diff --git a/psrqpy/__init__.py b/psrqpy/__init__.py
index 92b81572..6814c776 100644
--- a/psrqpy/__init__.py
+++ b/psrqpy/__init__.py
@@ -2,7 +2,7 @@
 
 """ A Python tool for interacting with the ATNF pulsar catalogue """
 
-__version__ = "0.3.6"
+__version__ = "0.4.1"
 
 from .search import QueryATNF
 from .pulsar import Pulsar, Pulsars
diff --git a/psrqpy/config.py b/psrqpy/config.py
index dd210bc3..0e818c42 100644
--- a/psrqpy/config.py
+++ b/psrqpy/config.py
@@ -57,7 +57,7 @@
               }
 
 # just return the parameter names
-PSR_GENERAL_PARS = PSR_GENERAL.keys()
+PSR_GENERAL_PARS = list(PSR_GENERAL.keys())
 
 # timing solution and profile parameters
 PSR_TIMING = {'P0':        {'ref': True, 'err': True, 'units': 's', 'format': 'f8'},           # Barycentric period of the pulsar (s)
@@ -79,7 +79,7 @@
               'S2000':     {'ref': True, 'err': True, 'units': 'mJy', 'format': 'f8'}          # Mean flux density at 2000 MHz (mJy)
              }
 
-PSR_TIMING_PARS = PSR_TIMING.keys()
+PSR_TIMING_PARS = list(PSR_TIMING.keys())
 
 # binary system parameters
 PSR_BINARY = {'BINARY':    {'ref': True, 'err': False, 'units': None, 'format': 'S5'},         # Binary model (usually one of several recognised by the pulsar timing programs TEMPO or TEMPO2). Modified versions of standard models are often used - refer to the source paper for details of the binary model used.
@@ -96,7 +96,7 @@
               'BINCOMP':   {'ref': True, 'err': False, 'units': None, 'format': 'S4'}          # Companion type
              }
 
-PSR_BINARY_PARS = PSR_BINARY.keys()
+PSR_BINARY_PARS = list(PSR_BINARY.keys())
 
 # derived parameters
 PSR_DERIVED = {'R_LUM':    {'ref': False, 'err': False, 'units': 'mJy kpc^2', 'format': 'f8'},      # Radio luminosity at 400 MHz (mJy kpc2)
@@ -113,7 +113,7 @@
                'B_LC':     {'ref': False, 'err': False, 'units': 'G', 'format': 'f8'}               # Magnetic field at light cylinder (gauss)
               }
 
-PSR_DERIVED_PARS = PSR_DERIVED.keys()
+PSR_DERIVED_PARS = list(PSR_DERIVED.keys())
 
 # a list of all allowed parameters for querying
 PSR_ALL = dict(itertools.chain(PSR_GENERAL.items(), PSR_TIMING.items(), PSR_BINARY.items(), PSR_DERIVED.items()))
diff --git a/psrqpy/pulsar.py b/psrqpy/pulsar.py
index 80cbf8c5..b4291db7 100644
--- a/psrqpy/pulsar.py
+++ b/psrqpy/pulsar.py
@@ -6,7 +6,8 @@
 from __future__ import print_function, division
 
 import warnings
-import six
+
+from six import string_types, iteritems
 
 from .config import PSR_ALL_PARS, PSR_ALL
 from .utils import get_version
@@ -28,7 +29,7 @@ def __init__(self, psrname, version=None, **kwargs):
         self._version = version if not version else get_version()
         self._ephemeris = None
 
-        for key, value in six.iteritems(kwargs):
+        for key, value in iteritems(kwargs):
             setattr(self, key, value)
 
     def __repr__(self):
@@ -51,7 +52,7 @@ def items(self):
         """
         Return a list of the class attribute values
         """
-        return [value for key, value in six.iteritems(self.__dict__) if key in PSR_ALL_PARS+[par+'_ERR' for par in PSR_ALL_PARS]]
+        return [value for key, value in iteritems(self.__dict__) if key in PSR_ALL_PARS+[par+'_ERR' for par in PSR_ALL_PARS]]
 
     @property
     def name(self):
@@ -185,7 +186,7 @@ def set_ephemeris(self, ephem=None):
         if not self._ephemeris and ephem:
             self._ephemeris = ephem # set ephemeris if it doesn't already exist
 
-        assert isinstance(self._ephemeris, basestring), 'Ephemeris must be a string'
+        assert isinstance(self._ephemeris, string_types), 'Ephemeris must be a string'
 
         # get ephemeris values
         ephemvals = [ev.split() for ev in ephem.split('\n') if len(ev.split()) > 1]
@@ -336,7 +337,7 @@ def remove_pulsar(self, psrname):
             psrname (str): a string with the name of a pulsar
         """
 
-        assert isinstance(psrname, basestring), 'psrname is not a string'
+        assert isinstance(psrname, string_types), 'psrname is not a string'
 
         if psrname in self._psrs:
             del self._psrs[psrname]
@@ -348,7 +349,7 @@ def pop(self, psrname):
         Args:
             psrname (str): a string with the name of a pulsar
         """
-        assert isinstance(psrname, basestring), 'psrname is not a string'
+        assert isinstance(psrname, string_types), 'psrname is not a string'
 
         if psrname in self._psrs:
             return self._psrs.pop(psrname)
diff --git a/psrqpy/search.py b/psrqpy/search.py
index a4393f73..96d36bd8 100644
--- a/psrqpy/search.py
+++ b/psrqpy/search.py
@@ -9,9 +9,11 @@
 import warnings
 from collections import OrderedDict
 import re
-import cPickle as pickle
 import six
 
+from six.moves import cPickle as pickle
+from six import string_types
+
 import numpy as np
 import requests
 from bs4 import BeautifulSoup
@@ -107,12 +109,12 @@ def __init__(self, params=None, condition=None, psrtype=None, assoc=None, bincom
                 raise Exception("No parameters in list")
 
             for p in params:
-                if not isinstance(p, basestring):
+                if not isinstance(p, string_types):
                     raise Exception("Non-string value '{}' found in params list".format(p))
 
             self._query_params = [p.upper() for p in params] # make sure parameter names are all upper case
         else:
-            if isinstance(params, basestring):
+            if isinstance(params, string_types):
                 self._query_params = [params.upper()] # make sure parameter is all upper case
             else:
                 if self._psrs and self._get_ephemeris: # if getting ephemerides then param can be None 
@@ -211,7 +213,7 @@ def generate_query(self, version='', params=None, condition='', sortorder='asc',
         query_dict['version'] = self._atnf_version
 
         if params:
-            if isinstance(params, basestring):
+            if isinstance(params, string_types):
                 params = [params] # convert to list
             else:
                 if not isinstance(params, list):
@@ -236,7 +238,7 @@ def generate_query(self, version='', params=None, condition='', sortorder='asc',
         query_dict['sortattr'] = self._sort_attr
 
         if psrnames:
-            if isinstance(psrnames, basestring):
+            if isinstance(psrnames, string_types):
                 self._psrs = [psrnames] # convert to list
             else:
                 if not isinstance(psrnames, list):
@@ -245,7 +247,7 @@ def generate_query(self, version='', params=None, condition='', sortorder='asc',
 
         qpulsars = '' # pulsar name query string
         if self._psrs is not None:
-            if isinstance(self._psrs, basestring):
+            if isinstance(self._psrs, string_types):
                 self._psrs = [self._psrs] # if a string pulsar name then convert to list
 
             for psr in self._psrs:
@@ -555,7 +557,7 @@ def parse_conditions(self, condition, psrtype=None, assoc=None, bincomp=None, ex
         if not condition:
             conditionparse = ''
         else:
-            if not isinstance(condition, basestring):
+            if not isinstance(condition, string_types):
                 warnings.warn('Condition "{}" must be a string. No condition being set'.format(condition), UserWarning)
                 return ''
 
@@ -602,11 +604,11 @@ def parse_conditions(self, condition, psrtype=None, assoc=None, bincomp=None, ex
                     raise Exception("No pulsar types in list")
 
                 for p in psrtype:
-                    if not isinstance(p, basestring):
+                    if not isinstance(p, string_types):
                         raise Exception("Non-string value '{}' found in pulsar type list".format(p))
                 self._query_psr_types = psrtype
             else:
-                if isinstance(psrtype, basestring):
+                if isinstance(psrtype, string_types):
                     self._query_psr_types = [psrtype]
                 else:
                     raise Exception("'psrtype' must be a list or string")
@@ -628,11 +630,11 @@ def parse_conditions(self, condition, psrtype=None, assoc=None, bincomp=None, ex
                     raise Exception("No pulsar types in list")
 
                 for p in assoc:
-                    if not isinstance(p, basestring):
+                    if not isinstance(p, string_types):
                         raise Exception("Non-string value '{}' found in associations list".format(p))
                 self._query_assocs = assoc
             else:
-                if isinstance(assoc, basestring):
+                if isinstance(assoc, string_types):
                     self._query_assocs = [assoc]
                 else:
                     raise Exception("'assoc' must be a list or string")
@@ -654,11 +656,11 @@ def parse_conditions(self, condition, psrtype=None, assoc=None, bincomp=None, ex
                     raise Exception("No pulsar types in list")
 
                 for p in bincomp:
-                    if not isinstance(p, basestring):
+                    if not isinstance(p, string_types):
                         raise Exception("Non-string value '{}' found in binary companions list".format(p))
                 self._query_bincomps = bincomp
             else:
-                if isinstance(bincomp, basestring):
+                if isinstance(bincomp, string_types):
                     self._query_bincomps = [bincomp]
                 else:
                     raise Exception("'bincomp' must be a list or string")
@@ -772,7 +774,7 @@ def ppdot(self, intrinsicpdot=False, excludeGCs=False, showtypes=[], showGCs=Fal
         if intrinsicpdot and 'P1_I' not in self._query_params:
             self._query_params.append('P1_I')
 
-        if isinstance(showtypes, basestring):
+        if isinstance(showtypes, string_types):
             nshowtypes = [showtypes]
         else:
             nshowtypes = showtypes
diff --git a/psrqpy/utils.py b/psrqpy/utils.py
index 158358d0..7a0e8f15 100644
--- a/psrqpy/utils.py
+++ b/psrqpy/utils.py
@@ -9,9 +9,10 @@
 import datetime
 import numpy as np
 import requests
-
 from bs4 import BeautifulSoup
 
+from six import string_types
+
 from .config import ATNF_BASE_URL, ATNF_VERSION, ADS_URL
 
 # problematic references that are hard to parse
@@ -117,7 +118,7 @@ def get_references(useads=False):
                                'Sov. Astron. Lett.': 'SvAL',
                                'ATel.': 'ATel'}
 
-                if isinstance(rdf, basestring): # only run on string values
+                if isinstance(rdf, string_types): # only run on string values
                     rdfs = re.sub(r'\s+', ' ', rdf) # make sure only single spaces are present
                     for js in journalsubs:
                         if js in rdfs:
@@ -160,7 +161,7 @@ def get_references(useads=False):
             if '&' in sepauthors[-1] or 'and' in sepauthors[-1]: # split any authors that are seperated by an ampersand
                 lastauthors = [a.strip() for a in re.split(r'& | and ', sepauthors.pop(-1))]
                 sepauthors = sepauthors + lastauthors
-                for i in xrange(len(sepauthors)-2):
+                for i in range(len(sepauthors)-2):
                     sepauthors[i] += '.' # re-add final full stops where needed
                 sepauthors[-1] += '.'
             else: