Removed adding a space into empty docstrings. (#2249)

Resolves #2168 by disabling the insertion of a " " when the docstring is entirely empty.

Note that this PR is focussed only on the case of empty docstrings. In particular this does not make any changes to the behaviour that a " " is inserted if a non-empty docstring begins with the quoting character. That is, black still prefers:

    """ "something" """

to:

    """"something" """

and that:

    """"Something""""

is not a legal docstring.

CHANGES.md

@@ -4,6 +4,7 @@
 
 ### _Black_
 
+- A space is no longer inserted into empty docstrings (#2249)
 - Fix handling of .gitignore files containing non-ASCII characters on Windows (#2229)
 - Respect `.gitignore` files in all levels, not only `root/.gitignore` file (apply
   `.gitignore` rules like `git` does) (#2225)

src/black/linegen.py

@@ -236,6 +236,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
             # characters but only if they are the same as the first.
             quote_len = 1 if docstring[1] != quote_char else 3
             docstring = docstring[quote_len:-quote_len]
+            docstring_started_empty = not docstring
 
             if is_multiline_string(leaf):
                 indent = " " * 4 * self.current_line.depth
@@ -255,8 +256,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
                         # Odd number of tailing backslashes, add some padding to
                         # avoid escaping the closing string quote.
                         docstring += " "
-            else:
+            elif not docstring_started_empty:
-                # Add some padding if the docstring is empty.
                 docstring = " "
 
             # We could enforce triple quotes at this point.

tests/data/docstring.py

@@ -102,7 +102,7 @@ def and_this():
   "hey yah"'''
 
 
-def empty():
+def multiline_whitespace():
     '''
     
     
@@ -111,11 +111,11 @@ def empty():
     '''
 
 
-def oneline_empty():
+def oneline_whitespace():
     '''      '''
 
 
-def oneline_nothing():
+def empty():
     """"""
 
 
@@ -293,16 +293,16 @@ def and_this():
     "hey yah"'''
 
 
+def multiline_whitespace():
+    """ """
+
+
+def oneline_whitespace():
+    """ """
+
+
 def empty():
-    """ """
+    """"""
-
-
-def oneline_empty():
-    """ """
-
-
-def oneline_nothing():
-    """ """
 
 
 def single_quotes():
@@ -374,4 +374,4 @@ def my_god_its_full_of_stars_1():
 
 # the space below is actually a \u2001, removed in output
 def my_god_its_full_of_stars_2():
-    "I'm sorry Dave"
+    "I'm sorry Dave"