Added function filter to template system, closes #429

tests/template_filter.py

@@ -0,0 +1,17 @@
+""" Example of using a custom python function as an osxphotos template filter 
+
+    Use in formath:
+    "{template_field|template_filter.py::myfilter}"
+
+    Your filter function will receive a list of strings even if the template renders to a single value.  
+    You should expect a list and return a list and be able to handle multi-value templates like {keyword}
+    as well as single-value templates like {original_name}
+"""
+
+from typing import List
+
+def myfilter(values: List[str]) -> List[str]:
+    """ Custom filter to append "foo-" to template value """
+    values = ["foo-" + val for val in values]
+    return values
+

tests/test_template.py

@@ -982,3 +982,31 @@ def test_function_bad(photosdb):
             "{function:tests/template_function.py::foobar}"
         )
 
+
+def test_function_filter(photosdb):
+    """ Test {field|function} filter"""
+    photo = photosdb.get_photo(UUID_MULTI_KEYWORDS)
+
+    rendered, _ = photo.render_template(
+        "{photo.original_filename|function:tests/template_filter.py::myfilter}"
+    )
+    assert rendered == [f"foo-{photo.original_filename}"]
+
+    rendered, _ = photo.render_template(
+        "{photo.original_filename|lower|function:tests/template_filter.py::myfilter}"
+    )
+    assert rendered == [f"foo-{photo.original_filename.lower()}"]
+
+    rendered, _ = photo.render_template(
+        "{photo.original_filename|function:tests/template_filter.py::myfilter|lower}"
+    )
+    assert rendered == [f"foo-{photo.original_filename.lower()}"]
+
+
+def test_function_filter_bad(photosdb):
+    """ Test invalid {field|function} filter"""
+    photo = photosdb.get_photo(UUID_MULTI_KEYWORDS)
+    with pytest.raises(ValueError):
+        rendered, _ = photo.render_template(
+            "{photo.original_filename|function:tests/template_filter.py::foobar}"
+        )