Fix split filter compatibility issues (#135)

* Add failing `split` filter test cases. See #134.

* Add compatible implementation of `split`.

* Fix left matches repr of argument.

* Simplify by combining condition.

* Add more to doc comment.

* Update change log and bump version number.

CHANGES.md

@@ -1,5 +1,11 @@
 # Python Liquid Change Log
 
+## Version 1.11.0 (unreleased)
+
+**Fixes**
+
+- Added an additional implementation of the `split` filter, which resolves some compatibility issues between Python Liquid's and the reference implementation. Previously, when given an empty string to split or when the string and the delimiter were equal, we used Python's `str.split()` behavior of producing one or two element lists containing empty strings. We now match Shopify/Liquid in returning an empty list for such cases. The new `split` filter will be enabled by default when using [`liquid.future.Environment`](https://jg-rp.github.io/liquid/api/future-environment), and can optionally be registered with `liquid.Environment` for those that don't mind the behavioral change. See [#135](https://github.com/jg-rp/liquid/pull/135).
+
 ## Version 1.10.1
 
 **Fixes**

liquid/__init__.py

@@ -42,7 +42,7 @@
 
 from . import future
 
-__version__ = "1.10.1"
+__version__ = "1.11.0"
 
 __all__ = (
     "AwareBoundTemplate",

liquid/future/environment.py

@@ -5,6 +5,7 @@
 """
 from ..environment import Environment as DefaultEnvironment
 from ..template import FutureBoundTemplate
+from .filters import split
 
 
 class Environment(DefaultEnvironment):
@@ -19,3 +20,8 @@ class Environment(DefaultEnvironment):
     """
 
     template_class = FutureBoundTemplate
+
+    def setup_tags_and_filters(self) -> None:
+        """Add future tags and filters to this environment."""
+        super().setup_tags_and_filters()
+        self.add_filter("split", split)

liquid/future/filters/__init__.py

@@ -0,0 +1,3 @@
+from ._split import split  # noqa: D104
+
+__all__ = ("split",)

liquid/future/filters/_split.py

@@ -0,0 +1,23 @@
+"""An implementation of the `split` filter."""
+
+from typing import List
+
+from liquid import soft_str
+from liquid.filter import string_filter
+
+
+@string_filter
+def split(val: str, sep: str) -> List[str]:
+    """Split string _val_ on delimiter _sep_.
+
+    If _sep_ is empty or _undefined_, _val_ is split into a list of single
+    characters. If _val_ is empty or equal to _sep_, an empty list is returned.
+    """
+    if not sep:
+        return list(val)
+
+    sep = soft_str(sep)
+    if not val or val == sep:
+        return []
+
+    return val.split(sep)

liquid/golden/split_filter.py

@@ -40,4 +40,58 @@
         template=r'{{ "Hello there" | split: nosuchthing | join: "#" }}',
         expect="H#e#l#l#o# #t#h#e#r#e",
     ),
+    Case(
+        description="empty string argument",
+        template=(
+            r'{% assign a = "abc" | split: "" %}'
+            r"{% for i in a %}#{{ forloop.index0 }}{{ i }}{% endfor %}"
+        ),
+        expect="#0a#1b#2c",
+        future=True,
+    ),
+    Case(
+        description="argument does not appear in string",
+        template=(
+            r'{% assign a = "abc" | split: "," %}'
+            r"{% for i in a %}#{{ forloop.index0 }}{{ i }}{% endfor %}"
+        ),
+        expect="#0abc",
+        future=True,
+    ),
+    Case(
+        description="empty string and empty argument",
+        template=(
+            r'{% assign a = "" | split: "" %}'
+            r"{% for i in a %}{{ forloop.index0 }}{{ i }}{% endfor %}"
+        ),
+        expect="",
+        future=True,
+    ),
+    Case(
+        description="empty string and single char argument",
+        template=(
+            r'{% assign a = "" | split: "," %}'
+            r"{% for i in a %}{{ forloop.index0 }}{{ i }}{% endfor %}"
+        ),
+        expect="",
+        future=True,
+    ),
+    Case(
+        description="left matches argument",
+        template=(
+            r'{% assign a = "," | split: "," %}'
+            r"{% for i in a %}{{ forloop.index0 }}{{ i }}{% endfor %}"
+        ),
+        expect="",
+        future=True,
+    ),
+    Case(
+        description="left matches string repr of argument",
+        template=(
+            r'{% assign a = "1" | split: 1 %}'
+            r"{% for i in a %}{{ forloop.index0 }}{{ i }}{% endfor %}"
+        ),
+        expect="",
+        future=True,
+    ),
 ]