add new error E204 for whitespace after decorator @

docs/intro.rst

@@ -262,6 +262,8 @@ This is the current list of error and warning codes:
 +------------+----------------------------------------------------------------------+
 | E203       | whitespace before ',', ';', or ':'                                   |
 +------------+----------------------------------------------------------------------+
+| E204       | whitespace after decorator '@'                                       |
++------------+----------------------------------------------------------------------+
 +------------+----------------------------------------------------------------------+
 | E211       | whitespace before '('                                                |
 +------------+----------------------------------------------------------------------+

pycodestyle.py

@@ -120,6 +120,7 @@
 ERRORCODE_REGEX = re.compile(r'\b[A-Z]\d{3}\b')
 DOCSTRING_REGEX = re.compile(r'u?r?["\']')
 EXTRANEOUS_WHITESPACE_REGEX = re.compile(r'[\[({][ \t]|[ \t][\]}),;:](?!=)')
+WHITESPACE_AFTER_DECORATOR_REGEX = re.compile(r'@\s')
 WHITESPACE_AFTER_COMMA_REGEX = re.compile(r'[,;:]\s*(?:  |\t)')
 COMPARE_SINGLETON_REGEX = re.compile(r'(\bNone|\bFalse|\bTrue)?\s*([=!]=)'
                                      r'\s*(?(1)|(None|False|True))\b')
@@ -438,6 +439,9 @@ def extraneous_whitespace(logical_line):
     E203: if x == 4: print x, y; x, y = y , x
     E203: if x == 4: print x, y ; x, y = y, x
     E203: if x == 4 : print x, y; x, y = y, x
+
+    Okay: @decorator
+    E204: @ decorator
     """
     line = logical_line
     for match in EXTRANEOUS_WHITESPACE_REGEX.finditer(line):
@@ -451,6 +455,9 @@ def extraneous_whitespace(logical_line):
             code = ('E202' if char in '}])' else 'E203')  # if char in ',;:'
             yield found, f"{code} whitespace before '{char}'"
 
+    if WHITESPACE_AFTER_DECORATOR_REGEX.match(logical_line):
+        yield 1, "E204 whitespace after decorator '@'"
+
 
 @register_check
 def whitespace_around_keywords(logical_line):

testing/data/E20.py

@@ -75,4 +75,21 @@
     x, y = y, x
 a[b1, :] == a[b1, ...]
 b = a[:, b1]
+#: E204:1:2
+@ decorator
+def f():
+    pass
+#: E204:1:2
+@	decorator
+def f():
+    pass
+#: E204:1:2
+@         decorator
+def f():
+    pass
+#: E204:2:6
+if True:
+    @ decorator
+    def f():
+        pass
 #: