Fix split / rsplit collision (#394)

Only mutate split to rsplit if the `maxsplit` arg is provided.
Otherwise it would be an equivalent mutation.

Author: CHB-0r1s

mutmut/node_mutation.py

@@ -96,7 +96,7 @@ def operator_arg_removal(
             yield node.with_changes(args=[*node.args[:i], *node.args[i + 1 :]])
 
 
-supported_str_methods_swap = [
+supported_symmetric_str_methods_swap = [
          ("lower", "upper"),
          ("upper", "lower"),
          ("lstrip", "rstrip"),
@@ -107,24 +107,42 @@ def operator_arg_removal(
          ("rjust", "ljust"),
          ("index", "rindex"),
          ("rindex", "index"),
-         ("split", "rsplit"),
-         ("rsplit", "split"),
          ("removeprefix", "removesuffix"),
          ("removesuffix", "removeprefix"),
          ("partition", "rpartition"),
          ("rpartition", "partition")
-     ]
+]
+
+supported_unsymmetrical_str_methods_swap = [
+    ("split", "rsplit"),
+    ("rsplit", "split")
+]
 
-def operator_string_methods_swap(
+def operator_symmetric_string_methods_swap(
      node: cst.Call
  ) -> Iterable[cst.Call]:
      """try to swap string method to opposite e.g. a.lower() -> a.upper()"""
 
-     for old_call, new_call in supported_str_methods_swap:
+     for old_call, new_call in supported_symmetric_str_methods_swap:
          if m.matches(node.func, m.Attribute(value=m.DoNotCare(),  attr=m.Name(value=old_call))):
             func_name = cst.ensure_type(node.func, cst.Attribute).attr
             yield node.with_deep_changes(func_name, value=new_call)
 
+def operator_unsymmetrical_string_methods_swap(
+    node: cst.Call
+) -> Iterable[cst.Call]:
+    """Try to handle specific mutations of string, which useful only in specific args combination."""
+    for old_call, new_call in supported_unsymmetrical_str_methods_swap:
+        if m.matches(node.func, m.Attribute(attr=m.Name(value=old_call))):
+            if old_call in {"split", "rsplit"}:
+                # The logic of this "if" operator described here:
+                # https://github.com/boxed/mutmut/pull/394#issuecomment-2977890188
+                key_args: set[str] = {a.keyword.value for a in node.args if a.keyword} # sep or maxsplit or nothing
+                if len(node.args) == 2 or "maxsplit" in key_args:
+                    func_name = cst.ensure_type(node.func, cst.Attribute).attr
+                    yield node.with_deep_changes(func_name, value=new_call)
+
+
 
 def operator_remove_unary_ops(
     node: cst.UnaryOperation
@@ -238,7 +256,8 @@ def operator_match(node: cst.Match) -> Iterable[cst.CSTNode]:
     (cst.UnaryOperation, operator_remove_unary_ops),
     (cst.Call, operator_dict_arguments),
     (cst.Call, operator_arg_removal),
-    (cst.Call, operator_string_methods_swap),
+    (cst.Call, operator_symmetric_string_methods_swap),
+    (cst.Call, operator_unsymmetrical_string_methods_swap),
     (cst.Lambda, operator_lambda),
     (cst.CSTNode, operator_keywords),
     (cst.CSTNode, operator_swap_op),

tests/test_mutation.py

@@ -62,8 +62,47 @@ def mutated_module(source: str) -> str:
         ]),
         ('a.index("+")', ['a.rindex("+")', 'a.index("XX+XX")', 'a.index(None)']),
         ('a.rindex("+")', ['a.index("+")', 'a.rindex("XX+XX")', 'a.rindex(None)']),
-        ('a.split()', 'a.rsplit()'),
-        ('a.rsplit()', 'a.split()'),
+        ('a.split()', []),
+        ('a.rsplit()', []),
+        ('a.split(" ")', ['a.split("XX XX")', 'a.split(None)']),
+        ('a.rsplit(" ")', ['a.rsplit("XX XX")', 'a.rsplit(None)']),
+        ('a.split(sep="")', ['a.split(sep="XXXX")', 'a.split(sep=None)']),
+        ('a.rsplit(sep="")', ['a.rsplit(sep="XXXX")', 'a.rsplit(sep=None)']),
+        ('a.split(maxsplit=-1)', [
+            'a.rsplit(maxsplit=-1)', 'a.split(maxsplit=+1)', 'a.split(maxsplit=-2)', 'a.split(maxsplit=None)'
+        ]),
+        ('a.rsplit(maxsplit=-1)', [
+            'a.split(maxsplit=-1)', 'a.rsplit(maxsplit=+1)', 'a.rsplit(maxsplit=-2)', 'a.rsplit(maxsplit=None)'
+        ]),
+        ('a.split(" ", maxsplit=-1)', [
+            'a.split(" ", )', 'a.split(" ", maxsplit=+1)', 'a.split(" ", maxsplit=-2)',
+            'a.split(" ", maxsplit=None)', 'a.split("XX XX", maxsplit=-1)', 'a.split(None, maxsplit=-1)',
+            'a.split(maxsplit=-1)', 'a.rsplit(" ", maxsplit=-1)'
+        ]),
+        ('a.rsplit(" ", maxsplit=-1)', [
+            'a.rsplit(" ", )', 'a.rsplit(" ", maxsplit=+1)', 'a.rsplit(" ", maxsplit=-2)',
+            'a.rsplit(" ", maxsplit=None)', 'a.rsplit("XX XX", maxsplit=-1)', 'a.rsplit(None, maxsplit=-1)',
+            'a.rsplit(maxsplit=-1)', 'a.split(" ", maxsplit=-1)'
+        ]),
+        ('a.split(maxsplit=1)', ['a.split(maxsplit=2)', 'a.split(maxsplit=None)', 'a.rsplit(maxsplit=1)']),
+        ('a.rsplit(maxsplit=1)', ['a.rsplit(maxsplit=2)', 'a.rsplit(maxsplit=None)', 'a.split(maxsplit=1)']),
+        ('a.split(" ", 1)', [
+            'a.rsplit(" ", 1)', 'a.split(" ", )', 'a.split(" ", 2)', 'a.split(" ", None)',
+            'a.split("XX XX", 1)', 'a.split(1)', 'a.split(None, 1)'
+        ]),
+        ('a.rsplit(" ", 1)', [
+            'a.rsplit(" ", )', 'a.rsplit(" ", 2)', 'a.rsplit(" ", None)', 'a.rsplit("XX XX", 1)',
+            'a.rsplit(1)', 'a.rsplit(None, 1)', 'a.split(" ", 1)'
+        ]),
+        ('a.split(" ", maxsplit=1)', [
+            'a.rsplit(" ", maxsplit=1)', 'a.split(" ", )', 'a.split(" ", maxsplit=2)', 'a.split(" ", maxsplit=None)',
+            'a.split("XX XX", maxsplit=1)', 'a.split(None, maxsplit=1)', 'a.split(maxsplit=1)'
+        ]),
+        ('a.rsplit(" ", maxsplit=1)', [
+            'a.rsplit(" ", )', 'a.rsplit(" ", maxsplit=2)', 'a.rsplit(" ", maxsplit=None)',
+            'a.rsplit("XX XX", maxsplit=1)', 'a.rsplit(None, maxsplit=1)', 'a.rsplit(maxsplit=1)',
+            'a.split(" ", maxsplit=1)'
+        ]),
         ('a.removeprefix("+")', ['a.removesuffix("+")', 'a.removeprefix("XX+XX")', 'a.removeprefix(None)']),
         ('a.removesuffix("+")', ['a.removeprefix("+")', 'a.removesuffix("XX+XX")', 'a.removesuffix(None)']),
         ('a.partition("++")', ['a.rpartition("++")', 'a.partition("XX++XX")', 'a.partition(None)']),