diff --git a/mitmproxy/console/select.py b/mitmproxy/console/select.py
index 928a7ca5e..7c6d1e965 100644
--- a/mitmproxy/console/select.py
+++ b/mitmproxy/console/select.py
@@ -72,7 +72,8 @@ class Heading:
         return opt
 
 
-_neg = lambda: False
+def _neg(*args):
+    return False
 
 
 class Option:
diff --git a/mitmproxy/flow.py b/mitmproxy/flow.py
index 997a78305..80569a3c6 100644
--- a/mitmproxy/flow.py
+++ b/mitmproxy/flow.py
@@ -418,12 +418,16 @@ class FlowList(object):
         return
 
 
+def _pos(*args):
+    return True
+
+
 class FlowView(FlowList):
 
     def __init__(self, store, filt=None):
         super(FlowView, self).__init__()
         if not filt:
-            filt = lambda flow: True
+            filt = _pos
         self._build(store, filt)
 
         self.store = store
diff --git a/netlib/http/headers.py b/netlib/http/headers.py
index 5cf15b6fb..6165fd616 100644
--- a/netlib/http/headers.py
+++ b/netlib/http/headers.py
@@ -2,11 +2,6 @@ from __future__ import absolute_import, print_function, division
 
 import re
 
-try:
-    from collections.abc import MutableMapping
-except ImportError:  # pragma: no cover
-    from collections import MutableMapping  # Workaround for Python < 3.3
-
 import six
 from ..multidict import MultiDict
 from ..utils import always_bytes