diff --git a/pycodestyle.py b/pycodestyle.py
index 83069c50..cbbcf4a5 100755
--- a/pycodestyle.py
+++ b/pycodestyle.py
@@ -422,8 +422,69 @@ def blank_lines(logical_line, blank_lines, indent_level, line_number,
                 top_level_lines, blank_before)
 
 
+def _E203(tokens, count):
+    """Additional check for E203 to allow whitespace in slices"""
+
+    sqbrackets = 0
+    last = 0
+
+    col_count = 0
+
+    in_lambda = False
+    delim_history = []
+
+    for token in tokens:
+        if token.exact_type == tokenize.LSQB:
+            sqbrackets += 1
+        elif token.exact_type == tokenize.RSQB:
+            sqbrackets -= 1
+        if token.exact_type in (
+            tokenize.LBRACE, tokenize.RBRACE, tokenize.RSQB, tokenize.LSQB
+        ):
+            delim_history.append(token.exact_type)
+
+        if token.exact_type == tokenize.NAME and token[1] == "lambda":
+            in_lambda = True
+
+        if token.exact_type == tokenize.COLON and col_count == count:
+
+            # Colon not inside brackets or
+            # following another colon or in lambda expression
+            if sqbrackets <= 0 or last == tokenize.COLON or in_lambda:
+                return True
+
+            # Colon in dict
+            if tokenize.LBRACE not in delim_history:
+                break
+            needs = 0
+            count = 0
+            for d in delim_history[::-1]:
+                if needs == 0:
+                    if d == tokenize.LBRACE:
+                        return True
+                    if d == tokenize.LSQB:
+                        return False
+                    needs = d - 1
+                else:
+                    if d == needs:
+                        if count == 0:
+                            needs = 0
+                        else:
+                            count -= 1
+                    if d == needs + 1:
+                        count += 1
+            break
+
+        if token.exact_type == tokenize.COLON:
+            col_count += 1
+            in_lambda = False
+        last = token.exact_type
+
+    return False
+
+
 @register_check
-def extraneous_whitespace(logical_line):
+def extraneous_whitespace(logical_line, tokens):
     r"""Avoid extraneous whitespace.
 
     Avoid extraneous whitespace in these situations:
@@ -442,7 +503,8 @@ 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
     """
-    line = logical_line
+    line: str = logical_line
+    colons = [m.end() for m in re.compile(r":").finditer(line)]
     for match in EXTRANEOUS_WHITESPACE_REGEX.finditer(line):
         text = match.group()
         char = text.strip()
@@ -451,6 +513,9 @@ def extraneous_whitespace(logical_line):
             # assert char in '([{'
             yield found + 1, "E201 whitespace after '%s'" % char
         elif line[found - 1] != ',':
+            if (char == ":" and line[found - 1] != " " and
+                    not _E203(tokens, colons.index(match.end()))):
+                continue
             code = ('E202' if char in '}])' else 'E203')  # if char in ',;:'
             yield found, f"{code} whitespace before '{char}'"
 
diff --git a/testsuite/E20.py b/testsuite/E20.py
index 20c6dfd8..bd20ebe0 100644
--- a/testsuite/E20.py
+++ b/testsuite/E20.py
@@ -69,10 +69,28 @@
 if x == 4:
     print x, y
     x, y = y	, x
+#: E203:1:20
+ham[lower + offset  : upper + offset]
+#: E203:1:12
+ham[lower : : upper]
+#: E203:1:14
+ham[(lambda x : x)(5) :]
+#: E203:1:9
+ham[{"a" : 6}["a"] :]
+#: E203:1:38
+ham[{"a": [1, {}, {"a": [{}, []], "b" :[3]}, 3][1 : 3]}["a"] :]
 #: 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[lower:upper], ham[lower:upper:], ham[lower::step]
+ham[: upper_fn(x) : step_fn(x)], ham[:: step_fn(x)]
+ham[{"a": [1, {}, {"a": [{}, []], "b": [3]}, 3][1 : 3]}["a"] :]
+append_leaves(
+    new_line, line, LL[string_idx + 1 : rpar_idx] + LL[rpar_idx + 1 :]
+)
+if token.PERCENT in {leaf.type for leaf in LL[idx - 1 : next_idx]}
 #:
diff --git a/testsuite/python35.py b/testsuite/python35.py
index 1bedee5d..d653fc85 100644
--- a/testsuite/python35.py
+++ b/testsuite/python35.py
@@ -4,3 +4,5 @@ def bar(a, b)->int:
 #: Okay
 def baz(a, b) -> int:
     pass
+#: E203
+a : int