diff --git a/src/calibre/ebooks/lrf/html/convert_from.py b/src/calibre/ebooks/lrf/html/convert_from.py
index 6aad099979..e2ce8491d5 100644
--- a/src/calibre/ebooks/lrf/html/convert_from.py
+++ b/src/calibre/ebooks/lrf/html/convert_from.py
@@ -11,6 +11,8 @@ import os, re, sys, copy, glob, tempfile
 from collections import deque
 from math import ceil, floor
 from functools import partial
+from polyglot.builtins import string_or_bytes, itervalues
+from itertools import chain
 
 try:
     from PIL import Image as PILImage
@@ -258,6 +260,8 @@ class HTMLConverter(object):
                 src = open(self._override_css, 'rb').read()
             else:
                 src = self._override_css
+            if isinstance(src, bytes):
+                src = src.decode('utf-8', 'replace')
             match = self.PAGE_BREAK_PAT.search(src)
             if match and not re.match('avoid', match.group(1), re.IGNORECASE):
                 self.page_break_found = True
@@ -1112,6 +1116,8 @@ class HTMLConverter(object):
             ans['sidemargin'] = int((factor*int(self.current_block.blockStyle.attrs['blockwidth']))/2.)
 
         for prop in ('topskip', 'footskip', 'sidemargin'):
+            if isinstance(ans[prop], string_or_bytes):
+                ans[prop] = int(ans[prop])
             if ans[prop] < 0:
                 ans[prop] = 0
 
@@ -1520,9 +1526,8 @@ class HTMLConverter(object):
                 elif (tag.has_attr('type') and tag['type'] in ("text/css", "text/x-oeb1-css") and tag.has_attr('href')):
                     path = munge_paths(self.target_prefix, tag['href'])[0]
                     try:
-                        f = open(path, 'rb')
-                        src = f.read()
-                        f.close()
+                        with open(path, 'rb') as f:
+                            src = f.read().decode('utf-8', 'replace')
                         match = self.PAGE_BREAK_PAT.search(src)
                         if match and not re.match('avoid', match.group(1), re.IGNORECASE):
                             self.page_break_found = True
@@ -1792,7 +1797,7 @@ class HTMLConverter(object):
         self.book.renderLrs(path) if lrs else self.book.renderLrf(path)
 
     def cleanup(self):
-        for _file in self.scaled_images.values() + self.rotated_images.values():
+        for _file in chain(itervalues(self.scaled_images), itervalues(self.rotated_images)):
             _file.__del__()
 
 
diff --git a/src/calibre/ebooks/lrf/pylrs/pylrf.py b/src/calibre/ebooks/lrf/pylrs/pylrf.py
index a1b4a51223..f99eea6be9 100644
--- a/src/calibre/ebooks/lrf/pylrs/pylrf.py
+++ b/src/calibre/ebooks/lrf/pylrs/pylrf.py
@@ -108,11 +108,11 @@ def writeQWord(f, qword):
 
 
 def writeZeros(f, nZeros):
-    f.write("\x00" * nZeros)
+    f.write(b"\0" * nZeros)
 
 
-def writeString(f, str):
-    f.write(str)
+def writeString(f, s):
+    f.write(s)
 
 
 def writeIdList(f, idList):
@@ -177,7 +177,7 @@ def writeRuledLine(f, lineInfo):
     writeColor(f, lineColor)
 
 
-LRF_SIGNATURE = "L\x00R\x00F\x00\x00\x00"
+LRF_SIGNATURE = b"L\x00R\x00F\x00\x00\x00"
 
 # XOR_KEY = 48
 XOR_KEY = 65024  # that's what lrf2lrs says -- not used, anyway...
diff --git a/src/calibre/ebooks/lrf/pylrs/pylrs.py b/src/calibre/ebooks/lrf/pylrs/pylrs.py
index ebe3aa6e37..207de6e6b7 100644
--- a/src/calibre/ebooks/lrf/pylrs/pylrs.py
+++ b/src/calibre/ebooks/lrf/pylrs/pylrs.py
@@ -53,7 +53,7 @@ DEFAULT_GENREADING      = "fs"          # default is yes to both lrf and lrs
 
 from calibre import __appname__, __version__
 from calibre import entity_to_unicode
-from polyglot.builtins import string_or_bytes, unicode_type
+from polyglot.builtins import string_or_bytes, unicode_type, iteritems
 
 
 class LrsError(Exception):
@@ -421,7 +421,7 @@ class Book(Delegator):
         LrsObject.nextObjId += 1
 
         styledefault = StyleDefault()
-        if settings.has_key('setdefault'):  # noqa
+        if 'setdefault' in settings:
             styledefault = settings.pop('setdefault')
         Delegator.__init__(self, [BookInformation(), Main(),
             Template(), Style(styledefault), Solos(), Objects()])
@@ -569,12 +569,12 @@ class Book(Delegator):
 
         text_blocks = list(main.get_all(lambda x: isinstance(x, TextBlock)))
         for tb in text_blocks:
-            if tb.textSettings.has_key('fontsize'):  # noqa
+            if 'fontsize' in tb.textSettings:
                 tb.textSettings['fontsize'] = rescale(tb.textSettings['fontsize'])
             for span in tb.get_all(lambda x: isinstance(x, Span)):
-                if span.attrs.has_key('fontsize'):  # noqa
+                if 'fontsize' in span.attrs:
                     span.attrs['fontsize'] = rescale(span.attrs['fontsize'])
-                if span.attrs.has_key('baselineskip'):  # noqa
+                if 'baselineskip' in span.attrs:
                     span.attrs['baselineskip'] = rescale(span.attrs['baselineskip'])
 
         text_styles = set(tb.textStyle for tb in text_blocks)
@@ -1835,7 +1835,7 @@ class Span(LrsSimpleChar1, LrsContainer):
         oldTextStyle = self.findCurrentTextStyle()
 
         # set the attributes we want changed
-        for (name, value) in self.attrs.items():
+        for (name, value) in tuple(iteritems(self.attrs)):
             if name in oldTextStyle.attrs and oldTextStyle.attrs[name] == self.attrs[name]:
                 self.attrs.pop(name)
             else: