Reworked logic based on review

ChangeLog

@@ -51,7 +51,7 @@ modules are added.
 
   Closes #4412
 
-* New checker ``consider-using-str-partition``. Emitted either when accessing only the first or last
+* New checker ``consider-using-maxsplit-arg``. Emitted either when accessing only the first or last
   element of ``str.split()``.
 
   Closes #4440

doc/whatsnew/2.9.rst

@@ -16,7 +16,7 @@ New checkers
   indexing the same dictionary with the key within loop body.
 
 
-* ``consider-using-str-partition``: Emitted either when accessing only the first or last
+* ``consider-using-maxsplit-arg``: Emitted either when accessing only the first or last
   element of ``str.split()``.
 
 * An ``ignore_signatures`` option has been added to the similarity checker. It will permits  to reduce false positives when multiple functions have the same parameters.

pylint/checkers/refactoring/recommendation_checker.py

@@ -37,10 +37,11 @@ class RecommendationChecker(checkers.BaseChecker):
         ),
         "C0207": (
             "Consider using %s instead",
-            "consider-using-str-partition",
+            "consider-using-maxsplit-arg",
             "Emitted when accessing only the first or last element of str.split(). "
-            "The first and last element can be accessed by using str.partition()[0] "
-            "or str.rpartition()[-1] instead.",
+            "The first and last element can be accessed by using "
+            "str.split(sep,maxsplit=1)[0] or str.rpartition(sep,maxsplit=1)[-1] "
+            "instead.",
         ),
     }
 
@@ -52,11 +53,11 @@ def _is_builtin(node, function):
         return utils.is_builtin_object(inferred) and inferred.name == function
 
     @utils.check_messages(
-        "consider-iterating-dictionary", "consider-using-str-partition"
+        "consider-iterating-dictionary", "consider-using-maxsplit-arg"
     )
     def visit_call(self, node: astroid.Call) -> None:
         self._check_consider_iterating_dictionary(node)
-        self._check_consider_using_str_partition(node)
+        self._check_consider_using_maxsplit_arg(node)
 
     def _check_consider_iterating_dictionary(self, node: astroid.Call) -> None:
         if not isinstance(node.func, astroid.Attribute):
@@ -75,7 +76,7 @@ def _check_consider_iterating_dictionary(self, node: astroid.Call) -> None:
         if isinstance(node.parent, (astroid.For, astroid.Comprehension)):
             self.add_message("consider-iterating-dictionary", node=node)
 
-    def _check_consider_using_str_partition(self, node: astroid.Call) -> None:
+    def _check_consider_using_maxsplit_arg(self, node: astroid.Call) -> None:
         """Add message when accessing first or last elements of a str.split() or str.rsplit(),
         or when split/rsplit with max_split=1 is used"""
 
@@ -86,14 +87,14 @@ def _check_consider_using_str_partition(self, node: astroid.Call) -> None:
             and isinstance(utils.safe_infer(node.func), astroid.BoundMethod)
         ):
             try:
-                seperator = utils.get_argument_from_call(node, 0, "sep").value
+                sep = utils.get_argument_from_call(node, 0, "sep").value
             except utils.NoSuchArgumentError:
                 return
 
             # Check if maxsplit is set, and ignore checking if maxsplit is > 1
             try:
                 maxsplit = utils.get_argument_from_call(node, 1, "maxsplit").value
-                if maxsplit > 1:
+                if maxsplit > 0:
                     return
             except utils.NoSuchArgumentError:
                 maxsplit = 0
@@ -106,21 +107,16 @@ def _check_consider_using_str_partition(self, node: astroid.Call) -> None:
                 except utils.InferredTypeError:
                     return
 
-                if maxsplit == 1 and subscript_value == 1:
-                    new_func = node.func.attrname.replace("split", "partition")
-                    new_name = f"{node.as_string().rpartition('.')[0]}.{new_func}({seperator})[2]"
-
-                elif maxsplit == 0 and subscript_value in (-1, 0):
+                if subscript_value in (-1, 0):
                     fn_name = node.func.attrname
-                    new_fn = "rpartition" if subscript_value == -1 else "partition"
-                    new_name = f"{new_fn.join(node.as_string().rsplit(fn_name, maxsplit=1))}[{subscript_value}]"
-
-                else:
-                    return
-
-                self.add_message(
-                    "consider-using-str-partition", node=node, args=(new_name,)
-                )
+                    new_fn = "rsplit" if subscript_value == -1 else "split"
+                    new_name = (
+                        f"{node.as_string().rsplit(fn_name, maxsplit=1)[0]}{new_fn}"
+                        f"(sep='{sep}',maxsplit=1)[{subscript_value}]"
+                    )
+                    self.add_message(
+                        "consider-using-maxsplit-arg", node=node, args=(new_name,)
+                    )
 
     @utils.check_messages("consider-using-enumerate", "consider-using-dict-items")
     def visit_for(self, node: astroid.For) -> None:

tests/functional/c/consider/consider_using_maxsplit_arg.py

@@ -0,0 +1,77 @@
+"""Emit a message for accessing first/last element of string.split"""
+# pylint: disable=line-too-long,missing-docstring,unsubscriptable-object,too-few-public-methods,invalid-name,redefined-builtin
+
+# Test subscripting .split()
+get_first = '1,2,3'.split(',')[0]  # [consider-using-maxsplit-arg]
+get_last = '1,2,3'[::-1].split(',')[0]  # [consider-using-maxsplit-arg]
+
+SEQ = '1,2,3'
+get_first = SEQ.split(',')[0]  # [consider-using-maxsplit-arg]
+get_last = SEQ.split(',')[-1]  # [consider-using-maxsplit-arg]
+get_first = SEQ.rsplit(',')[0]  # [consider-using-maxsplit-arg]
+get_last = SEQ.rsplit(',')[-1]  # [consider-using-maxsplit-arg]
+
+get_mid = SEQ.split(',')[1]  # This is okay
+get_mid = SEQ.split(',')[-2]  # This is okay
+
+
+# Test varying maxsplit argument -- all these will be okay
+## str.split() tests
+good_split = '1,2,3'.split(sep=',', maxsplit=1)[-1]  # This is fine
+good_split = '1,2,3'.split(sep=',', maxsplit=1)[0]  # This is fine
+good_split = '1,2,3'.split(sep=',', maxsplit=2)[-1]  # This is fine
+good_split = '1,2,3'.split(sep=',', maxsplit=2)[0]  # This is fine
+good_split = '1,2,3'.split(sep=',', maxsplit=2)[1]  # This is fine
+
+## str.rsplit() tests
+good_split = '1,2,3'.rsplit(sep=',', maxsplit=1)[-1]  # This is fine
+good_split = '1,2,3'.rsplit(sep=',', maxsplit=1)[0]  # This is fine
+good_split = '1,2,3'.rsplit(sep=',', maxsplit=2)[-1]  # This is fine
+good_split = '1,2,3'.rsplit(sep=',', maxsplit=2)[0]  # This is fine
+good_split = '1,2,3'.rsplit(sep=',', maxsplit=2)[1]  # This is fine
+
+
+# Tests on class attributes
+class Foo():
+    class_str = '1,2,3'
+    def __init__(self):
+        self.my_str = '1,2,3'
+
+    def get_string(self) -> str:
+        return self.my_str
+
+# Class attributes
+get_first = Foo.class_str.split(',')[0]  # [consider-using-maxsplit-arg]
+get_last = Foo.class_str.split(',')[-1]  # [consider-using-maxsplit-arg]
+get_first = Foo.class_str.rsplit(',')[0]  # [consider-using-maxsplit-arg]
+get_last = Foo.class_str.rsplit(',')[-1]  # [consider-using-maxsplit-arg]
+
+get_mid = Foo.class_str.split(',')[1]
+get_mid = Foo.class_str.split(',')[-2]
+
+
+# Test with accessors
+bar = Foo()
+get_first = bar.get_string().split(',')[0]  # [consider-using-maxsplit-arg]
+get_last = bar.get_string().split(',')[-1]  # [consider-using-maxsplit-arg]
+
+get_mid = bar.get_string().split(',')[1]
+get_mid = bar.get_string().split(',')[-2]
+
+
+# Test with iterating over strings
+list_of_strs = ["a", "b", "c", "d", "e", "f"]
+for s in list_of_strs:
+    print(s.split(" ")[0])  # [consider-using-maxsplit-arg]
+    print(s.split(" ")[-1])  # [consider-using-maxsplit-arg]
+    print(s.split(" ")[-2])
+
+
+# Test warning messages (matching and replacing .split / .rsplit)
+class Bar():
+    split = '1,2,3'
+
+print(Bar.split.split(",")[0])  # [consider-using-maxsplit-arg] (Error message should show Bar.split.partition)
+print(Bar.split.split(",")[-1])  # [consider-using-maxsplit-arg] (Error message should show Bar.split.rpartition)
+print(Bar.split.rsplit(",")[0])  # [consider-using-maxsplit-arg] (Error message should show Bar.split.partition)
+print(Bar.split.rsplit(",")[-1])  # [consider-using-maxsplit-arg] (Error message should show Bar.split.rpartition)

tests/functional/c/consider/consider_using_maxsplit_arg.txt

@@ -0,0 +1,18 @@
+consider-using-maxsplit-arg:5:12::Consider using '1,2,3'.split(sep=',',maxsplit=1)[0] instead
+consider-using-maxsplit-arg:6:11::"Consider using '1,2,3'[::-1].split(sep=',',maxsplit=1)[0] instead"
+consider-using-maxsplit-arg:9:12::Consider using SEQ.split(sep=',',maxsplit=1)[0] instead
+consider-using-maxsplit-arg:10:11::Consider using SEQ.rsplit(sep=',',maxsplit=1)[-1] instead
+consider-using-maxsplit-arg:11:12::Consider using SEQ.split(sep=',',maxsplit=1)[0] instead
+consider-using-maxsplit-arg:12:11::Consider using SEQ.rsplit(sep=',',maxsplit=1)[-1] instead
+consider-using-maxsplit-arg:44:12::Consider using Foo.class_str.split(sep=',',maxsplit=1)[0] instead
+consider-using-maxsplit-arg:45:11::Consider using Foo.class_str.rsplit(sep=',',maxsplit=1)[-1] instead
+consider-using-maxsplit-arg:46:12::Consider using Foo.class_str.split(sep=',',maxsplit=1)[0] instead
+consider-using-maxsplit-arg:47:11::Consider using Foo.class_str.rsplit(sep=',',maxsplit=1)[-1] instead
+consider-using-maxsplit-arg:55:12::Consider using bar.get_string().split(sep=',',maxsplit=1)[0] instead
+consider-using-maxsplit-arg:56:11::Consider using bar.get_string().rsplit(sep=',',maxsplit=1)[-1] instead
+consider-using-maxsplit-arg:65:10::Consider using s.split(sep=' ',maxsplit=1)[0] instead
+consider-using-maxsplit-arg:66:10::Consider using s.rsplit(sep=' ',maxsplit=1)[-1] instead
+consider-using-maxsplit-arg:74:6::Consider using Bar.split.split(sep=',',maxsplit=1)[0] instead
+consider-using-maxsplit-arg:75:6::Consider using Bar.split.rsplit(sep=',',maxsplit=1)[-1] instead
+consider-using-maxsplit-arg:76:6::Consider using Bar.split.split(sep=',',maxsplit=1)[0] instead
+consider-using-maxsplit-arg:77:6::Consider using Bar.split.rsplit(sep=',',maxsplit=1)[-1] instead