Understand starred expression used in string formatting RHS tuple (#1299

)

Fixes #957

ChangeLog

@@ -288,6 +288,12 @@ Release date: tba
 
       Fixes #831
 
+    * `too-many-format-args` and `too-few-format-args` are emitted correctly when
+      starred expression are used in RHS tuple.
+
+      Fixes #957
+
+
 What's new in Pylint 1.6.3?
 ===========================
 

doc/whatsnew/2.0.rst

@@ -877,6 +877,16 @@ Bug fixes
 
      class MyError(Exception): pass
 
+  * `too-many-format-args` and `too-few-format-args` are emitted correctly
+    (or not emitted at all, when exact count of elements in RHS cannot be
+    inferred) when starred expressions are used in RHS tuple. For example,
+    code block as shown below detects correctly that the used tuple has in
+    fact three elements, not two.
+
+  .. code-block:: python
+
+    meat = ['spam', 'ham']
+    print('%s%s%s' % ('eggs', *meat))
 
 Removed Changes
 ===============

pylint/checkers/strings.py

@@ -304,7 +304,10 @@ def visit_binop(self, node):
             # the % operator matches the number required by the format
             # string.
             if isinstance(args, astroid.Tuple):
-                num_args = len(args.elts)
+                rhs_tuple = utils.safe_infer(args)
+                num_args = None
+                if rhs_tuple not in (None, astroid.Uninferable):
+                    num_args = len(rhs_tuple.elts)
             elif isinstance(args, OTHER_NODES + (astroid.Dict, astroid.DictComp)):
                 num_args = 1
             else:

pylint/test/functional/string_formatting_py3.py

@@ -0,0 +1,21 @@
+# pylint: disable=missing-docstring,import-error
+
+
+def issue_957_good():
+    meat = ['spam', 'ham']
+    print('%s%s%s' % ('eggs', *meat))
+
+
+def issue_957_bad1():
+    meat = ['spam', 'ham', 'monthy python']
+    print('%s%s%s' % ('eggs', *meat))  # [too-many-format-args]
+
+
+def issue_957_bad2():
+    meat = ['spam']
+    print('%s%s%s' % ('eggs', *meat))  # [too-few-format-args]
+
+
+def issue_957_uninferable():
+    from butchery import meat
+    print('%s%s%s' % ('eggs', *meat))

pylint/test/functional/string_formatting_py3.rc

@@ -0,0 +1,2 @@
+[testoptions]
+min_pyver=3.0

pylint/test/functional/string_formatting_py3.txt

@@ -0,0 +1,2 @@
+too-many-format-args:11:issue_957_bad1:Too many arguments for format string
+too-few-format-args:16:issue_957_bad2:Not enough arguments for format string