Add format-string-without-interpolation checker (#4794)

* Add ``format-string-without-interpolation`` checker
This adds a checker that checks strings with '%' or format() applied to them.
If no variables to be replaced are found the warning is emitted.
Closes #4042

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

ChangeLog

@@ -75,6 +75,10 @@ Release date: TBA
   Closes #3608
   Closes #4346
 
+* Added ``format-string-without-interpolation`` checker: Emitted when formatting is applied to a string without any variables to be replaced
+
+  Closes #4042
+
 
 * Refactor of ``--list-msgs`` & ``--list-msgs-enabled``: both options now show whether messages are emittable with the current interpreter.
 

doc/whatsnew/2.10.rst

@@ -70,3 +70,7 @@ Other Changes
   in an exception handler, but used outside of the handler.
 
   Closes #626
+
+* Added ``format-string-without-interpolation`` checker: Emitted when formatting is applied to a string without any variables to be replaced
+
+  Closes #4042

pylint/checkers/strings.py

@@ -204,6 +204,12 @@
         "Used when we detect an f-string that does not use any interpolation variables, "
         "in which case it can be either a normal string or a bug in the code.",
     ),
+    "W1310": (
+        "Using formatting for a string that does not have any interpolated variables",
+        "format-string-without-interpolation",
+        "Used when we detect a string that does not have any interpolation variables, "
+        "in which case it can be either a normal string without formatting or a bug in the code.",
+    ),
 }
 
 OTHER_NODES = (
@@ -273,6 +279,7 @@ class StringFormatChecker(BaseChecker):
         "too-many-format-args",
         "too-few-format-args",
         "bad-string-format-type",
+        "format-string-without-interpolation",
     )
     def visit_binop(self, node):
         if node.op != "%":
@@ -301,6 +308,9 @@ def visit_binop(self, node):
         except utils.IncompleteFormatString:
             self.add_message("truncated-format-string", node=node)
             return
+        if not required_keys and not required_num_args:
+            self.add_message("format-string-without-interpolation", node=node)
+            return
         if required_keys and required_num_args:
             # The format string uses both named and unnamed format
             # specifiers.
@@ -518,6 +528,9 @@ def _check_new_format(self, node, func):
         if check_args:
             # num_args can be 0 if manual_pos is not.
             num_args = num_args or manual_pos
+            if not num_args:
+                self.add_message("format-string-without-interpolation", node=node)
+                return
             if len(positional_arguments) > num_args:
                 self.add_message("too-many-format-args", node=node)
             elif len(positional_arguments) < num_args:

pylint/config/__init__.py

@@ -115,8 +115,7 @@ def find_pylintrc():
 
 PYLINTRC = find_pylintrc()
 
-ENV_HELP = (
-    """
+ENV_HELP = """
 The following environment variables are used:
     * PYLINTHOME
     Path to the directory where persistent data for the run will be stored. If
@@ -126,5 +125,3 @@ def find_pylintrc():
     Path to the configuration file. See the documentation for the method used
 to search for configuration file.
 """
-    % globals()  # type: ignore
-)

tests/functional/s/string/string_formatting.py

@@ -65,7 +65,7 @@ def print_good():
 
 def pprint_bad():
     """Test string format """
-    "{{}}".format(1) # [too-many-format-args]
+    "{{}}".format(1) # [format-string-without-interpolation]
     "{} {".format() # [bad-format-string]
     "{} }".format() # [bad-format-string]
     "{0} {}".format(1, 2) # [format-combined-specification]
@@ -98,6 +98,10 @@ def pprint_bad():
     "{[0]} {}".format([4], 5, 6) # [too-many-format-args]
     logging.debug("%s %s", 42) # [logging-too-few-args]
     logging.debug("%s", 42, 43) # [logging-too-many-args]
+    "String".format(1)  # [format-string-without-interpolation]
+    "String".format(())  # [format-string-without-interpolation]
+    "String".format([])  # [format-string-without-interpolation]
+    "String".format(None)  # [format-string-without-interpolation]
 
 
 def good_issue288(*args, **kwargs):

tests/functional/s/string/string_formatting.txt

@@ -1,4 +1,4 @@
-too-many-format-args:68:4:pprint_bad:Too many arguments for format string
+format-string-without-interpolation:68:4:pprint_bad:Using formatting for a string that does not have any interpolated variables
 bad-format-string:69:4:pprint_bad:Invalid format string
 bad-format-string:70:4:pprint_bad:Invalid format string
 format-combined-specification:71:4:pprint_bad:Format string contains both automatic field numbering and manual field specification
@@ -31,13 +31,17 @@ too-few-format-args:97:4:pprint_bad:Not enough arguments for format string
 too-many-format-args:98:4:pprint_bad:Too many arguments for format string
 logging-too-few-args:99:4:pprint_bad:Not enough arguments for logging format string
 logging-too-many-args:100:4:pprint_bad:Too many arguments for logging format string
-too-few-format-args:128:4:nested_issue294:Not enough arguments for format string
-too-many-format-args:129:4:nested_issue294:Too many arguments for format string
-missing-format-argument-key:130:4:nested_issue294:Missing keyword argument 'a' for format string
-missing-format-attribute:131:4:nested_issue294:Missing format attribute 'x' in format specifier 'a.x'
-too-few-format-args:137:4:issue310:Not enough arguments for format string
-too-many-format-args:145:4:issue322:Too many arguments for format string
-too-few-format-args:146:4:issue322:Not enough arguments for format string
-too-few-format-args:171:4:issue351:Not enough arguments for format string
-too-many-format-args:173:4:issue351:Too many arguments for format string
-bad-format-string:209:11:avoid_empty_attribute:Invalid format string
+format-string-without-interpolation:101:4:pprint_bad:Using formatting for a string that does not have any interpolated variables
+format-string-without-interpolation:102:4:pprint_bad:Using formatting for a string that does not have any interpolated variables
+format-string-without-interpolation:103:4:pprint_bad:Using formatting for a string that does not have any interpolated variables
+format-string-without-interpolation:104:4:pprint_bad:Using formatting for a string that does not have any interpolated variables
+too-few-format-args:132:4:nested_issue294:Not enough arguments for format string
+too-many-format-args:133:4:nested_issue294:Too many arguments for format string
+missing-format-argument-key:134:4:nested_issue294:Missing keyword argument 'a' for format string
+missing-format-attribute:135:4:nested_issue294:Missing format attribute 'x' in format specifier 'a.x'
+too-few-format-args:141:4:issue310:Not enough arguments for format string
+too-many-format-args:149:4:issue322:Too many arguments for format string
+too-few-format-args:150:4:issue322:Not enough arguments for format string
+too-few-format-args:175:4:issue351:Not enough arguments for format string
+too-many-format-args:177:4:issue351:Too many arguments for format string
+bad-format-string:213:11:avoid_empty_attribute:Invalid format string

tests/functional/s/string/string_formatting_error.py

@@ -18,3 +18,7 @@ def pprint():
     print("%2z" % PARG_1)  # [bad-format-character]
     print("strange format %2" % PARG_2) # [truncated-format-string]
     print("works in 3 %a" % 1)
+    print("String" % PARG_1) # [format-string-without-interpolation]
+    print("String" % ())  # [format-string-without-interpolation]
+    print("String" % [])  # [format-string-without-interpolation]
+    print("String" % None)  # [format-string-without-interpolation]

tests/functional/s/string/string_formatting_error.txt

@@ -9,3 +9,7 @@ format-needs-mapping:16:10:pprint:Expected mapping for format string, not Tuple
 format-needs-mapping:17:10:pprint:Expected mapping for format string, not List
 bad-format-character:18:10:pprint:Unsupported format character 'z' (0x7a) at index 2
 truncated-format-string:19:10:pprint:Format string ends in middle of conversion specifier
+format-string-without-interpolation:21:10:pprint:Using formatting for a string that does not have any interpolated variables
+format-string-without-interpolation:22:10:pprint:Using formatting for a string that does not have any interpolated variables
+format-string-without-interpolation:23:10:pprint:Using formatting for a string that does not have any interpolated variables
+format-string-without-interpolation:24:10:pprint:Using formatting for a string that does not have any interpolated variables