Fix false alarm in E203 rule.

E203 checks for extraneous whitespace before a colon.
Yet the colon can be used as a "slice" operator, and as such PEP8 asks for 0 or 1 whitespaces on both sides of the operator.

Author: DesbyP

pycodestyle.py

@@ -413,6 +413,12 @@ def extraneous_whitespace(logical_line):
     - Immediately inside parentheses, brackets or braces.
     - Immediately before a comma, semicolon, or colon.
 
+    Exceptions:
+    - When the colon acts as a slice, the rule of binary operators
+      applies and we should have the same amount of space on either side
+    - When the colon acts as a slice but a parameter is omitted, then
+      the space is omitted
+
     Okay: spam(ham[1], {eggs: 2})
     E201: spam( ham[1], {eggs: 2})
     E201: spam(ham[ 1], {eggs: 2})
@@ -421,10 +427,32 @@ def extraneous_whitespace(logical_line):
     E202: spam(ham[1 ], {eggs: 2})
     E202: spam(ham[1], {eggs: 2 })
 
+    Okay: ham[8 : 2]
+    Okay: ham[: 2]
     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
     """
+
+    def is_a_slice(line, colon_position):
+        """Check colon acts as a slice
+
+        Return True if the colon is directly contained within
+        square brackets.
+        """
+        parentheses_brackets_braces = list()
+        for i in range(colon_position):
+            c = line[i]
+            if c in '[({':
+                parentheses_brackets_braces += c
+            elif c in '])}':
+                last_opened = parentheses_brackets_braces.pop()
+                expected_close = {'{': '}', '(': ')', '[': ']'}[last_opened]
+                if c != expected_close:  # invalid Python code
+                    return False
+        return (len(parentheses_brackets_braces) > 0 and
+                parentheses_brackets_braces.pop() == '[')
+
     line = logical_line
     for match in EXTRANEOUS_WHITESPACE_REGEX.finditer(line):
         text = match.group()
@@ -433,7 +461,8 @@ def extraneous_whitespace(logical_line):
         if text == char + ' ':
             # assert char in '([{'
             yield found + 1, "E201 whitespace after '%s'" % char
-        elif line[found - 1] != ',':
+        elif (line[found - 1] != ',' and
+              not (char == ':' and is_a_slice(line, found))):
             code = ('E202' if char in '}])' else 'E203')  # if char in ',;:'
             yield found, "%s whitespace before '%s'" % (code, char)
 

testsuite/E20.py

@@ -46,10 +46,16 @@
 if x == 4:
     print x, y
     x, y = y , x
+#: E203:1:37
+foo[idxs[2 : 6] : spam(ham[1], {eggs : a[2 : 4]})]
 #: Okay
 if x == 4:
     print x, y
     x, y = y, x
 a[b1, :] == a[b1, ...]
 b = a[:, b1]
+ham[1:9], ham[1:9:3], ham[:9:3], ham[1::3], ham[1:9:]
+ham[: upper_fn(x) : step_fn(x)], ham[:: step_fn(x)]
+ham[lower + offset : upper + offset]
+foo[idxs[2 : 6] : spam(ham[1], {eggs: a[2 : 4]})]
 #: