Handle set/dictionary comprehensions in format strings

test/simple_source/bug36/10_fstring.py

@@ -102,7 +102,17 @@ assert g.__doc__ is None
 
 import decimal
 width, precision, value = (10, 4, decimal.Decimal('12.34567'))
+
+# Make sure we don't have additional f'..' inside the format strings below.
 assert f'result: {value:{width}.{precision}}' == 'result:      12.35'
 assert f'result: {value:{width:0}.{precision:1}}' == 'result:      12.35'
 assert f'{2}\t' ==  '2\t'
+
+# But below we *do* need the additional f".."
 assert f'{f"{0}"*3}' == "000"
+
+# We need to make sure we have { {x:... not {{x: ...
+#                               ^
+# The former, {{ confuses the format strings so dictionary/set comprehensions
+# don't work.
+assert f'expr={ {x: y for x, y in [(1, 2), ]}}' == 'expr={1: 2}'

test/stdlib/3.6-exclude.sh

@@ -69,7 +69,6 @@ SKIP_TESTS=(
     [test_fractions.py]=1 # doesn't terminate
     [test_float.py]=1 # it fails on its own
     [test_frame.py]=1 # doesn't terminate
-    [test_fstring.py]=1 # syntax error: Investigate
     [test_functools.py]=1 # it fails on its own
     [test___future__.py]=1 # syntax error: Investigate
 

uncompyle6/semantics/customize36.py

@@ -453,7 +453,13 @@ def customize_for_version36(self, version):
         conversion = f_conversion(node)
         if (self.in_format_string and self.in_format_string != "formatted_value1"):
             value = self.traverse(expr, indent="")
-            es = escape_string("{%s%s}" % (value, conversion))
+            if value[0] == "{":
+                # If we have a set or dictionary comprehension, then we need to add a space
+                # so as not to confuse the format string with {{.
+                fmt = "{ %s%s }"
+            else:
+                fmt = "{%s%s}"
+            es = escape_string( fmt % (value, conversion))
             f_str = "%s" % es
         else:
             old_in_format_string = self.in_format_string