diff --git a/src/pylorax/__init__.py b/src/pylorax/__init__.py
index 01f3d65c..7d947831 100644
--- a/src/pylorax/__init__.py
+++ b/src/pylorax/__init__.py
@@ -33,21 +33,19 @@ import locale
 from subprocess import CalledProcessError
 import selinux
 
-from base import BaseLoraxClass, DataHolder
-import output
+from pylorax.base import BaseLoraxClass, DataHolder
+import pylorax.output as output
 
 import yum
-import ltmpl
 
-import imgutils
-from sysutils import *
+from pylorax.sysutils import joinpaths, remove, linktree
 from rpmUtils.arch import getBaseArch
 
-from treebuilder import RuntimeBuilder, TreeBuilder
-from buildstamp import BuildStamp
-from treeinfo import TreeInfo
-from discinfo import DiscInfo
-from executils import runcmd, runcmd_output
+from pylorax.treebuilder import RuntimeBuilder, TreeBuilder
+from pylorax.buildstamp import BuildStamp
+from pylorax.treeinfo import TreeInfo
+from pylorax.discinfo import DiscInfo
+from pylorax.executils import runcmd, runcmd_output
 
 # List of drivers to remove on ppc64 arch to keep initrd < 32MiB
 REMOVE_PPC64_DRIVERS = "floppy scsi_debug nouveau radeon cirrus mgag200"
@@ -60,6 +58,7 @@ class ArchData(DataHolder):
                     arm="arm", armhfp="arm")
 
     def __init__(self, buildarch):
+        super(ArchData, self).__init__()
         self.buildarch = buildarch
         self.basearch = getBaseArch(buildarch)
         self.libdir = "lib64" if self.basearch in self.lib64_arches else "lib"
@@ -70,6 +69,13 @@ class Lorax(BaseLoraxClass):
     def __init__(self):
         BaseLoraxClass.__init__(self)
         self._configured = False
+        self.product = None
+        self.workdir = None
+        self.arch = None
+        self.conf = None
+        self.inroot = None
+        self.debug = False
+        self.outputdir = None
 
         # set locale to C
         locale.setlocale(locale.LC_ALL, 'C')
@@ -129,7 +135,7 @@ class Lorax(BaseLoraxClass):
 
         # remove some environmental variables that can cause problems with package scripts
         env_remove = ('DISPLAY', 'DBUS_SESSION_BUS_ADDRESS')
-        [os.environ.pop(k) for k in env_remove if k in os.environ]
+        map(os.environ.pop, (k for k in env_remove if k in os.environ))
 
         self._configured = True
 
@@ -229,7 +235,7 @@ class Lorax(BaseLoraxClass):
         product = DataHolder(name=product, version=version, release=release,
                              variant=variant, bugurl=bugurl, isfinal=isfinal)
         self.product = product
-        logger.debug("product data: %s" % product)
+        logger.debug("product data: %s", product)
 
         # NOTE: if you change isolabel, you need to change pungi to match, or
         # the pungi images won't boot.
diff --git a/src/pylorax/base.py b/src/pylorax/base.py
index 651c5b17..b7593b95 100644
--- a/src/pylorax/base.py
+++ b/src/pylorax/base.py
@@ -22,7 +22,7 @@
 from abc import ABCMeta, abstractmethod
 import sys
 
-import output
+import pylorax.output as output
 
 
 class BaseLoraxClass(object):
diff --git a/src/pylorax/executils.py b/src/pylorax/executils.py
index 6da74c8e..c837de47 100644
--- a/src/pylorax/executils.py
+++ b/src/pylorax/executils.py
@@ -112,7 +112,7 @@ def execWithRedirect(command, argv, stdin = None, stdout = None,
     elif stderr is None or not isinstance(stderr, file):
         stderr = sys.stderr.fileno()
 
-    program_log.info("Running... %s" % (" ".join([command] + argv),))
+    program_log.info("Running... %s", " ".join([command] + argv))
 
     #prepare os pipes for feeding tee proceses
     pstdout, pstdin = os.pipe()
@@ -124,9 +124,9 @@ def execWithRedirect(command, argv, stdin = None, stdout = None,
     if root:
         preexec_fn = chroot
         cwd = root
-        program_log.info("chrooting into %s" % (cwd,))
+        program_log.info("chrooting into %s", cwd)
     elif cwd:
-        program_log.info("chdiring into %s" % (cwd,))
+        program_log.info("chdiring into %s", cwd)
 
     try:
         #prepare tee proceses
@@ -173,7 +173,7 @@ def execWithRedirect(command, argv, stdin = None, stdout = None,
         stdinclose()
         stdoutclose()
         stderrclose()
-        raise RuntimeError, errstr
+        raise RuntimeError(errstr)
 
     if ret and raise_err:
         raise subprocess.CalledProcessError(ret, [command]+argv)
@@ -223,7 +223,7 @@ def execWithCapture(command, argv, stdin = None, stderr = None, root=None,
     elif stderr is None or not isinstance(stderr, file):
         stderr = sys.stderr.fileno()
 
-    program_log.info("Running... %s" % (" ".join([command] + argv),))
+    program_log.info("Running... %s", " ".join([command] + argv))
 
     env = os.environ.copy()
     env.update({"LC_ALL": "C"})
@@ -231,9 +231,9 @@ def execWithCapture(command, argv, stdin = None, stderr = None, root=None,
     if root:
         preexec_fn = chroot
         cwd = root
-        program_log.info("chrooting into %s" % (cwd,))
+        program_log.info("chrooting into %s", cwd)
     elif cwd:
-        program_log.info("chdiring into %s" % (cwd,))
+        program_log.info("chdiring into %s", cwd)
 
     try:
         proc = subprocess.Popen([command] + argv, stdin=stdin,
@@ -254,9 +254,9 @@ def execWithCapture(command, argv, stdin = None, stderr = None, root=None,
             if proc.returncode is not None:
                 break
     except OSError as e:
-        log.error ("Error running " + command + ": " + e.strerror)
+        log.error("Error running %s: %s", command, e.strerror)
         closefds()
-        raise RuntimeError, "Error running " + command + ": " + e.strerror
+        raise RuntimeError("Error running %s: %s" % (command, e.strerror))
 
     closefds()
     if proc.returncode and raise_err:
@@ -267,9 +267,6 @@ def execWithCapture(command, argv, stdin = None, stderr = None, root=None,
 def execWithCallback(command, argv, stdin = None, stdout = None,
                      stderr = None, echo = True, callback = None,
                      callback_data = None, root = '/'):
-    def chroot():
-        os.chroot(root)
-
     def closefds ():
         stdinclose()
         stdoutclose()
@@ -305,7 +302,7 @@ def execWithCallback(command, argv, stdin = None, stdout = None,
     elif stderr is None or not isinstance(stderr, file):
         stderr = sys.stderr.fileno()
 
-    program_log.info("Running... %s" % (" ".join([command] + argv),))
+    program_log.info("Running... %s", " ".join([command] + argv))
 
     p = os.pipe()
     p_stderr = os.pipe()
@@ -333,7 +330,7 @@ def execWithCallback(command, argv, stdin = None, stdout = None,
         except OSError as e:
             if e.errno != 4:
                 map(program_log.info, log_output.splitlines())
-                raise IOError, e.args
+                raise IOError(e.args)
 
         if echo:
             os.write(stdout, s)
@@ -349,7 +346,7 @@ def execWithCallback(command, argv, stdin = None, stdout = None,
             if pid != 0:
                 break
         except OSError as e:
-            log.critical("exception from waitpid: %s %s" %(e.errno, e.strerror))
+            log.critical("exception from waitpid: %s %s", e.errno, e.strerror)
 
         if len(s) < 1:
             break
@@ -363,7 +360,7 @@ def execWithCallback(command, argv, stdin = None, stdout = None,
         except OSError as e:
             if e.errno != 4:
                 map(program_log.error, log_errors.splitlines())
-                raise IOError, e.args
+                raise IOError(e.args)
             break
         log_errors += err
         if len(err) < 1:
@@ -379,7 +376,7 @@ def execWithCallback(command, argv, stdin = None, stdout = None,
         if not pid:
             (pid, status) = os.waitpid(childpid, 0)
     except OSError as e:
-        log.critical("exception from waitpid: %s %s" %(e.errno, e.strerror))
+        log.critical("exception from waitpid: %s %s", e.errno, e.strerror)
 
     closefds()
 
@@ -405,7 +402,7 @@ def execConsole():
         proc = subprocess.Popen(["/bin/sh"])
         proc.wait()
     except OSError as e:
-        raise RuntimeError, "Error running /bin/sh: " + e.strerror
+        raise RuntimeError("Error running /bin/sh: %s" % e.strerror)
 
 def runcmd(cmd, **kwargs):
     """ run execWithRedirect with raise_err=True
diff --git a/src/pylorax/imgutils.py b/src/pylorax/imgutils.py
index 9fa0fe1d..5f0a8f52 100644
--- a/src/pylorax/imgutils.py
+++ b/src/pylorax/imgutils.py
@@ -34,13 +34,14 @@ from pylorax.executils import runcmd, runcmd_output
 
 ######## Functions for making container images (cpio, tar, squashfs) ##########
 
-def compress(command, rootdir, outfile, compression="xz", compressargs=["-9"]):
+def compress(command, rootdir, outfile, compression="xz", compressargs=None):
     '''Make a compressed archive of the given rootdir.
     command is a list of the archiver commands to run
     compression should be "xz", "gzip", "lzma", "bzip2", or None.
     compressargs will be used on the compression commandline.'''
     if compression not in (None, "xz", "gzip", "lzma", "bzip2"):
-        raise ValueError, "Unknown compression type %s" % compression
+        raise ValueError("Unknown compression type %s" % compression)
+    compressargs = compressargs or ["-9"]
     if compression == "xz":
         compressargs.insert(0, "--check=crc32")
     if compression is None:
@@ -70,19 +71,22 @@ def compress(command, rootdir, outfile, compression="xz", compressargs=["-9"]):
     except OSError as e:
         logger.error(e)
         # Kill off any hanging processes
-        [p.kill() for p in (find, archive, comp) if p]
+        map(lambda p: p.kill(), (p for p in (find, archive, comp) if p))
         return 1
 
-def mkcpio(rootdir, outfile, compression="xz", compressargs=["-9"]):
+def mkcpio(rootdir, outfile, compression="xz", compressargs=None):
+    compressargs = compressargs or ["-9"]
     return compress(["cpio", "--null", "--quiet", "-H", "newc", "-o"],
                     rootdir, outfile, compression, compressargs)
 
-def mktar(rootdir, outfile, compression="xz", compressargs=["-9"]):
+def mktar(rootdir, outfile, compression="xz", compressargs=None):
+    compressargs = compressargs or ["-9"]
     return compress(["tar", "--selinux", "--acls", "--xattrs", "-cf-", "--null", "-T-"],
                     rootdir, outfile, compression, compressargs)
 
-def mksquashfs(rootdir, outfile, compression="default", compressargs=[]):
+def mksquashfs(rootdir, outfile, compression="default", compressargs=None):
     '''Make a squashfs image containing the given rootdir.'''
+    compressargs = compressargs or []
     if compression != "default":
         compressargs = ["-comp", compression] + compressargs
     return execWithRedirect("mksquashfs", [rootdir, outfile] + compressargs)
@@ -150,24 +154,24 @@ def mount(dev, opts="", mnt=None):
     if mnt is None:
         mnt = tempfile.mkdtemp(prefix="lorax.imgutils.")
         logger.debug("make tmp mountdir %s", mnt)
-    mount = ["mount"]
+    cmd = ["mount"]
     if opts:
-        mount += ["-o", opts]
-    mount += [dev, mnt]
-    runcmd(mount)
+        cmd += ["-o", opts]
+    cmd += [dev, mnt]
+    runcmd(cmd)
     return mnt
 
 def umount(mnt,  lazy=False, maxretry=3, retrysleep=1.0):
     '''Unmount the given mountpoint. If lazy is True, do a lazy umount (-l).
     If the mount was a temporary dir created by mount, it will be deleted.
     raises CalledProcessError if umount fails.'''
-    umount = ["umount"]
-    if lazy: umount += ["-l"]
-    umount += [mnt]
+    cmd = ["umount"]
+    if lazy: cmd += ["-l"]
+    cmd += [mnt]
     count = 0
     while maxretry > 0:
         try:
-            rv = runcmd(umount)
+            rv = runcmd(cmd)
         except CalledProcessError:
             count += 1
             if count == maxretry:
@@ -220,7 +224,8 @@ def round_to_blocks(size, blocksize):
     return size
 
 # TODO: move filesystem data outside this function
-def estimate_size(rootdir, graft={}, fstype=None, blocksize=4096, overhead=128):
+def estimate_size(rootdir, graft=None, fstype=None, blocksize=4096, overhead=128):
+    graft = graft or {}
     getsize = lambda f: os.lstat(f).st_size
     if fstype == "btrfs":
         overhead = 64*1024 # don't worry, it's all sparse
@@ -245,22 +250,24 @@ def estimate_size(rootdir, graft={}, fstype=None, blocksize=4096, overhead=128):
 
 class LoopDev(object):
     def __init__(self, filename, size=None):
+        self.loopdev = None
         self.filename = filename
         if size:
             mksparse(self.filename, size)
     def __enter__(self):
         self.loopdev = loop_attach(self.filename)
         return self.loopdev
-    def __exit__(self, exc_type, exc_value, traceback):
+    def __exit__(self, exc_type, exc_value, tracebk):
         loop_detach(self.loopdev)
 
 class DMDev(object):
     def __init__(self, dev, size, name=None):
+        self.mapperdev = None
         (self.dev, self.size, self.name) = (dev, size, name)
     def __enter__(self):
         self.mapperdev = dm_attach(self.dev, self.size, self.name)
         return self.mapperdev
-    def __exit__(self, exc_type, exc_value, traceback):
+    def __exit__(self, exc_type, exc_value, tracebk):
         dm_detach(self.mapperdev)
 
 class Mount(object):
@@ -269,7 +276,7 @@ class Mount(object):
     def __enter__(self):
         self.mnt = mount(self.dev, self.opts, self.mnt)
         return self.mnt
-    def __exit__(self, exc_type, exc_value, traceback):
+    def __exit__(self, exc_type, exc_value, tracebk):
         umount(self.mnt)
 
 class PartitionMount(object):
@@ -280,6 +287,8 @@ class PartitionMount(object):
         mount_ok is a function that is passed the mount point and
         returns True if it should be mounted.
         """
+        self.mount_dev = None
+        self.mount_size = None
         self.mount_dir = None
         self.disk_img = disk_img
         self.mount_ok = mount_ok
@@ -325,7 +334,7 @@ class PartitionMount(object):
             os.rmdir(mount_dir)
         return self
 
-    def __exit__(self, exc_type, exc_value, traceback):
+    def __exit__(self, exc_type, exc_value, tracebk):
         if self.mount_dir:
             umount( self.mount_dir )
             os.rmdir(self.mount_dir)
@@ -335,12 +344,14 @@ class PartitionMount(object):
 
 ######## Functions for making filesystem images ##########################
 
-def mkfsimage(fstype, rootdir, outfile, size=None, mkfsargs=[], mountargs="", graft={}):
+def mkfsimage(fstype, rootdir, outfile, size=None, mkfsargs=None, mountargs="", graft=None):
     '''Generic filesystem image creation function.
     fstype should be a filesystem type - "mkfs.${fstype}" must exist.
     graft should be a dict: {"some/path/in/image": "local/file/or/dir"};
       if the path ends with a '/' it's assumed to be a directory.
     Will raise CalledProcessError if something goes wrong.'''
+    mkfsargs = mkfsargs or []
+    graft = graft or {}
     preserve = (fstype not in ("msdos", "vfat"))
     if not size:
         size = estimate_size(rootdir, graft, fstype)
@@ -348,7 +359,7 @@ def mkfsimage(fstype, rootdir, outfile, size=None, mkfsargs=[], mountargs="", gr
         try:
             runcmd(["mkfs.%s" % fstype] + mkfsargs + [loopdev])
         except CalledProcessError as e:
-            logger.error("mkfs exited with a non-zero return code: %d" % e.returncode)
+            logger.error("mkfs exited with a non-zero return code: %d", e.returncode)
             logger.error(e.output)
             sys.exit(e.returncode)
 
@@ -361,18 +372,22 @@ def mkfsimage(fstype, rootdir, outfile, size=None, mkfsargs=[], mountargs="", gr
     runcmd(["sync"])
 
 # convenience functions with useful defaults
-def mkdosimg(rootdir, outfile, size=None, label="", mountargs="shortname=winnt,umask=0077", graft={}):
+def mkdosimg(rootdir, outfile, size=None, label="", mountargs="shortname=winnt,umask=0077", graft=None):
+    graft = graft or {}
     mkfsimage("msdos", rootdir, outfile, size, mountargs=mountargs,
               mkfsargs=["-n", label], graft=graft)
 
-def mkext4img(rootdir, outfile, size=None, label="", mountargs="", graft={}):
+def mkext4img(rootdir, outfile, size=None, label="", mountargs="", graft=None):
+    graft = graft or {}
     mkfsimage("ext4", rootdir, outfile, size, mountargs=mountargs,
               mkfsargs=["-L", label, "-b", "1024", "-m", "0"], graft=graft)
 
-def mkbtrfsimg(rootdir, outfile, size=None, label="", mountargs="", graft={}):
+def mkbtrfsimg(rootdir, outfile, size=None, label="", mountargs="", graft=None):
+    graft = graft or {}
     mkfsimage("btrfs", rootdir, outfile, size, mountargs=mountargs,
                mkfsargs=["-L", label], graft=graft)
 
-def mkhfsimg(rootdir, outfile, size=None, label="", mountargs="", graft={}):
+def mkhfsimg(rootdir, outfile, size=None, label="", mountargs="", graft=None):
+    graft = graft or {}
     mkfsimage("hfsplus", rootdir, outfile, size, mountargs=mountargs,
               mkfsargs=["-v", label], graft=graft)
diff --git a/src/pylorax/ltmpl.py b/src/pylorax/ltmpl.py
index 9fcc8a73..abbdcb19 100644
--- a/src/pylorax/ltmpl.py
+++ b/src/pylorax/ltmpl.py
@@ -27,9 +27,9 @@ import os, re, glob, shlex, fnmatch
 from os.path import basename, isdir
 from subprocess import CalledProcessError
 
-from sysutils import joinpaths, cpfile, mvfile, replace, remove
-from yumhelper import * # Lorax*Callback classes
-from base import DataHolder
+from pylorax.sysutils import joinpaths, cpfile, mvfile, replace, remove
+from pylorax.yumhelper import LoraxDownloadCallback, LoraxTransactionCallback, LoraxRpmCallback
+from pylorax.base import DataHolder
 from pylorax.executils import runcmd, runcmd_output
 
 from mako.lookup import TemplateLookup
@@ -38,7 +38,8 @@ import sys, traceback
 import struct
 
 class LoraxTemplate(object):
-    def __init__(self, directories=["/usr/share/lorax"]):
+    def __init__(self, directories=None):
+        directories = directories or ["/usr/share/lorax"]
         # we have to add ["/"] to the template lookup directories or the
         # file includes won't work properly for absolute paths
         self.directories = ["/"] + directories
@@ -67,7 +68,6 @@ class LoraxTemplate(object):
         # split with shlex and perform brace expansion
         lines = map(split_and_expand, lines)
 
-        self.lines = lines
         return lines
 
 def split_and_expand(line):
@@ -92,7 +92,7 @@ def rglob(pathname, root="/", fatal=False):
             seen.add(f)
             yield f[rootlen:] # remove the root to produce relative path
     if fatal and not seen:
-        raise IOError, "nothing matching %s in %s" % (pathname, root)
+        raise IOError("nothing matching %s in %s" % (pathname, root))
 
 def rexists(pathname, root=""):
     # Generator is always True, even with no values;
@@ -145,16 +145,17 @@ class LoraxTemplateRunner(object):
     * Commands should raise exceptions for errors - don't use sys.exit()
     '''
     def __init__(self, inroot, outroot, yum=None, fatalerrors=True,
-                                        templatedir=None, defaults={}):
+                                        templatedir=None, defaults=None):
         self.inroot = inroot
         self.outroot = outroot
         self.yum = yum
         self.fatalerrors = fatalerrors
         self.templatedir = templatedir or "/usr/share/lorax"
+        self.templatefile = None
         # some builtin methods
         self.builtins = DataHolder(exists=lambda p: rexists(p, root=inroot),
                                    glob=lambda g: list(rglob(g, root=inroot)))
-        self.defaults = defaults
+        self.defaults = defaults or {}
         self.results = DataHolder(treeinfo=dict()) # just treeinfo for now
         # TODO: set up custom logger with a filter to add line info
 
@@ -195,9 +196,9 @@ class LoraxTemplateRunner(object):
                 # grab the method named in cmd and pass it the given arguments
                 f = getattr(self, cmd, None)
                 if cmd[0] == '_' or cmd == 'run' or not callable(f):
-                    raise ValueError, "unknown command %s" % cmd
+                    raise ValueError("unknown command %s" % cmd)
                 f(*args)
-            except Exception:
+            except Exception: # pylint: disable=broad-except
                 if skiperror:
                     logger.debug("ignoring error")
                     continue
@@ -258,7 +259,7 @@ class LoraxTemplateRunner(object):
                 match = True
                 replace(f, pat, repl)
         if not match:
-            raise IOError, "no files matched %s" % " ".join(fileglobs)
+            raise IOError("no files matched %s" % " ".join(fileglobs))
 
     def append(self, filename, data):
         '''
@@ -453,7 +454,7 @@ class LoraxTemplateRunner(object):
         for p in pkgs:
             try:
                 self.yum.install(pattern=p)
-            except Exception as e:
+            except Exception as e: # pylint: disable=broad-except
                 # FIXME: save exception and re-raise after the loop finishes
                 logger.error("installpkg %s failed: %s",p,str(e))
                 if required:
@@ -524,15 +525,15 @@ class LoraxTemplateRunner(object):
                 logger.debug("removefrom %s %s: no files matched!", pkg, g)
         # are we removing the matches, or keeping only the matches?
         if keepmatches:
-            remove = filelist.difference(matches)
+            remove_files = filelist.difference(matches)
         else:
-            remove = matches
+            remove_files = matches
         # remove the files
-        if remove:
+        if remove_files:
             logger.debug("%s: removed %i/%i files, %ikb/%ikb", cmd,
-                             len(remove), len(filelist),
-                             self._getsize(*remove)/1024, self._getsize(*filelist)/1024)
-            self.remove(*remove)
+                             len(remove_files), len(filelist),
+                             self._getsize(*remove_files)/1024, self._getsize(*filelist)/1024)
+            self.remove(*remove_files)
         else:
             logger.debug("%s: no files to remove!", cmd)
 
diff --git a/src/pylorax/output.py b/src/pylorax/output.py
index e60a300f..a9a4bc02 100644
--- a/src/pylorax/output.py
+++ b/src/pylorax/output.py
@@ -22,7 +22,7 @@
 import sys
 import re
 
-import decorators
+import pylorax.decorators as decorators
 
 
 # output levels
@@ -88,44 +88,44 @@ class LinuxTerminalOutput(object):
         if self._indent_level > 0:
             self._indent_level -= 1
 
-    def write(self, s, file=sys.stdout):
+    def write(self, s, fout=sys.stdout):
         if self._colors:
             s = self.__format(s)
         else:
             s = self.__raw(s)
 
-        file.write(s)
-        file.flush()
+        fout.write(s)
+        fout.flush()
 
-    def writeline(self, s, file=sys.stdout):
+    def writeline(self, s, fout=sys.stdout):
         s = "{0}{1}\n".format("    " * self._indent_level, s)
-        self.write(s, file=file)
+        self.write(s, fout=fout)
 
-    def critical(self, s, file=sys.stdout):
+    def critical(self, s, fout=sys.stdout):
         s = "** critical: {0}".format(s)
         if (self._output_level <= CRITICAL and
             self.__raw(s) not in self._ignored_messages):
-            self.writeline(s, file=file)
+            self.writeline(s, fout=fout)
 
-    def error(self, s, file=sys.stdout):
+    def error(self, s, fout=sys.stdout):
         s = "** error: {0}".format(s)
         if (self._output_level <= ERROR and
             self.__raw(s) not in self._ignored_messages):
-            self.writeline(s, file=file)
+            self.writeline(s, fout=fout)
 
-    def warning(self, s, file=sys.stdout):
+    def warning(self, s, fout=sys.stdout):
         s = "** warning: {0}".format(s)
         if (self._output_level <= WARNING and
             self.__raw(s) not in self._ignored_messages):
-            self.writeline(s, file=file)
+            self.writeline(s, fout=fout)
 
-    def info(self, s, file=sys.stdout):
+    def info(self, s, fout=sys.stdout):
         if self._output_level <= INFO:
-            self.writeline(s, file=file)
+            self.writeline(s, fout=fout)
 
-    def debug(self, s, file=sys.stdout):
+    def debug(self, s, fout=sys.stdout):
         if self._output_level <= DEBUG:
-            self.writeline(s, file=file)
+            self.writeline(s, fout=fout)
 
     def __format(self, s):
         for tag, ccode in TAGS:
diff --git a/src/pylorax/sysutils.py b/src/pylorax/sysutils.py
index 8743b9a4..914eebfc 100644
--- a/src/pylorax/sysutils.py
+++ b/src/pylorax/sysutils.py
@@ -30,7 +30,6 @@ import pwd
 import grp
 import glob
 import shutil
-import subprocess
 
 from pylorax.executils import runcmd
 
@@ -48,12 +47,12 @@ def touch(fname):
         pass
 
 
-def replace(fname, find, replace):
+def replace(fname, find, sub):
     fin = fileinput.input(fname, inplace=1)
     pattern = re.compile(find)
 
     for line in fin:
-        line = pattern.sub(replace, line)
+        line = pattern.sub(sub, line)
         sys.stdout.write(line)
 
     fin.close()
diff --git a/src/pylorax/treebuilder.py b/src/pylorax/treebuilder.py
index dc8bb48b..1b71604b 100644
--- a/src/pylorax/treebuilder.py
+++ b/src/pylorax/treebuilder.py
@@ -21,13 +21,13 @@ import logging
 logger = logging.getLogger("pylorax.treebuilder")
 
 import os, re
-from os.path import basename, isdir
-
-from sysutils import joinpaths, remove
+from os.path import basename
 from shutil import copytree, copy2
-from base import DataHolder
-from ltmpl import LoraxTemplateRunner
-import imgutils
+
+from pylorax.sysutils import joinpaths, remove
+from pylorax.base import DataHolder
+from pylorax.ltmpl import LoraxTemplateRunner
+import pylorax.imgutils as imgutils
 from pylorax.executils import runcmd, runcmd_output
 
 templatemap = {
@@ -53,10 +53,10 @@ def generate_module_info(moddir, outfile=None):
                'eth':read_module_set("modules.networking")}
 
     modinfo = list()
-    for root, dirs, files in os.walk(moddir):
+    for root, _dirs, files in os.walk(moddir):
         for modtype, modset in modsets.items():
             for mod in modset.intersection(files):  # modules in this dir
-                (name, ext) = os.path.splitext(mod) # foo.ko -> (foo, .ko)
+                (name, _ext) = os.path.splitext(mod) # foo.ko -> (foo, .ko)
                 desc = module_desc(joinpaths(root,mod)) or "%s driver" % name
                 modinfo.append(dict(name=name, type=modtype, desc=desc))
 
@@ -146,8 +146,9 @@ class RuntimeBuilder(object):
             runcmd(["depmod", "-a", "-F", ksyms, "-b", root, kver])
             generate_module_info(moddir+kver, outfile=moddir+"module-info")
 
-    def create_runtime(self, outfile="/var/tmp/squashfs.img", compression="xz", compressargs=[], size=2):
+    def create_runtime(self, outfile="/var/tmp/squashfs.img", compression="xz", compressargs=None, size=2):
         # make live rootfs image - must be named "LiveOS/rootfs.img" for dracut
+        compressargs = compressargs or []
         workdir = joinpaths(os.path.dirname(outfile), "runtime-workdir")
         if size:
             fssize = size * (1024*1024*1024) # 2GB sparse file compresses down to nothin'
@@ -181,18 +182,20 @@ class TreeBuilder(object):
         self._runner = LoraxTemplateRunner(inroot, outroot, templatedir=templatedir)
         self._runner.defaults = self.vars
         self.templatedir = templatedir
+        self.treeinfo_data = None
 
     @property
     def kernels(self):
         return findkernels(root=self.vars.inroot)
 
-    def rebuild_initrds(self, add_args=[], backup="", prefix=""):
+    def rebuild_initrds(self, add_args=None, backup="", prefix=""):
         '''Rebuild all the initrds in the tree. If backup is specified, each
         initrd will be renamed with backup as a suffix before rebuilding.
         If backup is empty, the existing initrd files will be overwritten.
         If suffix is specified, the existing initrd is untouched and a new
         image is built with the filename "${prefix}-${kernel.version}.img"
         '''
+        add_args = add_args or []
         dracut = ["dracut", "--nomdadmconf", "--nolvmconf"] + add_args
         if not backup:
             dracut.append("--force")
@@ -231,7 +234,7 @@ class TreeBuilder(object):
         self.implantisomd5()
 
     def implantisomd5(self):
-        for section, data in self.treeinfo_data.items():
+        for _section, data in self.treeinfo_data.items():
             if 'boot.iso' in data:
                 iso = joinpaths(self.vars.outroot, data['boot.iso'])
                 runcmd(["implantisomd5", iso])
@@ -261,7 +264,7 @@ class TreeBuilder(object):
         for hook_script, dracut_path in hooks:
             src = joinpaths(self.dracut_hooks_path, hook_script)
             if not os.path.exists(src):
-                logger.error("Missing lorax dracut hook script %s" % (src))
+                logger.error("Missing lorax dracut hook script %s", (src))
                 continue
             dst = joinpaths(self.vars.inroot, "/tmp/", hook_script)
             copy2(src, dst)
@@ -289,13 +292,13 @@ def findkernels(root="/", kdir="boot"):
     for kernel in kernels:
         for f in bootfiles:
             if f.endswith('-'+kernel.version+'.img'):
-                imgtype, rest = f.split('-',1)
+                imgtype, _rest = f.split('-',1)
                 # special backwards-compat case
                 if imgtype == 'initramfs':
                     imgtype = 'initrd'
                 kernel[imgtype] = DataHolder(path=joinpaths(kdir, f))
 
-    logger.debug("kernels=%s" % kernels)
+    logger.debug("kernels=%s", kernels)
     return kernels
 
 # udev whitelist: 'a-zA-Z0-9#+.:=@_-' (see is_whitelisted in libudev-util.c)
diff --git a/src/pylorax/yumhelper.py b/src/pylorax/yumhelper.py
index 6ad4bbe2..7ac4e439 100644
--- a/src/pylorax/yumhelper.py
+++ b/src/pylorax/yumhelper.py
@@ -23,7 +23,7 @@ import logging
 logger = logging.getLogger("pylorax.yumhelper")
 import sys
 import yum, yum.callbacks, yum.rpmtrans
-import output
+import pylorax.output as output
 
 __all__ = ['LoraxDownloadCallback', 'LoraxTransactionCallback',
            'LoraxRpmCallback']