diff --git a/.gitignore b/.gitignore
index 2f295e5f..bbfcc8c8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,3 +15,6 @@ eval*
 .pytest_cache
 .tox
 docs_api
+.idea
+venv
+dump.txt.gz
diff --git a/.travis.yml b/.travis.yml
index 705f3ee6..b08023f0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,10 +1,8 @@
 language: python
 
 python:
-    - "2.7"
-    - "3.4"
-    - "3.5"
-    - "3.6"
+    - "3.7"
+    - "3.8"
 
 install:
     - pip install tox-travis
diff --git a/dump.txt.gz b/dump.txt.gz
deleted file mode 100644
index b3b4af48..00000000
Binary files a/dump.txt.gz and /dev/null differ
diff --git a/examples/advanced.py b/examples/advanced.py
index 25d17076..08776949 100644
--- a/examples/advanced.py
+++ b/examples/advanced.py
@@ -1,5 +1,3 @@
-from __future__ import print_function
-
 from koala.ExcelCompiler import ExcelCompiler
 from koala.Spreadsheet import Spreadsheet
 from koala.excellib import xsum
diff --git a/examples/basic.py b/examples/basic.py
index ddeb323c..280934bd 100644
--- a/examples/basic.py
+++ b/examples/basic.py
@@ -1,5 +1,3 @@
-from __future__ import print_function
-
 from koala.ExcelCompiler import ExcelCompiler
 from koala.Spreadsheet import Spreadsheet
 
diff --git a/koala/Cell.py b/koala/Cell.py
index df893ccd..3dd97d9c 100644
--- a/koala/Cell.py
+++ b/koala/Cell.py
@@ -1,13 +1,9 @@
 # cython: profile=True
 
-from __future__ import absolute_import, division
-
 from koala.CellBase import CellBase
 from koala.Range import RangeCore
 from koala.utils import *
 
-from openpyxl.compat import unicode
-
 
 class Cell(CellBase):
     ctr = 0
@@ -62,11 +58,7 @@ def __init__(
             self.__row = None
             self.__col_idx = None
 
-        # `unicode` != `str` in Python2. See `from openpyxl.compat import unicode`
-        if type(formula) == str and str != unicode:
-            self.__formula = unicode(formula, 'utf-8') if formula else None
-        else:
-            self.__formula = formula if formula else None
+        self.__formula = formula if formula else None
 
         self.__value = value
         self.python_expression = None
diff --git a/koala/ExcelCompiler.py b/koala/ExcelCompiler.py
index 8d228a34..a5eab3b8 100644
--- a/koala/ExcelCompiler.py
+++ b/koala/ExcelCompiler.py
@@ -1,15 +1,5 @@
-from __future__ import print_function
 # cython: profile=True
 
-import os.path
-
-import networkx
-
-from koala.reader import read_archive, read_named_ranges, read_cells
-from koala.utils import *
-from koala.ast import graph_from_seeds, shunting_yard, build_ast, prepare_pointer
-from koala.Cell import Cell
-from koala.Range import RangeFactory
 from koala.Spreadsheet import Spreadsheet
 import warnings
 
@@ -20,8 +10,6 @@ class ExcelCompiler(object):
     """
 
     def __init__(self, file, ignore_sheets = [], ignore_hidden = False, debug = False):
-        # print("___### Initializing Excel Compiler ###___")
-
         warnings.warn(
             "The ExcelCompiler class will disappear in a future version. Please use Spreadsheet instead.",
             PendingDeprecationWarning
diff --git a/koala/Range.py b/koala/Range.py
index f3c2c0cd..a4809caa 100644
--- a/koala/Range.py
+++ b/koala/Range.py
@@ -1,12 +1,8 @@
-from __future__ import absolute_import, division, print_function
-
+import logging
 from koala.CellBase import CellBase
 from koala.ExcelError import ErrorCodes, ExcelError
 from koala.utils import *
 
-from openpyxl.compat import unicode
-
-
 # WARNING: Range should never be imported directly. Import Range from excelutils instead.
 
 ### Range Utils ###
@@ -58,7 +54,7 @@ def check_value(a):
         return ExcelError(a)
 
     try:  # This is to avoid None or Exception returned by Range operations
-        if isinstance(a, (unicode, str)):
+        if isinstance(a, str):
             return a
         elif float(a):
             return a
@@ -122,7 +118,6 @@ def __build(
                 cells, nrows, ncols = resolve_range(
                     reference, should_flatten=True)
             except Exception as e:
-                print('Pb with ref', reference, e)
                 return ValueError('Range ERROR')  # Will still be considered as a Range object, since we are inside __init__...
 
         origin = parse_cell_address(cells[0]) if len(cells) > 0 else None  # origin of Range
@@ -170,7 +165,7 @@ def __build(
         elif not self.is_pointer:  # when building pointers, name shouldn't be updated, but in that case reference is not a dict
             self.__name = reference
         else:
-            print('Pb with Name', reference, name)
+            logging.debug('Pb with Name', reference, name)
         self.__origin = origin
         self.__addresses = cells
         self.__order = order
@@ -570,7 +565,7 @@ def multiply(a, b):
     @staticmethod
     def divide(a, b):
         try:
-            return old_div(float(check_value(a)), float(check_value(b)))
+            return float(check_value(a)) / float(check_value(b))
         except Exception as e:
             return ExcelError('#DIV/0!', e)
 
@@ -584,9 +579,9 @@ def power(a, b):
     @staticmethod
     def is_equal(a, b):
         try:
-            if not isinstance(a, (str, unicode)):
+            if not isinstance(a, str):
                 a = check_value(a)
-            if not isinstance(b, (str, unicode)):
+            if not isinstance(b, str):
                 b = check_value(b)
 
             return is_almost_equal(a, b, precision=0.00001)
@@ -596,9 +591,9 @@ def is_equal(a, b):
     @staticmethod
     def is_not_equal(a, b):
         try:
-            if not isinstance(a, (str, unicode)):
+            if not isinstance(a, str):
                 a = check_value(a)
-            if not isinstance(a, (str, unicode)):
+            if not isinstance(a, str):
                 b = check_value(b)
 
             return a != b
diff --git a/koala/Spreadsheet.py b/koala/Spreadsheet.py
index f4ac5db5..d5ab35e4 100644
--- a/koala/Spreadsheet.py
+++ b/koala/Spreadsheet.py
@@ -1,4 +1,3 @@
-from __future__ import absolute_import, print_function
 # cython: profile=True
 
 from koala.Range import get_cell_address, parse_cell_address
@@ -13,17 +12,13 @@
 from koala.utils import *
 
 import warnings
+import logging
 import os.path
 import networkx
 from networkx.readwrite import json_graph
 
-from openpyxl.compat import unicode
-
-
 class Spreadsheet(object):
-    def __init__(self, file=None, ignore_sheets=[], ignore_hidden=False, debug=False):
-        # print("___### Initializing Excel Compiler ###___")
-
+    def __init__(self, file=None, ignore_sheets=[], ignore_hidden=False, include_only_sheets=None, debug=False):
         if file is None:
             # create empty version of this object
             self.cells = None  # precursor for cellmap: dict that link addresses (str) to Cell objects.
@@ -56,7 +51,7 @@ def __init__(self, file=None, ignore_sheets=[], ignore_hidden=False, debug=False
             else:                       # assume file path
                 archive = read_archive(os.path.abspath(file))
             # Parse cells
-            self.cells = read_cells(archive, ignore_sheets, ignore_hidden)
+            self.cells, self.sheets = read_cells(archive, ignore_sheets, ignore_hidden, include_only_sheets)
             # Parse named_range { name (ExampleName) -> address (Sheet!A1:A10)}
             self.named_ranges = read_named_ranges(archive)
             self.range = RangeFactory(self.cells)
@@ -83,8 +78,6 @@ def gen_graph(self, outputs=[], inputs=[]):
         :param outputs: can be used to specify the outputs. All not affected cells are removed from the graph.
         :param inputs: can be used to specify the inputs. All not affected cells are removed from the graph.
         """
-        # print('___### Generating Graph ###___')
-
         if len(outputs) == 0:
             preseeds = set(list(flatten(self.cells.keys())) + list(self.named_ranges.keys())) # to have unicity
         else:
@@ -128,7 +121,6 @@ def gen_graph(self, outputs=[], inputs=[]):
                     seeds.append(self.cells[o])
 
         seeds = set(seeds)
-        # print("Seeds %s cells" % len(seeds))
         outputs = set(preseeds) if len(outputs) > 0 else [] # seeds and outputs are the same when you don't specify outputs
 
         cellmap, G = graph_from_seeds(seeds, self)
@@ -169,12 +161,6 @@ def gen_graph(self, outputs=[], inputs=[]):
 
             inputs = set(inputs)
 
-
-        # print("Graph construction done, %s nodes, %s edges, %s cellmap entries" % (len(G.nodes()),len(G.edges()),len(cellmap)))
-
-        # undirected = networkx.Graph(G)
-        # print "Number of connected components %s", str(number_connected_components(undirected))
-
         if inputs == [] and outputs == []:
             self.build_spreadsheet(G, cellmap, self.named_ranges, pointers = self.pointers, outputs = outputs, inputs = inputs, debug = self.debug)
         else:
@@ -277,7 +263,7 @@ def cell_add(self, address=None, cell=None, value=None, formula=None):
         self.cellmap = cellmap
         self.G = G
 
-        print("Graph construction updated, %s nodes, %s edges, %s cellmap entries" % (len(G.nodes()),len(G.edges()),len(cellmap)))
+        logging.debug("Graph construction updated, %s nodes, %s edges, %s cellmap entries" % (len(G.nodes()),len(G.edges()),len(cellmap)))
 
     def set_formula(self, addr, formula):
         # previously set_formula was used. Capture this behaviour.
@@ -306,7 +292,7 @@ def cell_set_formula(self, address, formula):
             for index, c in enumerate(cell.range.cells): # for each cell of the range, translate the formula
                 if index == 0:
                     c.formula = formula
-                    translator = Translator(unicode('=' +    formula), c.address().split('!')[1]) # the Translator needs a reference without sheet
+                    translator = Translator('=' + formula, c.address().split('!')[1]) # the Translator needs a reference without sheet
                 else:
                     translated = translator.translate_formula(c.address().split('!')[1]) # the Translator needs a reference without sheet
                     c.formula = translated[1:] # to get rid of the '='
@@ -325,11 +311,11 @@ def cell_set_formula(self, address, formula):
         self.evaluate(address)
         self.cellmap[address].should_eval = should_eval
 
-        print("Graph construction updated, %s nodes, %s edges, %s cellmap entries" % (len(G.nodes()),len(G.edges()),len(cellmap)))
+        logging.debug("Graph construction updated, %s nodes, %s edges, %s cellmap entries" % (len(G.nodes()),len(G.edges()),len(cellmap)))
 
 
     def prune_graph(self, *args):
-        print('___### Pruning Graph ###___')
+        logging.debug('___### Pruning Graph ###___')
 
         G = self.G
 
@@ -338,7 +324,6 @@ def prune_graph(self, *args):
         for input_address in self.inputs:
             child = self.cellmap[input_address]
             if child == None:
-                print("Not found ", input_address)
                 continue
             g = make_subgraph(G, child, "descending")
             dependencies = dependencies.union(g.nodes())
@@ -393,7 +378,6 @@ def prune_graph(self, *args):
                     subgraph.add_edge(const_node, current)
 
 
-        print("Graph pruning done, %s nodes, %s edges, %s cellmap entries" % (len(subgraph.nodes()),len(subgraph.edges()),len(new_cellmap)))
         undirected = networkx.Graph(subgraph)
         # print "Number of connected components %s", str(number_connected_components(undirected))
         # print map(lambda x: x.address(), subgraph.nodes())
@@ -430,7 +414,7 @@ def prune_graph(self, *args):
         return spreadsheet.build_spreadsheet(subgraph, new_cellmap, self.named_ranges, self.pointers, self.outputs, self.inputs, debug = self.debug)
 
     def clean_pointer(self):
-        print('___### Cleaning Pointers ###___')
+        logging.debug('___### Cleaning Pointers ###___')
 
         new_named_ranges = self.named_ranges.copy()
         new_cells = self.cellmap.copy()
@@ -484,7 +468,7 @@ def clean_pointer(self):
                 for repl in replacements:
                     if type(repl["value"]) == ExcelError:
                         if self.debug:
-                            print('WARNING: Excel error found => replacing with #N/A')
+                            logging.debug('WARNING: Excel error found => replacing with #N/A')
                         repl["value"] = "#N/A"
 
                     if repl["expression_type"] == "value":
@@ -503,7 +487,7 @@ def clean_pointer(self):
         return new_cells, new_named_ranges
 
     def print_value_ast(self, ast,node,indent):
-        print("%s %s %s %s" % (" "*indent, str(node.token.tvalue), str(node.token.ttype), str(node.token.tsubtype)))
+        logging.debug("%s %s %s %s" % (" "*indent, str(node.token.tvalue), str(node.token.ttype), str(node.token.tsubtype)))
         for c in node.children(ast):
             self.print_value_ast(ast, c, indent+1)
 
@@ -525,7 +509,7 @@ def eval_pointers_from_ast(self, ast, node, cell):
 
             except Exception as e:
                 if self.debug:
-                    print('EXCEPTION raised in eval_pointers: EXPR', expression, cell["address"])
+                    logging.debug('EXCEPTION raised in eval_pointers: EXPR', expression, cell["address"])
                 raise Exception("Problem evalling: %s for %s, %s" % (e, cell["address"], expression))
 
             return {"formula":pointer_string, "value": pointer_value, "expression_type": expression_type}
@@ -701,7 +685,7 @@ def cell_set_value(self, address, value):
             cell = self.cellmap[address]
 
             # when you set a value on cell, its should_eval flag is set to 'never' so its formula is not used until set free again => sp.activate_formula()
-            self.fix_cell(address)
+            self.cell_fix(address)
 
             # case where the address refers to a range
             if cell.is_range:
@@ -854,7 +838,7 @@ def cell_free(self, address=None):
 
     def print_value_tree(self,addr,indent):
         cell = self.cellmap[addr]
-        print("%s %s = %s" % (" "*indent,addr,cell.value))
+        logging.debug("%s %s = %s" % (" "*indent,addr,cell.value))
         for c in self.G.predecessors_iter(cell):
             self.print_value_tree(c.address(), indent+1)
 
@@ -892,7 +876,7 @@ def eval_ref(self, addr1, addr2 = None, ref = None):
                 cell1 = self.cellmap[addr1]
             else:
                 if self.debug:
-                    print('WARNING in eval_ref: address %s not found in cellmap, returning #NULL' % addr1)
+                    logging.warning('WARNING in eval_ref: address %s not found in cellmap, returning #NULL' % addr1)
                 return ExcelError('#NULL', 'Cell %s is empty' % addr1)
             if addr2 == None:
                 if cell1.is_range:
@@ -976,7 +960,7 @@ def cell_evaluate(self, address):
             cell = self.cellmap[address]
         except:
             if self.debug:
-                print('WARNING: Empty cell at ' + address)
+                logging.warning('WARNING: Empty cell at ' + address)
             return ExcelError('#NULL', 'Cell %s is empty' % address)
 
         # no formula, fixed value
diff --git a/koala/__init__.py b/koala/__init__.py
index 012c30aa..322478f4 100644
--- a/koala/__init__.py
+++ b/koala/__init__.py
@@ -1,5 +1,3 @@
-from __future__ import absolute_import
-
 from openpyxl import *
 from .ast import *
 from .Cell import *
diff --git a/koala/ast/__init__.py b/koala/ast/__init__.py
index 6d1f8d21..03efb107 100644
--- a/koala/ast/__init__.py
+++ b/koala/ast/__init__.py
@@ -1,12 +1,9 @@
-from __future__ import absolute_import
 # cython: profile=True
 
 import collections
-import six
 
 import networkx
 from networkx.classes.digraph import DiGraph
-from openpyxl.compat import unicode
 
 from koala.utils import uniqueify, flatten, max_dimension, col2num, resolve_range
 from koala.Cell import Cell
@@ -118,7 +115,7 @@ def shunting_yard(expression, named_ranges, ref = None, tokenize_range = False):
         for index, token in enumerate(tokens):
             new_tokens.append(token)
 
-            if type(token.tvalue) == str or type(token.tvalue) == unicode:
+            if type(token.tvalue) == str:
 
                 if token.tvalue.startswith(':'): # example -> :OFFSET( or simply :A10
                     depth = 0
@@ -139,7 +136,7 @@ def shunting_yard(expression, named_ranges, ref = None, tokenize_range = False):
                         if depth == 0:
                             new_tokens.pop() # these 2 lines are needed to remove INDEX()
                             new_tokens.pop()
-                            expr = six.next(rev).tvalue + expr
+                            expr = next(rev).tvalue + expr
                             break
 
                     expr += token.tvalue
@@ -377,13 +374,11 @@ def cell2code(cell, named_ranges):
 
     else:
         ast = None
-        if isinstance(cell.value, unicode):
-            code = u'u"' + cell.value.replace(u'"', u'\\"') + u'"'
-        elif isinstance(cell.value, str):
-            raise RuntimeError("Got unexpected non-unicode str")
+        if isinstance(cell.value, str):
+            code = '"' + cell.value.replace('"', r'\"') + '"'
         else:
             code = str(cell.value)
-    return code,ast
+    return code, ast
 
 
 def prepare_pointer(code, names, ref_cell = None):
@@ -455,7 +450,8 @@ def graph_from_seeds(seeds, cell_source):
         # directed graph
         G = networkx.DiGraph()
         # match the info in cellmap
-        for c in cellmap.values(): G.add_node(c)
+        for c in cellmap.values():
+            G.add_node(c)
 
     # cells to analyze: only formulas
     todo = [s for s in seeds if s.formula]
diff --git a/koala/ast/astnodes.py b/koala/ast/astnodes.py
index 03bceb44..4d10d3d3 100644
--- a/koala/ast/astnodes.py
+++ b/koala/ast/astnodes.py
@@ -1,26 +1,23 @@
-from __future__ import print_function
 # cython: profile=True
+import logging
 
 from networkx import NetworkXError
 
-from openpyxl.compat import unicode
-
 from koala.excellib import FUNCTION_MAP, IND_FUN
 from koala.utils import is_range, split_range, split_address, resolve_range
 from koala.ExcelError import *
 
 
 def to_str(my_string):
-    # `unicode` != `str` in Python2. See `from openpyxl.compat import unicode`
-    if type(my_string) == str and str != unicode:
-        return unicode(my_string, 'utf-8')
-    elif type(my_string) == unicode:
+    if isinstance(my_string, bytes):
+        return my_string.decode("utf-8")
+    elif isinstance(my_string, str):
         return my_string
     else:
         try:
             return str(my_string)
-        except:
-            print('Couldnt parse as string', type(my_string))
+        except Exception:
+            logging.warning('Couldnt parse as string', type(my_string))
             return my_string
     # elif isinstance(my_string, (int, float, tuple, Ra):
     #     return str(my_string)
@@ -45,7 +42,7 @@ def __getattr__(self, name):
     def children(self, ast):
         try:
             args = ast.predecessors(self)
-            args = sorted(args, key=lambda x: ast.node[x]['pos'])
+            args = sorted(args, key=lambda x: ast.nodes[x]['pos'])
         except NetworkXError:
             args = ''
         return args
@@ -204,7 +201,7 @@ def get_cells(self):
     def emit(self,ast,context=None, pointer = False):
         if isinstance(self.tvalue, ExcelError):
             if self.debug:
-                print('WARNING: Excel Error Code found', self.tvalue)
+                logging.debug('WARNING: Excel Error Code found', self.tvalue)
             return self.tvalue
 
         is_a_range = False
@@ -228,7 +225,7 @@ def emit(self,ast,context=None, pointer = False):
                         sh,col,row = split_address(rng)
                     except:
                         if self.debug:
-                            print('WARNING: Unknown address: %s is not a cell/range reference, nor a named range' % to_str(rng))
+                            logging.warning('WARNING: Unknown address: %s is not a cell/range reference, nor a named range' % to_str(rng))
                         sh = None
 
                 if sh:
diff --git a/koala/excellib.py b/koala/excellib.py
index 46b85008..01172e5a 100644
--- a/koala/excellib.py
+++ b/koala/excellib.py
@@ -6,10 +6,9 @@
 
 # source: https://github.com/dgorissen/pycel/blob/master/src/pycel/excellib.py
 
-from __future__ import absolute_import, division
-
 import itertools
 import numpy as np
+import numpy_financial as npf
 import scipy.optimize
 import datetime
 import random
@@ -18,8 +17,6 @@
 from calendar import monthrange
 from dateutil.relativedelta import relativedelta
 
-from openpyxl.compat import unicode
-
 from koala.utils import *
 from koala.Range import RangeCore as Range
 from koala.ExcelError import *
@@ -427,7 +424,7 @@ def irr(values, guess = None):
         raise ValueError('guess value for excellib.irr() is %s and not 0' % guess)
     else:
         try:
-            return np.irr(values)
+            return npf.irr(values)
         except Exception as e:
             return ExcelError('#NUM!', e)
 
@@ -729,7 +726,7 @@ def randbetween(bottom, top):
 
 def right(text,n):
     #TODO: hack to deal with naca section numbers
-    if isinstance(text, unicode) or isinstance(text,str):
+    if isinstance(text, str):
         return text[-n:]
     else:
         # TODO: get rid of the decimal
diff --git a/koala/reader.py b/koala/reader.py
index 8a679204..4a4a9076 100644
--- a/koala/reader.py
+++ b/koala/reader.py
@@ -1,6 +1,5 @@
-from __future__ import print_function
-
 from io import BytesIO
+import logging
 import re
 import os
 import json
@@ -101,13 +100,13 @@ def read_named_ranges(archive):
 
     return dict
 
-def read_cells(archive, ignore_sheets = [], ignore_hidden = False):
+def read_cells(archive, ignore_sheets = [], ignore_hidden = False, include_only_sheets=None):
     global debug
 
-    # print('___### Reading Cells from XLSX ###___')
-
     cells = {}
 
+    sheets = []
+
     functions = set()
 
     cts = dict(read_content_types(archive))
@@ -120,12 +119,20 @@ def read_cells(archive, ignore_sheets = [], ignore_hidden = False):
     else:
         shared_strings = []
 
+    ignore_sheets = frozenset(ignore_sheets)
+
+    if include_only_sheets is not None:
+        include_only_sheets = frozenset(include_only_sheets)
+
     for sheet in detect_worksheets(archive):
         sheet_name = sheet['title']
 
         function_map = {}
 
-        if sheet_name in ignore_sheets: continue
+        if sheet_name in ignore_sheets or (include_only_sheets is not None and sheet_name not in include_only_sheets):
+            continue
+
+        sheets.append(sheet_name)
 
         root = fromstring(archive.read(sheet['path'])) # it is necessary to use cElementTree from xml module, otherwise root.findall doesn't work as it should
 
@@ -159,21 +166,21 @@ def read_cells(archive, ignore_sheets = [], ignore_hidden = False):
             if not skip:
                 cell = {'a': '%s!%s' % (sheet_name, cell_address), 'f': None, 'v': None}
                 if debug:
-                    print('Cell', cell['a'])
+                    logging.debug('Cell', cell['a'])
                 for child in c:
                     child_data_type = child.get('t', 'n') # if no type assigned, assign 'number'
 
                     if child.tag == '{%s}f' % SHEET_MAIN_NS :
                         if 'ref' in child.attrib: # the first cell of a shared formula has a 'ref' attribute
                             if debug:
-                                print('*** Found definition of shared formula ***', child.text, child.attrib['ref'])
+                                logging.debug('*** Found definition of shared formula ***', child.text, child.attrib['ref'])
                             if "si" in child.attrib:
                                 function_map[child.attrib['si']] = (child.attrib['ref'], Translator(str('=' + child.text), cell_address)) # translator of openpyxl needs a unicode argument that starts with '='
                             # else:
                             #     print "Encountered cell with ref but not si: ", sheet_name, child.attrib['ref']
                         if child_data_type == 'shared':
                             if debug:
-                                print('*** Found child %s of shared formula %s ***' % (cell_address, child.attrib['si']))
+                                logging.debug('*** Found child %s of shared formula %s ***' % (cell_address, child.attrib['si']))
 
                             ref = function_map[child.attrib['si']][0]
                             formula = function_map[child.attrib['si']][1]
@@ -215,17 +222,7 @@ def read_cells(archive, ignore_sheets = [], ignore_hidden = False):
                     else:
                         cells[sheet_name + "!" + cell_address] = Cell(cell_address, sheet_name, value = cell['v'], formula = cleaned_formula, should_eval=should_eval)
 
-        # if nb_hidden > 0:
-            # print('Ignored %i hidden cells in sheet %s' % (nb_hidden, sheet_name))
-
-    # print('Nb of different functions %i' % len(functions))
-    # print(functions)
-
-    # for f in functions:
-    #     if f not in existing:
-    #         print('== Missing function: %s' % f)
-
-    return cells
+    return cells, sheets
 
 
 def read_rels(archive):
diff --git a/koala/serializer.py b/koala/serializer.py
index 27dd2d79..dec4c7ad 100644
--- a/koala/serializer.py
+++ b/koala/serializer.py
@@ -1,13 +1,10 @@
-from __future__ import absolute_import, print_function
-
 import json
 import gzip
+import logging
 import networkx
 
 from networkx.classes.digraph import DiGraph
-from networkx.readwrite import json_graph
 from networkx.drawing.nx_pydot import write_dot
-from openpyxl.compat import unicode
 
 from koala.Cell import Cell
 from koala.Range import RangeCore, RangeFactory
@@ -46,15 +43,15 @@ def parse_cell_info(cell):
             is_named_range,
             is_pointer,
             should_eval
-        ]) + u"\n").encode('utf-8'))
+        ]) + "\n").encode('utf-8'))
 
     for cell in simple_cells:
         parse_cell_info(cell)
         value = cell.value
-        if isinstance(value, unicode):
+        if isinstance(value, str):
             outfile.write(cell.value.encode('utf-8') + b"\n")
         else:
-            outfile.write((str(cell.value) + u"\n").encode('utf-8'))
+            outfile.write((str(cell.value) + "\n").encode('utf-8'))
         outfile.write(b"====" + b"\n")
 
     outfile.write(b"-----" + b"\n")
@@ -63,9 +60,9 @@ def parse_cell_info(cell):
         parse_cell_info(cell)
 
         if cell.range.is_pointer:
-            outfile.write((json.dumps(cell.range.reference) + u"\n").encode('utf-8'))
+            outfile.write((json.dumps(cell.range.reference) + "\n").encode('utf-8'))
         else:
-            outfile.write((cell.range.name + u"\n").encode('utf-8'))
+            outfile.write((cell.range.name + "\n").encode('utf-8'))
 
         outfile.write(b"====" + b"\n")
         outfile.write(b"====" + b"\n")
@@ -73,15 +70,15 @@ def parse_cell_info(cell):
     # writing the edges
     outfile.write(b"edges" + b"\n")
     for source, target in self.G.edges():
-        outfile.write((source.address() + SEP + target.address() + u"\n").encode('utf-8'))
+        outfile.write((source.address() + SEP + target.address() + "\n").encode('utf-8'))
 
     # writing the rest
     if self.outputs is not None:
         outfile.write(b"outputs" + b"\n")
-        outfile.write((SEP.join(self.outputs) + u"\n").encode('utf-8'))
+        outfile.write((SEP.join(self.outputs) + "\n").encode('utf-8'))
     if self.inputs is not None:
         outfile.write(b"inputs" + b"\n")
-        outfile.write((SEP.join(self.inputs) + u"\n").encode('utf-8'))
+        outfile.write((SEP.join(self.inputs) + "\n").encode('utf-8'))
     outfile.write(b"named_ranges" + b"\n")
     for k in self.named_ranges:
         outfile.write((k + SEP + self.named_ranges[k] + u"\n").encode('utf-8'))
@@ -191,7 +188,7 @@ def to_float(string):
     G.add_nodes_from(nodes)
     G.add_edges_from(edges)
 
-    print("Graph loading done, %s nodes, %s edges, %s cellmap entries" % (len(G.nodes()),len(G.edges()),len(cellmap)))
+    logging.debug("Graph loading done, %s nodes, %s edges, %s cellmap entries" % (len(G.nodes()),len(G.edges()),len(cellmap)))
 
     return (G, cellmap, named_ranges, pointers, outputs, inputs)
 
@@ -205,41 +202,15 @@ def dump_json(self, fname):
 
 def load_json(fname):
 
-    def _decode_list(data):
-        rv = []
-        for item in data:
-            if isinstance(item, unicode) and unicode != str:
-                item = item.encode('utf-8')
-            elif isinstance(item, list) and unicode != str:
-                item = _decode_list(item)
-            elif isinstance(item, dict):
-                item = _decode_dict(item)
-            rv.append(item)
-        return rv
-
-    def _decode_dict(data):
-        rv = {}
-        for key, value in data.items():
-            if isinstance(key, unicode) and unicode != str:
-                key = key.encode('utf-8')
-            if isinstance(value, unicode) and unicode != str:
-                value = value.encode('utf-8')
-            elif isinstance(value, list):
-                value = _decode_list(value)
-            elif isinstance(value, dict):
-                value = _decode_dict(value)
-            rv[key] = value
-        return rv
-
     with gzip.GzipFile(fname, 'r') as infile:
-        data = json.loads(infile.read().decode('utf-8'), object_hook=_decode_dict)
+        data = json.loads(infile.read().decode('utf-8'))
 
     return data
 
 
 ########### based on dot #################
-def export_to_dot(self,fname):
-    write_dot(self.G,fname)
+def export_to_dot(self, fname):
+    write_dot(self.G, fname)
 
 
 ########### plotting #################
diff --git a/koala/tokenizer.py b/koala/tokenizer.py
index 03fbdf45..acdfadf5 100644
--- a/koala/tokenizer.py
+++ b/koala/tokenizer.py
@@ -1,5 +1,3 @@
-from __future__ import division, print_function
-
 # cython: profile=True
 
 #========================================================================
@@ -27,10 +25,8 @@
 #========================================================================
 
 import re
-import six
 import collections
-
-from koala.utils import old_div
+import logging
 
 
 #========================================================================
@@ -433,7 +429,7 @@ def EOF():
             # standard postfix operators
             if ("%".find(currentChar()) != -1):
                 if (len(token) > 0):
-                    tokens.add(old_div(float(token), 100), self.TOK_TYPE_OPERAND)
+                    tokens.add(float(token) / 100, self.TOK_TYPE_OPERAND)
                     token = ""
                 else:
                     tokens.add('*', self.TOK_TYPE_OP_IN)
@@ -501,9 +497,9 @@ def EOF():
                   ):
                     pass
                 elif (not(
-                     ((six.next(tokens).ttype == self.TOK_TYPE_FUNCTION) and (tokens.next().tsubtype == self.TOK_SUBTYPE_START)) or
-                     ((six.next(tokens).ttype == self.TOK_TYPE_SUBEXPR) and (tokens.next().tsubtype == self.TOK_SUBTYPE_START)) or
-                     (six.next(tokens).ttype == self.TOK_TYPE_OPERAND)
+                     ((next(tokens).ttype == self.TOK_TYPE_FUNCTION) and (tokens.next().tsubtype == self.TOK_SUBTYPE_START)) or
+                     ((next(tokens).ttype == self.TOK_TYPE_SUBEXPR) and (tokens.next().tsubtype == self.TOK_SUBTYPE_START)) or
+                     (next(tokens).ttype == self.TOK_TYPE_OPERAND)
                      )
                    ):
                     pass
@@ -700,7 +696,7 @@ def shunting_yard(expression):
         else:
             tokens.append(t)
 
-    print("tokens: ", "|".join([x.tvalue for x in tokens]))
+    logging.debug("tokens: ", "|".join([x.tvalue for x in tokens]))
 
     #http://office.microsoft.com/en-us/excel-help/calculation-operators-and-precedence-HP010078886.aspx
     operators = {}
@@ -728,12 +724,12 @@ def shunting_yard(expression):
     arg_count = []
 
     def po():
-        print("output: ", "|".join([x.tvalue for x in output]))
+        logging.debug("output: ", "|".join([x.tvalue for x in output]))
     def so():
-        print("stack:", "|".join([x.tvalue for x in stack]))
+        logging.debug("stack:", "|".join([x.tvalue for x in stack]))
 
     for t in tokens:
-        print(t, t.type)
+        logging.debug(t, t.type)
         if t.ttype == "operand":
 
             output.append(create_node(t))
@@ -803,7 +799,7 @@ def so():
                 w = were_values.pop()
                 if w: a += 1
                 f.num_args = a
-                print(f, "has ",a," args")
+                logging.debug(f, "has ",a," args")
                 output.append(f)
 
     while stack:
@@ -813,5 +809,5 @@ def so():
         output.append(create_node(stack.pop()))
 
     #print "Stack is: ", "|".join(stack)
-    print("Output is: ", "|".join([x.tvalue for x in output]))
+    logging.debug("Output is: ", "|".join([x.tvalue for x in output]))
     return output
diff --git a/koala/utils.py b/koala/utils.py
index 626f1980..242d2062 100644
--- a/koala/utils.py
+++ b/koala/utils.py
@@ -1,20 +1,11 @@
 # cython: profile=True
 
-from __future__ import absolute_import, division
-
-import collections
-import numbers
+import collections.abc
 import re
 import datetime as dt
-try:
-    from functools import lru_cache
-except ImportError:  # fix for Python 2.7
-    from backports.functools_lru_cache import lru_cache
-from six import string_types
+from functools import lru_cache
 from copy import deepcopy
 
-from openpyxl.compat import unicode
-
 from .ExcelError import ExcelError
 
 ASCII = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
@@ -150,12 +141,6 @@ def resolve_range(rng, should_flatten = False, sheet=''):
     else:
         pass
 
-    # `unicode` != `str` in Python2. See `from openpyxl.compat import unicode`
-    if type(sheet) == str and str != unicode:
-        sheet = unicode(sheet, 'utf-8')
-    if type(rng) == str and str != unicode:
-        rng = unicode(rng, 'utf-8')
-
     key = rng+str(should_flatten)+sheet
 
     if key in resolve_range_cache:
@@ -347,10 +332,10 @@ def get_linest_degree(excel,cl):
     return (max(degree,1),coef)
 
 def flatten(l, only_lists = False):
-    instance = list if only_lists else collections.Iterable
+    instance = list if only_lists else collections.abc.Iterable
 
     for el in l:
-        if isinstance(el, instance) and not isinstance(el, string_types):
+        if isinstance(el, instance) and not isinstance(el, str):
             for sub in flatten(el, only_lists = only_lists):
                 yield sub
         else:
@@ -587,7 +572,7 @@ def extract_numeric_values(*args):
     values = []
 
     for arg in args:
-        if isinstance(arg, collections.Iterable) and type(arg) != list and type(arg) != tuple and type(arg) != str and type(arg) != unicode: # does not work fo other Iterable than RangeCore, but can t import RangeCore here for circular reference issues
+        if isinstance(arg, collections.abc.Iterable) and type(arg) != list and type(arg) != tuple and type(arg) != str: # does not work fo other Iterable than RangeCore, but can t import RangeCore here for circular reference issues
             values.extend([x for x in arg.values if is_number(x) and type(x) is not bool])
             # for x in arg.values:
             #     if is_number(x) and type(x) is not bool: # excludes booleans from nested ranges
@@ -602,21 +587,6 @@ def extract_numeric_values(*args):
 
     return values
 
-
-def old_div(a, b):
-    """
-    Equivalent to ``a / b`` on Python 2 without ``from __future__ import
-    division``.
-
-    Copied from:
-    https://github.com/PythonCharmers/python-future/blob/master/src/past/utils/__init__.py
-    """
-    if isinstance(a, numbers.Integral) and isinstance(b, numbers.Integral):
-        return a // b
-    else:
-        return a / b
-
-
 def safe_iterator(node, tag=None):
     """Return an iterator or an empty list"""
     if node is None:
diff --git a/requirements.txt b/requirements.txt
index c676f783..4486046f 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,9 +1,8 @@
-networkx==2.1
-openpyxl==2.5.3
+networkx>=2.4
+openpyxl>=3.0.3
 numpy>=1.14.2
-Cython==0.28.2
-lxml==4.1.1
-six==1.11.0
+numpy-financial>=1.0.0
+Cython>=0.29.15
+lxml>=4.5.0
 scipy>=1.0.0
-python-dateutil==2.8.0
-backports.functools_lru_cache==1.5
+python-dateutil>=2.8.0
diff --git a/setup.py b/setup.py
index ace79353..5cf93c7c 100644
--- a/setup.py
+++ b/setup.py
@@ -21,7 +21,7 @@
         packages=find_packages(),
 
         classifiers=[
-            "Programming Language :: Python :: 2.7 :: 3",
+            "Programming Language :: Python :: 3",
             "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
             'Operating System :: Microsoft :: Windows',
             'Operating System :: MacOS :: MacOS X',
@@ -63,14 +63,13 @@
 
 
         install_requires=[
-            'networkx >= 2.1',
-            'openpyxl >= 2.5.3',
+            'networkx >= 2.4',
+            'openpyxl >= 3.0.3',
             'numpy >= 1.14.2',
-            'Cython >= 0.28.2',
-            'lxml >= 4.1.1',
-            'six >= 1.11.0',
+            'numpy-financial>=1.0.0',
+            'Cython >= 0.29.15',
+            'lxml >= 4.5.0',
             'scipy>=1.0.0',
-            'python-dateutil==2.8.0',
-            'backports.functools_lru_cache==1.5'
+            'python-dateutil>=2.8.0'
         ]
     )
diff --git a/tests/ast/test_compile.py b/tests/ast/test_compile.py
index 7c6992b5..37133d2c 100644
--- a/tests/ast/test_compile.py
+++ b/tests/ast/test_compile.py
@@ -46,7 +46,7 @@ def test_string(self):
 
         code, ast = cell2code(cell, named_ranges=[])
 
-        assert code == u'u"hello world"'
+        assert code == u'"hello world"'
 
         RangeCore
         assert eval(code) == u"hello world"
@@ -57,7 +57,7 @@ def test_string_unicode(self):
 
         code, ast = cell2code(cell, named_ranges=[])
 
-        assert code == u'u"hello world ☺"'
+        assert code == u'"hello world ☺"'
 
         RangeCore
         assert eval(code) == u"hello world ☺"
@@ -68,7 +68,7 @@ def test_string_quotes(self):
 
         code, ast = cell2code(cell, named_ranges=[])
 
-        assert code == u'u"hello \\"world\'"'
+        assert code == u'"hello \\"world\'"'
 
         RangeCore
         assert eval(code) == u"hello \"world'"
diff --git a/tests/ast/test_range.py b/tests/ast/test_range.py
index c51be38e..3512196b 100644
--- a/tests/ast/test_range.py
+++ b/tests/ast/test_range.py
@@ -1,4 +1,3 @@
-from __future__ import print_function
 import unittest
 
 from koala.Range import RangeFactory
@@ -12,12 +11,10 @@ def test_Range_Factory(self):
         cellmap = {'A1': 25, 'B1': 34, 'C1': 79}
         Range = RangeFactory(cellmap)
         range = Range('A1:C1')
-        print('RANGE', range)
         self.assertEqual(range[(1, 'B')], 34)
 
     def test_Range_getter(self):
         range = Range('Sheet1!A1:A3', [10, 20, 30])
-        print('RANGE2', range)
         self.assertEqual(range.values, [10, 20, 30])
 
     def test_Range_setter(self):
diff --git a/tests/excel/test_excel.py b/tests/excel/test_excel.py
index 28348dff..0205a48a 100644
--- a/tests/excel/test_excel.py
+++ b/tests/excel/test_excel.py
@@ -12,7 +12,7 @@ class Test_SharedFormula(unittest.TestCase):
     def setUp(self):
         file_name = os.path.abspath("./tests/files/SharedFormula.xlsx")
         archive = read_archive(file_name)
-        self.cells = read_cells(archive)
+        self.cells, _ = read_cells(archive)
 
     @unittest.skip('This test fails.')
     def test_nb_formulas(self):
diff --git a/tests/excel/test_functions.py b/tests/excel/test_functions.py
index 1d1b76a9..7be69dee 100644
--- a/tests/excel/test_functions.py
+++ b/tests/excel/test_functions.py
@@ -1,6 +1,5 @@
-from __future__ import absolute_import
-
-import pyximport; pyximport.install()
+import pyximport
+pyximport.install()
 
 import unittest
 
diff --git a/tests/excel/test_utils.py b/tests/excel/test_utils.py
index c36b207f..7ad16685 100644
--- a/tests/excel/test_utils.py
+++ b/tests/excel/test_utils.py
@@ -1,6 +1,6 @@
-from __future__ import absolute_import
+import pyximport
 
-import pyximport; pyximport.install()
+pyximport.install()
 
 import unittest
 
diff --git a/tox.ini b/tox.ini
index 1ba3b0b3..fcee21e8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,13 +1,11 @@
 [tox]
-envlist = py27,py34,py35,py36
+envlist = py36,py37
 recreate = True
 
 [testenv]
 basepython =
-    py27: python2.7
-    py34: python3.4
-    py35: python3.5
-    py36: python3.6
+    py37: python3.7
+    py38: python3.8
 deps=
     pytest
     -r{toxinidir}/requirements.txt