Add redundant-u-string-prefix checker

This adds a checker for u-prefixes for strings, as used in Python 2.
Closes pylint-dev#4102

ChangeLog

@@ -89,10 +89,14 @@ Release date: TBA
 
   Closes #626
 
-* Add ``disable-next`` option: allows using `# pylint: disable-next=msgid` to disable a message for the following line
+* Added ``disable-next`` option: allows using `# pylint: disable-next=msgid` to disable a message for the following line
 
   Closes #1682
 
+* Added ``redundant-u-string-prefix`` checker: Emitted when the u prefix is added to a string
+
+  Closes #4102
+
 
 What's New in Pylint 2.9.6?
 ===========================

doc/whatsnew/2.10.rst

@@ -71,10 +71,14 @@ Other Changes
 
   Closes #626
 
-* Add ``disable-next`` option: allows using `# pylint: disable-next=msgid` to disable a message for the following line
+* Added ``disable-next`` option: allows using `# pylint: disable-next=msgid` to disable a message for the following line
 
   Closes #1682
 
 * Added ``format-string-without-interpolation`` checker: Emitted when formatting is applied to a string without any variables to be replaced
 
   Closes #4042
+
+* Added ``redundant-u-string-prefix`` checker: Emitted when the u prefix is added to a string
+
+  Closes #4102

pylint/checkers/strings.py

@@ -669,6 +669,13 @@ class StringConstantChecker(BaseTokenChecker):
             "Quote delimiters are not used consistently throughout a module "
             "(with allowances made for avoiding unnecessary escaping).",
         ),
+        "W1406": (
+            "The u prefix for strings is no longer necessary in Python >=3.0",
+            "redundant-u-string-prefix",
+            "Used when we detect a string with a u prefix. These prefixes were necessary "
+            "in Python 2 to indicate a string was Unicode, but since Python 3.0 strings "
+            "are Unicode by default.",
+        ),
     }
     options = (
         (
@@ -905,6 +912,22 @@ def process_non_raw_string_token(
             # character can never be the start of a new backslash escape.
             index += 2
 
+    @check_messages("redundant-u-string-prefix")
+    def visit_const(self, node: astroid.Const):
+        if node.pytype() == "builtins.str" and not isinstance(
+            node.parent, astroid.JoinedStr
+        ):
+            self._detect_u_string_prefix(node)
+
+    def _detect_u_string_prefix(self, node: astroid.Const):
+        """Check whether strings include a 'u' prefix like u'String'"""
+        if node.kind == "u":
+            self.add_message(
+                "redundant-u-string-prefix",
+                line=node.lineno,
+                col_offset=node.col_offset,
+            )
+
 
 def register(linter):
     """required method to auto register this checker"""

tests/functional/r/redundant_u_string_prefix.py

@@ -0,0 +1,14 @@
+""""Checks for redundant u-prefixes for strings"""
+# pylint: disable=missing-function-docstring
+
+def print_good():
+    print("String")
+    print(f"String{1 + 1}")
+
+
+def print_bad():
+    print(u"String")  # [redundant-u-string-prefix]
+    print(u'String')  # [redundant-u-string-prefix]
+    print([u"String", u"String2"])  # [redundant-u-string-prefix, redundant-u-string-prefix]
+    print((u"String", u"String2"))  # [redundant-u-string-prefix, redundant-u-string-prefix]
+    print({1: u"String"})  # [redundant-u-string-prefix]

tests/functional/r/redundant_u_string_prefix.txt

@@ -0,0 +1,7 @@
+redundant-u-string-prefix:10:10::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:11:10::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:12:11::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:12:22::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:13:11::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:13:22::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:14:14::The u prefix for strings is no longer necessary in Python >=3.0:HIGH