Remove left-over Python 2-isms (#1283)

This removes Python 2 compatibility shims and constructs that were not
addressed in a previous PR to remove Python 2 shims.

* Removed `from builtins import map, zip` in
`src/openfermion/utils/operator_utils.py`. These were legacy imports
from the `future` library to provide Python 3 behavior in Python 2.

* Removed explicit `(object)` inheritance from all class definitions.

* Removed the redundant `u` prefix from string literals, because in
Python 3, all strings are Unicode by default.

Author: mhucka

.pylintrc

@@ -19,9 +19,7 @@
 
 [MAIN]
 
-# Analyse import fallback blocks. This can be used to support both Python 2 and
-# 3 compatible code, which means that the block might have code that exists
-# only in one or another interpreter, leading to false positives when analysed.
+# Analyse import fallback blocks.
 analyse-fallback-blocks=no
 
 # Clear in-memory caches upon conclusion of linting. Useful if running pylint
@@ -1027,4 +1025,4 @@ init-import=yes
 
 # List of qualified module names which can have objects that can redefine
 # builtins.
-redefining-builtins-modules=six.moves,past.builtins,future.builtins,builtins,io
+redefining-builtins-modules=builtins,io

src/openfermion/chem/molecular_data.py

@@ -396,7 +396,7 @@ def spinorb_from_spatial(one_body_integrals, two_body_integrals):
     return one_body_coefficients, two_body_coefficients
 
 
-class MolecularData(object):
+class MolecularData:
     """Class for storing molecule data from a fixed basis set at a fixed
     geometry that is obtained from classical electronic structure
     packages. Not every field is filled in every calculation. All data
@@ -859,7 +859,7 @@ def load(self):
             # Load charge:
             self.charge = int(f["charge"][...])
             # Load description:
-            self.description = f["description"][...].tobytes().decode('utf-8').rstrip(u'\x00')
+            self.description = f["description"][...].tobytes().decode('utf-8').rstrip('\x00')
             # Load name:
             self.name = f["name"][...].tobytes().decode('utf-8')
             # Load n_atoms:

src/openfermion/contrib/representability/_dualbasis.py

@@ -2,7 +2,7 @@
 import copy
 
 
-class DualBasisElement(object):
+class DualBasisElement:
     """
     This object is named after the algebraic dual space or dual vector space
 
@@ -154,7 +154,7 @@ def __add__(self, other):
             raise TypeError("DualBasisElement can be added to same type or DualBasis")
 
 
-class DualBasis(object):
+class DualBasis:
     def __init__(self, elements: Optional[Union[None, List[DualBasisElement]]] = None):
         """
         A collection of DualBasisElements

src/openfermion/contrib/representability/_multitensor.py

@@ -3,7 +3,7 @@
 from openfermion.contrib.representability._dualbasis import DualBasisElement, DualBasis
 
 
-class TMap(object):
+class TMap:
     def __init__(self, tensors):
         """
         provide a map of tensor name to tensors
@@ -22,7 +22,7 @@ def __iter__(self):
             yield tt
 
 
-class MultiTensor(object):
+class MultiTensor:
     def __init__(self, tensors, dual_basis=None):
         """
         A collection of tensor objects with maps from name to tensor

src/openfermion/contrib/representability/_namedtensor.py

@@ -4,7 +4,7 @@
 from openfermion.contrib.representability._bijections import Bijection, index_index_basis
 
 
-class Tensor(object):
+class Tensor:
     """
     Instantiation of named tensor
     """

src/openfermion/hamiltonians/special_operators_test.py

@@ -173,7 +173,7 @@ def test_init(self):
         # Test 'c' operator
         op1 = majorana_operator((2, 0))
         op2 = majorana_operator('c2')
-        op3 = majorana_operator(u'c2')
+        op3 = majorana_operator('c2')
         correct = FermionOperator('2^') + FermionOperator('2')
         self.assertEqual(op1, op2)
         self.assertEqual(op1, op3)

src/openfermion/linalg/davidson.py

@@ -31,7 +31,7 @@ class DavidsonError(Exception):
     pass
 
 
-class DavidsonOptions(object):
+class DavidsonOptions:
     """Davidson algorithm iteration options."""
 
     def __init__(self, max_subspace=100, max_iterations=300, eps=1e-6, real_only=False):
@@ -68,7 +68,7 @@ def set_dimension(self, dimension):
         self.max_subspace = min(self.max_subspace, dimension + 1)
 
 
-class Davidson(object):
+class Davidson:
     """Davidson algorithm to get the n states with smallest eigenvalues."""
 
     def __init__(self, linear_operator, linear_operator_diagonal, options=None):
@@ -350,7 +350,7 @@ def __init__(self, qubit_operator, n_qubits=None, options=None):
             n_qubits(int): Number of qubits.
             options(DavidsonOptions): Iteration options.
         """
-        super(QubitDavidson, self).__init__(
+        super().__init__(
             generate_linear_qubit_operator(qubit_operator, n_qubits, options),
             get_linear_qubit_operator_diagonal(qubit_operator, n_qubits),
             options=options,
@@ -366,9 +366,7 @@ def __init__(self, sparse_matrix, options=None):
             sparse_matrix(scipy.sparse.spmatrix): A sparse matrix in scipy.
             options(DavidsonOptions): Iteration options.
         """
-        super(SparseDavidson, self).__init__(
-            sparse_matrix, sparse_matrix.diagonal(), options=options
-        )
+        super().__init__(sparse_matrix, sparse_matrix.diagonal(), options=options)
 
 
 def generate_random_vectors(row, col, real_only=False):

src/openfermion/linalg/linear_qubit_operator.py

@@ -23,7 +23,7 @@
 from openfermion.utils.operator_utils import count_qubits
 
 
-class LinearQubitOperatorOptions(object):
+class LinearQubitOperatorOptions:
     """Options for LinearQubitOperator."""
 
     def __init__(self, processes=10, pool=None):
@@ -94,7 +94,7 @@ def __init__(self, qubit_operator, n_qubits=None):
             )
 
         n_hilbert = 2**n_qubits
-        super(LinearQubitOperator, self).__init__(shape=(n_hilbert, n_hilbert), dtype=complex)
+        super().__init__(shape=(n_hilbert, n_hilbert), dtype=complex)
         self.qubit_operator = qubit_operator
         self.n_qubits = n_qubits
 
@@ -156,9 +156,7 @@ def __init__(self, qubit_operator, n_qubits=None, options=None):
         """
         n_qubits = n_qubits or count_qubits(qubit_operator)
         n_hilbert = 2**n_qubits
-        super(ParallelLinearQubitOperator, self).__init__(
-            shape=(n_hilbert, n_hilbert), dtype=complex
-        )
+        super().__init__(shape=(n_hilbert, n_hilbert), dtype=complex)
 
         self.qubit_operator = qubit_operator
         self.n_qubits = n_qubits

src/openfermion/ops/operators/binary_code.py

@@ -73,7 +73,7 @@ class BinaryCodeError(Exception):
     pass
 
 
-class BinaryCode(object):
+class BinaryCode:
     r"""The BinaryCode class provides a representation of an encoding-decoding
     pair for binary vectors of different lengths, where the decoding is allowed
     to be non-linear.

src/openfermion/ops/operators/binary_polynomial.py

@@ -23,7 +23,7 @@ class BinaryPolynomialError(Exception):
     pass
 
 
-class BinaryPolynomial(object):
+class BinaryPolynomial:
     r"""The BinaryPolynomial class provides an analytic representation
     of non-linear binary functions. An instance of this class describes
     a term of binary variables (variables of the values {0,1}, indexed