Handle python 3.6 f-strings without error (#2622)

Linked to #2265

Author: Adrien Chauve

mypy/fastparse.py

@@ -850,6 +850,26 @@ def visit_Str(self, n: ast3.Str) -> Union[UnicodeExpr, StrExpr]:
         else:
             return UnicodeExpr(n.s)
 
+    # Only available with typed_ast >= 0.6.2
+    if hasattr(ast3, 'JoinedStr'):
+        # JoinedStr(expr* values)
+        @with_line
+        def visit_JoinedStr(self, n: ast3.JoinedStr) -> Expression:
+            result_expression = StrExpr('')  # type: Expression
+            for value_expr in self.translate_expr_list(n.values):
+                string_method = MemberExpr(value_expr, '__str__')
+                string_method.set_line(value_expr)
+                stringified_value_expr = CallExpr(string_method, [], [])
+                stringified_value_expr.set_line(value_expr)
+                result_expression = OpExpr('+', result_expression, stringified_value_expr)
+                result_expression.set_line(value_expr)
+            return result_expression
+
+        # FormattedValue(expr value)
+        @with_line
+        def visit_FormattedValue(self, n: ast3.FormattedValue) -> Expression:
+            return self.visit(n.value)
+
     # Bytes(bytes s)
     @with_line
     def visit_Bytes(self, n: ast3.Bytes) -> Union[BytesExpr, StrExpr]:

test-data/unit/check-newsyntax.test

@@ -99,11 +99,46 @@ main:4: error: Unsupported target for indexed assignment
 main:5: error: Type cannot be declared in assignment to non-self attribute
 main:5: error: "str" has no attribute "x"
 
-[case testNewSyntaxFstringError]
-# flags: --fast-parser --python-version 3.5
-f''  # E: Format strings are only supported in Python 3.6 and greater
-
 [case testNewSyntaxAsyncComprehensionError]
 # flags: --fast-parser --python-version 3.5
 async def f():
     results = [i async for i in aiter() if i % 2]  # E: Async comprehensions are only supported in Python 3.6 and greater
+
+
+[case testNewSyntaxFstringError]
+# flags: --fast-parser --python-version 3.5
+f''  # E: Format strings are only supported in Python 3.6 and greater
+
+[case testNewSyntaxFStringBasics]
+# flags: --fast-parser --python-version 3.6
+f'foobar'
+f'{"foobar"}'
+f'foo{"bar"}'
+f'.{1}.'
+a: str
+a = f'foobar'
+a = f'{"foobar"}'
+[builtins fixtures/primitives.pyi]
+
+[case testNewSyntaxFStringExpressionsOk]
+# flags: --fast-parser --python-version 3.6
+f'.{1 + 1}.'
+f'.{1 + 1}.{"foo" + "bar"}'
+[builtins fixtures/primitives.pyi]
+
+[case testNewSyntaxFStringExpressionsErrors]
+# flags: --fast-parser --python-version 3.6
+f'{1 + ""}'
+f'.{1 + ""}'
+[builtins fixtures/primitives.pyi]
+[out]
+main:2: error: Unsupported operand types for + ("int" and "str")
+main:3: error: Unsupported operand types for + ("int" and "str")
+
+[case testNewSyntaxFStringParseFormatOptions]
+# flags: --fast-parser --python-version 3.6
+value = 10.5142
+width = 10
+precision = 4
+f'result: {value:{width}.{precision}}'
+[builtins fixtures/primitives.pyi]

test-data/unit/fixtures/primitives.pyi

@@ -2,15 +2,18 @@
 
 class object:
     def __init__(self) -> None: pass
+    def __str__(self) -> str: pass
 
 class type:
     def __init__(self, x) -> None: pass
 
-class int: pass
+class int:
+    def __add__(self, i: int) -> int: pass
 class float: pass
 class complex: pass
 class bool: pass
-class str: pass
+class str:
+    def __add__(self, s: str) -> str: pass
 class bytes: pass
 class bytearray: pass
 class tuple: pass