Add ``using-f-string-in-unsupported-version`` checker (#5139)

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

6 files changed, 31 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index e1699f0d2..567eaeaa1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -29,6 +29,9 @@ Release date: TBA
   ``use-implicit-booleaness-not-len`` in order to be consistent with
   ``use-implicit-booleaness-not-comparison``.
 
+* Added ``using-f-string-in-unsupported-version`` checker. Issued when ``py-version``
+  is set to a version that does not support f-strings (< 3.6)
+
 
 What's New in Pylint 2.11.2?
 ============================
diff --git a/doc/whatsnew/2.12.rst b/doc/whatsnew/2.12.rst
index f6d99185a..c143b52a5 100644
--- a/doc/whatsnew/2.12.rst
+++ b/doc/whatsnew/2.12.rst
@@ -27,6 +27,9 @@ New checkers
 
   Closes #3197
 
+* Added ``using-f-string-in-unsupported-version`` checker. Issued when ``py-version``
+  is set to a version that does not support f-strings (< 3.6)
+
 
 Removed checkers
 ================
diff --git a/pylint/checkers/strings.py b/pylint/checkers/strings.py
index f983e8afe..0c93076c5 100644
--- a/pylint/checkers/strings.py
+++ b/pylint/checkers/strings.py
@@ -47,6 +47,7 @@ from astroid import nodes
 from pylint.checkers import BaseChecker, BaseTokenChecker, utils
 from pylint.checkers.utils import check_messages
 from pylint.interfaces import IAstroidChecker, IRawChecker, ITokenChecker
+from pylint.utils import get_global_option
 
 if TYPE_CHECKING:
     from typing import Counter  # typing.Counter added in Python 3.6.1
@@ -211,6 +212,12 @@ MSGS = {  # pylint: disable=consider-using-namedtuple-or-dataclass
         "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.",
     ),
+    "W1311": (
+        "F-strings are not supported by all versions included in the py-version setting",
+        "using-f-string-in-unsupported-version",
+        "Used when the py-version set by the user is lower than 3.6 and pylint encounters "
+        "a f-string.",
+    ),
 }
 
 OTHER_NODES = (
@@ -410,7 +417,12 @@ class StringFormatChecker(BaseChecker):
                         )
 
     @check_messages("f-string-without-interpolation")
+    @check_messages("using-f-string-in-unsupported-version")
     def visit_joinedstr(self, node: nodes.JoinedStr) -> None:
+        self._check_interpolation(node)
+        self._check_unsupported_version(node)
+
+    def _check_interpolation(self, node: nodes.JoinedStr) -> None:
         if isinstance(node.parent, nodes.FormattedValue):
             return
         for value in node.values:
@@ -418,6 +430,10 @@ class StringFormatChecker(BaseChecker):
                 return
         self.add_message("f-string-without-interpolation", node=node)
 
+    def _check_unsupported_version(self, node: nodes.JoinedStr) -> None:
+        if get_global_option(self, "py-version") < (3, 6):
+            self.add_message("using-f-string-in-unsupported-version", node=node)
+
     @check_messages(*MSGS)
     def visit_call(self, node: nodes.Call) -> None:
         func = utils.safe_infer(node.func)
diff --git a/tests/functional/u/unsupported/unsupported_version_for_f_string.py b/tests/functional/u/unsupported/unsupported_version_for_f_string.py
new file mode 100644
index 000000000..c7ae5e385
--- /dev/null
+++ b/tests/functional/u/unsupported/unsupported_version_for_f_string.py
@@ -0,0 +1,5 @@
+"""Tests for the use of f-strings whenever the py-version is set < 3.6"""
+# pylint: disable=f-string-without-interpolation
+
+VAR = f"a simple f-string"  # [using-f-string-in-unsupported-version]
+VAR_TWO = f"a simple f-string {'with'} interpolation"  # [using-f-string-in-unsupported-version]
diff --git a/tests/functional/u/unsupported/unsupported_version_for_f_string.rc b/tests/functional/u/unsupported/unsupported_version_for_f_string.rc
new file mode 100644
index 000000000..d0a4fe2d4
--- /dev/null
+++ b/tests/functional/u/unsupported/unsupported_version_for_f_string.rc
@@ -0,0 +1,2 @@
+[typing]
+py-version=3.5
diff --git a/tests/functional/u/unsupported/unsupported_version_for_f_string.txt b/tests/functional/u/unsupported/unsupported_version_for_f_string.txt
new file mode 100644
index 000000000..a30eda226
--- /dev/null
+++ b/tests/functional/u/unsupported/unsupported_version_for_f_string.txt
@@ -0,0 +1,2 @@
+using-f-string-in-unsupported-version:4:6::F-strings are not supported by all versions included in the py-version setting:HIGH
+using-f-string-in-unsupported-version:5:10::F-strings are not supported by all versions included in the py-version setting:HIGH