]
 
-_TEMPLATE_LOADER = PackageLoader("flopy", "mf6/utils/codegen/templates/")
-_TEMPLATE_ENV = Environment(loader=_TEMPLATE_LOADER)
+jinja = import_optional_dependency("jinja2", errors="ignore")
+if jinja:
+    _TEMPLATES_PATH = "mf6/utils/codegen/templates/"
+    _TEMPLATE_LOADER = jinja.PackageLoader("flopy", _TEMPLATES_PATH)
+    _TEMPLATE_ENV = jinja.Environment(loader=_TEMPLATE_LOADER)
 
 
 def make_targets(dfn: Dfn, outdir: Path, verbose: bool = False):
     """Generate Python source file(s) from the given input definition."""
 
+    if not jinja:
+        warn("Jinja2 not installed, can't make targets")
+
     for context in Context.from_dfn(dfn):
         name = context.name
         target = outdir / name.target
@@ -27,6 +33,9 @@ def make_targets(dfn: Dfn, outdir: Path, verbose: bool = False):
 def make_all(dfndir: Path, outdir: Path, verbose: bool = False):
     """Generate Python source files from the DFN files in the given location."""
 
+    if not jinja:
+        warn("Jinja2 not installed, can't make targets")
+
     # find definition files
     paths = [
         p for p in dfndir.glob("*.dfn") if p.stem not in ["common", "flopy"]