More fstring bugs -- nested fstring grammar rules

test/simple_source/bug36/10_fstring.py

@@ -69,17 +69,8 @@ fields = ['a', 'b', 'c']
 assert _repr_fn(fields) == ['return xx + f"(a={self.a!r}, b={self.b!r}, c={self.c!r})"']
 
 
-# From Python 3.7 test_fstring. Why this kind of thing matter seems a bit
-# academic, but decompile an equivalent thing. For compatiblity with older
-# Python we'll use "%" instead of a format string
-def f():
-    f'''Not a docstring'''
-def g():
-    '''Not a docstring''' \
-    f''
-
-assert f.__doc__ is None
-assert g.__doc__ is None
+#################################
+# From Python 3.7 test_fstring.py
 
 x = 5
 assert f'{(lambda y:x*y)("8")!r}' == "'88888'"
@@ -92,3 +83,19 @@ except SyntaxError:
     pass
 else:
     assert False, "f'{lambda x:x}' should be a syntax error"
+
+(x, y, width) = ("foo", 2, 10)
+assert f'x={x*y:{width}}' == 'x=foofoo    '
+
+# Why the fact that the distinction of docstring versus stmt is a
+# string expression is important academic, but we will decompile an
+# equivalent thing. For compatiblity with older Python we'll use "%"
+# instead of a format string
+def f():
+    f'''Not a docstring'''
+def g():
+    '''Not a docstring''' \
+    f''
+
+assert f.__doc__ is None
+assert g.__doc__ is None

uncompyle6/semantics/customize36.py

@@ -450,11 +450,18 @@ def customize_for_version36(self, version):
     def n_formatted_value1(node):
         expr = node[0]
         assert expr == "expr"
-        self.in_format_string = True
-        value = self.traverse(expr, indent="")
-        self.in_format_string = False
         conversion = f_conversion(node)
-        f_str = "f%s" % escape_string("{%s%s}" % (value, conversion))
+        if (self.in_format_string):
+            value = self.traverse(expr, indent="")
+            es = escape_string("{%s%s}" % (value, conversion))
+            f_str = "%s" % es
+        else:
+            self.in_format_string = True
+            value = self.traverse(expr, indent="")
+            self.in_format_string = False
+            es = escape_string("{%s%s}" % (value, conversion))
+            f_str = "f%s" % es
+
         self.write(f_str)
         self.prune()
 
@@ -468,7 +475,6 @@ def customize_for_version36(self, version):
         assert expr == "expr"
         self.in_format_string = True
         value = self.traverse(expr, indent="")
-        self.in_format_string = False
         format_value_attr = node[-1]
         assert format_value_attr == "FORMAT_VALUE_ATTR"
         attr = format_value_attr.attr
@@ -483,6 +489,7 @@ def customize_for_version36(self, version):
         else:
             conversion = FSTRING_CONVERSION_MAP.get(attr, "")
 
+        self.in_format_string = False
         f_str = "f%s" % escape_string("{%s%s}" % (value, conversion))
         self.write(f_str)
 
@@ -495,14 +502,14 @@ def customize_for_version36(self, version):
         p = self.prec
         self.prec = 100
 
-        result = ''
+        result = ""
         for expr in node[:-1]:
-            assert expr == 'expr'
-            value = self.traverse(expr, indent='')
+            assert expr == "expr"
+            value = self.traverse(expr, indent="")
             if expr[0].kind.startswith('formatted_value'):
                 # remove leading 'f'
-                assert value.startswith('f')
-                value = value[1:]
+                if value.startswith("f"):
+                    value = value[1:]
                 pass
             else:
                 # {{ and }} in Python source-code format strings mean