Fix mangling pweave and Spyder IDE special comments

Fixes #532.

README.md

@@ -953,6 +953,9 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 * cache is now populated when `--check` is successful for a file which speeds up
   consecutive checks of properly formatted unmodified files (#448)
 
+* fixed mangling [pweave](http://mpastell.com/pweave/) and
+  [Spyder IDE](https://pythonhosted.org/spyder/) special comments (#532)
+
 * fixed unstable formatting when unpacking big tuples (#267)
 
 * fixed parsing of `__future__` imports with renames (#389)

black.py

@@ -2082,8 +2082,8 @@ def list_comments(prefix: str, *, is_endmarker: bool) -> List[ProtoComment]:
 def make_comment(content: str) -> str:
     """Return a consistently formatted comment from the given `content` string.
 
-    All comments (except for "##", "#!", "#:") should have a single space between
+    All comments (except for "##", "#!", "#:", '#'", "#%%") should have a single
-    the hash sign and the content.
+    space between the hash sign and the content.
 
     If `content` didn't start with a hash sign, one is provided.
     """
@@ -2093,7 +2093,7 @@ def make_comment(content: str) -> str:
 
     if content[0] == "#":
         content = content[1:]
-    if content and content[0] not in " !:#":
+    if content and content[0] not in " !:#'%":
         content = " " + content
     return "#" + content
 

tests/data/comments.py

@@ -70,6 +70,9 @@ def __init__(self):
         """Docstring for instance attribute spam."""
 
 
+#' <h1>This is pweave!</h1>
+
+
 @fast(really=True)
 async def wat():
     async with X.open_async() as x:  # Some more comments

tests/data/comments3.py

@@ -1,3 +1,6 @@
+# The percent-percent comments are Spyder IDE cells.
+
+#%%
 def func():
     x = """
     a really long string
@@ -39,3 +42,6 @@ def func():
         # shared between sub-exceptions are not omitted
         _seen=set(_seen),
     )
+
+
+#%%