Parse each format-string component separately (#3390)

Fixes #3385.

The old code evaluated each FormattedValue as an expression potentially
returning the type of the value, rather than a string. But that's wrong,
because a FormattedValue can exist on its own when it's not being joined
to any other format strings. The new code evaluates each FormattedValue
by synthesizing '{}'.format(expr) and then, if necessary, joins them in
JoinedStr using ''.join(items).

mypy/fastparse.py

@@ -858,22 +858,35 @@ def visit_Str(self, n: ast3.Str) -> Union[UnicodeExpr, StrExpr]:
         # JoinedStr(expr* values)
         @with_line
         def visit_JoinedStr(self, n: ast3.JoinedStr) -> Expression:
-            arg_count = len(n.values)
-            format_string = StrExpr('{}' * arg_count)
-            format_string.set_line(n.lineno, n.col_offset)
-            format_method = MemberExpr(format_string, 'format')
-            format_method.set_line(format_string)
-            format_args = self.translate_expr_list(n.values)
-            format_arg_kinds = [ARG_POS] * arg_count
-            result_expression = CallExpr(format_method,
-                                         format_args,
-                                         format_arg_kinds)
+            # Each of n.values is a str or FormattedValue; we just concatenate
+            # them all using ''.join.
+            empty_string = StrExpr('')
+            empty_string.set_line(n.lineno, n.col_offset)
+            strs_to_join = ListExpr(self.translate_expr_list(n.values))
+            strs_to_join.set_line(empty_string)
+            join_method = MemberExpr(empty_string, 'join')
+            join_method.set_line(empty_string)
+            result_expression = CallExpr(join_method,
+                                         [strs_to_join],
+                                         [ARG_POS])
             return result_expression
 
         # FormattedValue(expr value)
         @with_line
         def visit_FormattedValue(self, n: ast3.FormattedValue) -> Expression:
-            return self.visit(n.value)
+            # A FormattedValue is a component of a JoinedStr, or it can exist
+            # on its own. We translate them to individual '{}'.format(value)
+            # calls -- we don't bother with the conversion/format_spec fields.
+            exp = self.visit(n.value)
+            exp.set_line(n.lineno, n.col_offset)
+            format_string = StrExpr('{}')
+            format_string.set_line(n.lineno, n.col_offset)
+            format_method = MemberExpr(format_string, 'format')
+            format_method.set_line(format_string)
+            result_expression = CallExpr(format_method,
+                                         [exp],
+                                         [ARG_POS])
+            return result_expression
 
     # Bytes(bytes s)
     @with_line

test-data/unit/check-newsyntax.test

@@ -119,19 +119,19 @@ f'{type(1)}'
 a: str
 a = f'foobar'
 a = f'{"foobar"}'
-[builtins fixtures/primitives.pyi]
+[builtins fixtures/f_string.pyi]
 
 [case testNewSyntaxFStringExpressionsOk]
 # flags: --python-version 3.6
 f'.{1 + 1}.'
 f'.{1 + 1}.{"foo" + "bar"}'
-[builtins fixtures/primitives.pyi]
+[builtins fixtures/f_string.pyi]
 
 [case testNewSyntaxFStringExpressionsErrors]
 # flags: --python-version 3.6
 f'{1 + ""}'
 f'.{1 + ""}'
-[builtins fixtures/primitives.pyi]
+[builtins fixtures/f_string.pyi]
 [out]
 main:2: error: Unsupported operand types for + ("int" and "str")
 main:3: error: Unsupported operand types for + ("int" and "str")
@@ -142,4 +142,12 @@ value = 10.5142
 width = 10
 precision = 4
 f'result: {value:{width}.{precision}}'
-[builtins fixtures/primitives.pyi]
+[builtins fixtures/f_string.pyi]
+
+[case testNewSyntaxFStringSingleField]
+# flags: --python-version 3.6
+v = 1
+reveal_type(f'{v}') # E: Revealed type is 'builtins.str'
+reveal_type(f'{1}') # E: Revealed type is 'builtins.str'
+[builtins fixtures/f_string.pyi]
+

test-data/unit/fixtures/f_string.pyi

@@ -0,0 +1,36 @@
+# Builtins stub used for format-string-related test cases.
+# We need str and list, and str needs join and format methods.
+
+from typing import TypeVar, Generic, Iterable, Iterator, List, overload
+
+T = TypeVar('T')
+
+class object:
+    def __init__(self): pass
+
+class type:
+    def __init__(self, x) -> None: pass
+
+class ellipsis: pass
+
+class list(Iterable[T], Generic[T]):
+    @overload
+    def __init__(self) -> None: pass
+    @overload
+    def __init__(self, x: Iterable[T]) -> None: pass
+    def append(self, x: T) -> None: pass
+
+class tuple(Generic[T]): pass
+
+class function: pass
+class int:
+    def __add__(self, i: int) -> int: pass
+
+class float: pass
+class bool(int): pass
+
+class str:
+    def __add__(self, s: str) -> str: pass
+    def format(self, *args) -> str: pass
+    def join(self, l: List[str]) -> str: pass
+