New check: TypeVar name mismatch (#6168)

* TypeVar name mismatch check

* Fix double hashes in changelogs

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

ChangeLog

@@ -10,14 +10,18 @@ Release date: TBA
   Put new features here and also in 'doc/whatsnew/2.14.rst'
 
 
+* Added new checker ``typevar-name-mismatch``: TypeVar must be assigned to a variable with the same name as its name argument.
+
+  Closes #5224
+
 * ``invalid-enum-extension``: Used when a class tries to extend an inherited Enum class.
 
   Closes #5501
 
 * Added new checker ``typevar-double-variance``: The "covariant" and "contravariant" keyword arguments
   cannot both be set to "True" in a TypeVar.
 
-  Closes ##5895
+  Closes #5895
 
 * Re-enable checker ``bad-docstring-quotes`` for Python <= 3.7.
 

doc/data/messages/t/typevar-name-mismatch/bad.py

@@ -0,0 +1,3 @@
+from typing import TypeVar
+
+X = TypeVar("T")  # [typevar-name-mismatch]

doc/data/messages/t/typevar-name-mismatch/good.py

@@ -0,0 +1,3 @@
+from typing import TypeVar
+
+T = TypeVar("T")

doc/whatsnew/2.14.rst

@@ -12,14 +12,18 @@ Summary -- Release highlights
 New checkers
 ============
 
+* Added new checker ``typevar-name-mismatch``: TypeVar must be assigned to a variable with the same name as its name argument.
+
+  Closes #5224
+
 * ``invalid-enum-extension``: Used when a class tries to extend an inherited Enum class.
 
   Closes #5501
 
 * Added new checker ``typevar-double-variance``: The "covariant" and "contravariant" keyword arguments
   cannot both be set to "True" in a TypeVar.
 
-  Closes ##5895
+  Closes #5895
 
 * Add new check ``unnecessary-dunder-call`` for unnecessary dunder method calls.
 

pylint/checkers/base/name_checker/checker.py

@@ -158,12 +158,18 @@ class NameChecker(_BasicChecker):
             "do not require a suffix. The message is also emitted when invariant "
             "variables do have a suffix.",
         ),
-        "C0130": (
+        "C0131": (
             "TypeVar cannot be both covariant and contravariant",
             "typevar-double-variance",
             'Emitted when both the "covariant" and "contravariant" '
             'keyword arguments are set to "True" in a TypeVar.',
         ),
+        "C0132": (
+            'TypeVar name "%s" does not match assigned variable name "%s"',
+            "typevar-name-mismatch",
+            "Emitted when a TypeVar is assigned to a variable "
+            "that does not match its name argument.",
+        ),
         "W0111": (
             "Name %s will become a keyword in Python %s",
             "assign-to-new-keyword",
@@ -376,6 +382,7 @@ def visit_global(self, node: nodes.Global) -> None:
         "assign-to-new-keyword",
         "typevar-name-incorrect-variance",
         "typevar-double-variance",
+        "typevar-name-mismatch",
     )
     def visit_assignname(self, node: nodes.AssignName) -> None:
         """Check module level assigned names."""
@@ -564,12 +571,15 @@ def _check_typevar(self, name: str, node: nodes.AssignName) -> None:
         """Check for TypeVar lint violations."""
         if isinstance(node.parent, nodes.Assign):
             keywords = node.assign_type().value.keywords
+            args = node.assign_type().value.args
         elif isinstance(node.parent, nodes.Tuple):
             keywords = (
                 node.assign_type().value.elts[node.parent.elts.index(node)].keywords
             )
+            args = node.assign_type().value.elts[node.parent.elts.index(node)].args
 
         variance = TypeVarVariance.invariant
+        name_arg = None
         for kw in keywords:
             if variance == TypeVarVariance.double_variant:
                 pass
@@ -586,6 +596,12 @@ def _check_typevar(self, name: str, node: nodes.AssignName) -> None:
                     else TypeVarVariance.double_variant
                 )
 
+            if kw.arg == "name" and isinstance(kw.value, nodes.Const):
+                name_arg = kw.value.value
+
+        if name_arg is None and args and isinstance(args[0], nodes.Const):
+            name_arg = args[0].value
+
         if variance == TypeVarVariance.double_variant:
             self.add_message(
                 "typevar-double-variance",
@@ -624,3 +640,11 @@ def _check_typevar(self, name: str, node: nodes.AssignName) -> None:
                 args=(f'. "{name}" is invariant, use "{suggest_name}" instead'),
                 confidence=interfaces.INFERENCE,
             )
+
+        if name_arg is not None and name_arg != name:
+            self.add_message(
+                "typevar-name-mismatch",
+                node=node,
+                args=(name_arg, name),
+                confidence=interfaces.INFERENCE,
+            )

tests/functional/t/typevar_name_mismatch.py

@@ -0,0 +1,47 @@
+"""Test case for TypeVar name not matching assigned variable name."""
+from typing import TypeVar
+
+# Control examples
+T = TypeVar("T")
+T_co = TypeVar("T_co", covariant=True)
+T_contra = TypeVar("T_contra", contravariant=True)
+
+# Mismatching names
+X = TypeVar("T")  # [typevar-name-mismatch]
+X_co = TypeVar("T_co", covariant=True)  # [typevar-name-mismatch]
+X_contra = TypeVar("T_contra", contravariant=True)  # [typevar-name-mismatch]
+X_co, X_contra = (  # [typevar-name-mismatch,typevar-name-mismatch]
+    TypeVar("T", covariant=True),
+    TypeVar("T", contravariant=True),
+)
+
+# The user may also violate typevar-double-variance
+# or typevar-name-incorrect-variance
+# on top of not matching the variable and TypeVar names.
+# This rule does not suggest what the correct name is
+# (that's already handled by the aforementioned rules),
+# it just highlights that the names don't match.
+X = TypeVar("T", contravariant=True)  # [typevar-name-mismatch,typevar-name-incorrect-variance]
+X = TypeVar(  # [typevar-name-mismatch,typevar-name-incorrect-variance,typevar-double-variance]
+    "T",
+    covariant=True,
+    contravariant=True
+)
+X_co = TypeVar("T_co")  # [typevar-name-mismatch,typevar-name-incorrect-variance]
+X_contra = TypeVar(  # [typevar-name-mismatch,typevar-name-incorrect-variance]
+    "T_contra",
+    covariant=True
+)
+
+# name can be specified as a keyword argument as well.
+T = TypeVar(name="T")
+T_co = TypeVar(name="T_co", covariant=True)
+T_contra = TypeVar(name="T_contra", contravariant=True)
+T_co = TypeVar(covariant=True, name="T_co")
+T_contra = TypeVar(contravariant=True, name="T_contra")
+
+X = TypeVar(name="T")  # [typevar-name-mismatch]
+X_co = TypeVar(name="T_co", covariant=True)  # [typevar-name-mismatch]
+X_contra = TypeVar(name="T_contra", contravariant=True) # [typevar-name-mismatch]
+X_co = TypeVar(covariant=True, name="T_co")  # [typevar-name-mismatch]
+X_contra = TypeVar(contravariant=True, name="T_contra") # [typevar-name-mismatch]

tests/functional/t/typevar_name_mismatch.txt

@@ -0,0 +1,19 @@
+typevar-name-mismatch:10:0:10:1::"TypeVar name ""T"" does not match assigned variable name ""X""":INFERENCE
+typevar-name-mismatch:11:0:11:4::"TypeVar name ""T_co"" does not match assigned variable name ""X_co""":INFERENCE
+typevar-name-mismatch:12:0:12:8::"TypeVar name ""T_contra"" does not match assigned variable name ""X_contra""":INFERENCE
+typevar-name-mismatch:13:0:13:4::"TypeVar name ""T"" does not match assigned variable name ""X_co""":INFERENCE
+typevar-name-mismatch:13:6:13:14::"TypeVar name ""T"" does not match assigned variable name ""X_contra""":INFERENCE
+typevar-name-incorrect-variance:24:0:24:1::"Type variable name does not reflect variance. ""X"" is contravariant, use ""X_contra"" instead":INFERENCE
+typevar-name-mismatch:24:0:24:1::"TypeVar name ""T"" does not match assigned variable name ""X""":INFERENCE
+typevar-double-variance:25:0:25:1::TypeVar cannot be both covariant and contravariant:INFERENCE
+typevar-name-incorrect-variance:25:0:25:1::Type variable name does not reflect variance:INFERENCE
+typevar-name-mismatch:25:0:25:1::"TypeVar name ""T"" does not match assigned variable name ""X""":INFERENCE
+typevar-name-incorrect-variance:30:0:30:4::"Type variable name does not reflect variance. ""X_co"" is invariant, use ""X"" instead":INFERENCE
+typevar-name-mismatch:30:0:30:4::"TypeVar name ""T_co"" does not match assigned variable name ""X_co""":INFERENCE
+typevar-name-incorrect-variance:31:0:31:8::"Type variable name does not reflect variance. ""X_contra"" is covariant, use ""X_co"" instead":INFERENCE
+typevar-name-mismatch:31:0:31:8::"TypeVar name ""T_contra"" does not match assigned variable name ""X_contra""":INFERENCE
+typevar-name-mismatch:43:0:43:1::"TypeVar name ""T"" does not match assigned variable name ""X""":INFERENCE
+typevar-name-mismatch:44:0:44:4::"TypeVar name ""T_co"" does not match assigned variable name ""X_co""":INFERENCE
+typevar-name-mismatch:45:0:45:8::"TypeVar name ""T_contra"" does not match assigned variable name ""X_contra""":INFERENCE
+typevar-name-mismatch:46:0:46:4::"TypeVar name ""T_co"" does not match assigned variable name ""X_co""":INFERENCE
+typevar-name-mismatch:47:0:47:8::"TypeVar name ""T_contra"" does not match assigned variable name ""X_contra""":INFERENCE