diff --git a/pandas/compat/chainmap.py b/pandas/compat/chainmap.py
index 83f1da597d6a6..84824207de2a9 100644
--- a/pandas/compat/chainmap.py
+++ b/pandas/compat/chainmap.py
@@ -15,9 +15,3 @@ def __delitem__(self, key):
                 del mapping[key]
                 return
         raise KeyError(key)
-
-    # override because the m parameter is introduced in Python 3.4
-    def new_child(self, m=None):
-        if m is None:
-            m = {}
-        return self.__class__(m, *self.maps)
diff --git a/pandas/core/computation/align.py b/pandas/core/computation/align.py
index 1046401850963..3e1e5ed89d877 100644
--- a/pandas/core/computation/align.py
+++ b/pandas/core/computation/align.py
@@ -9,6 +9,7 @@
 from pandas.errors import PerformanceWarning
 
 import pandas as pd
+from pandas.core.base import PandasObject
 import pandas.core.common as com
 from pandas.core.computation.common import _result_type_many
 
@@ -34,7 +35,7 @@ def _zip_axes_from_type(typ, new_axes):
 
 def _any_pandas_objects(terms):
     """Check a sequence of terms for instances of PandasObject."""
-    return any(isinstance(term.value, pd.core.generic.PandasObject) for term in terms)
+    return any(isinstance(term.value, PandasObject) for term in terms)
 
 
 def _filter_special_cases(f):
@@ -132,7 +133,8 @@ def _align(terms):
 
 
 def _reconstruct_object(typ, obj, axes, dtype):
-    """Reconstruct an object given its type, raw value, and possibly empty
+    """
+    Reconstruct an object given its type, raw value, and possibly empty
     (None) axes.
 
     Parameters
@@ -157,7 +159,7 @@ def _reconstruct_object(typ, obj, axes, dtype):
 
     res_t = np.result_type(obj.dtype, dtype)
 
-    if not isinstance(typ, partial) and issubclass(typ, pd.core.generic.PandasObject):
+    if not isinstance(typ, partial) and issubclass(typ, PandasObject):
         return typ(obj, dtype=res_t, **axes)
 
     # special case for pathological things like ~True/~False
diff --git a/pandas/core/computation/common.py b/pandas/core/computation/common.py
index 2a4277e4e0a20..bd32c8bee1cdf 100644
--- a/pandas/core/computation/common.py
+++ b/pandas/core/computation/common.py
@@ -36,8 +36,3 @@ def _remove_spaces_column_name(name):
 
 class NameResolutionError(NameError):
     pass
-
-
-class StringMixin:
-    # TODO: delete this class. Removing this ATM caused a failure.
-    pass
diff --git a/pandas/core/computation/engines.py b/pandas/core/computation/engines.py
index 2c94b142a45b3..3cc34ea1f4ed7 100644
--- a/pandas/core/computation/engines.py
+++ b/pandas/core/computation/engines.py
@@ -17,7 +17,8 @@ class NumExprClobberingError(NameError):
 
 
 def _check_ne_builtin_clash(expr):
-    """Attempt to prevent foot-shooting in a helpful way.
+    """
+    Attempt to prevent foot-shooting in a helpful way.
 
     Parameters
     ----------
@@ -53,7 +54,8 @@ def convert(self):
         return printing.pprint_thing(self.expr)
 
     def evaluate(self):
-        """Run the engine on the expression
+        """
+        Run the engine on the expression.
 
         This method performs alignment which is necessary no matter what engine
         is being used, thus its implementation is in the base class.
@@ -78,7 +80,8 @@ def _is_aligned(self):
 
     @abc.abstractmethod
     def _evaluate(self):
-        """Return an evaluated expression.
+        """
+        Return an evaluated expression.
 
         Parameters
         ----------
@@ -94,7 +97,6 @@ def _evaluate(self):
 
 
 class NumExprEngine(AbstractEngine):
-
     """NumExpr engine class"""
 
     has_neg_frac = True
@@ -127,8 +129,8 @@ def _evaluate(self):
 
 
 class PythonEngine(AbstractEngine):
-
-    """Evaluate an expression in Python space.
+    """
+    Evaluate an expression in Python space.
 
     Mostly for testing purposes.
     """
diff --git a/pandas/core/computation/expr.py b/pandas/core/computation/expr.py
index d0d87c23e9346..a58f256cf61d4 100644
--- a/pandas/core/computation/expr.py
+++ b/pandas/core/computation/expr.py
@@ -41,7 +41,8 @@
 
 
 def tokenize_string(source):
-    """Tokenize a Python source code string.
+    """
+    Tokenize a Python source code string.
 
     Parameters
     ----------
diff --git a/pandas/core/computation/expressions.py b/pandas/core/computation/expressions.py
index 0ecf56cf6fe96..5b6d275001d36 100644
--- a/pandas/core/computation/expressions.py
+++ b/pandas/core/computation/expressions.py
@@ -99,15 +99,13 @@ def _evaluate_numexpr(op, op_str, a, b, truediv=True, reversed=False, **eval_kwa
     result = None
 
     if _can_use_numexpr(op, op_str, a, b, "evaluate"):
-        try:
-
-            # we were originally called by a reversed op
-            # method
-            if reversed:
-                a, b = b, a
+        if reversed:
+            # we were originally called by a reversed op method
+            a, b = b, a
 
-            a_value = getattr(a, "values", a)
-            b_value = getattr(b, "values", b)
+        a_value = getattr(a, "values", a)
+        b_value = getattr(b, "values", b)
+        try:
             result = ne.evaluate(
                 "a_value {op} b_value".format(op=op_str),
                 local_dict={"a_value": a_value, "b_value": b_value},
@@ -138,11 +136,11 @@ def _where_numexpr(cond, a, b):
     result = None
 
     if _can_use_numexpr(None, "where", a, b, "where"):
+        cond_value = getattr(cond, "values", cond)
+        a_value = getattr(a, "values", a)
+        b_value = getattr(b, "values", b)
 
         try:
-            cond_value = getattr(cond, "values", cond)
-            a_value = getattr(a, "values", a)
-            b_value = getattr(b, "values", b)
             result = ne.evaluate(
                 "where(cond_value, a_value, b_value)",
                 local_dict={
@@ -209,7 +207,8 @@ def evaluate(op, op_str, a, b, use_numexpr=True, **eval_kwargs):
     Parameters
     ----------
     op : the actual operand
-    op_str : the string version of the op
+    op_str : str
+        The string version of the op.
     a : left operand
     b : right operand
     use_numexpr : bool, default True
@@ -224,11 +223,11 @@ def evaluate(op, op_str, a, b, use_numexpr=True, **eval_kwargs):
 
 def where(cond, a, b, use_numexpr=True):
     """
-    Evaluate the where condition cond on a and b
+    Evaluate the where condition cond on a and b.
 
     Parameters
     ----------
-    cond : ndarray[bool]
+    cond : np.ndarray[bool]
     a : return if cond is True
     b : return if cond is False
     use_numexpr : bool, default True
diff --git a/pandas/core/computation/ops.py b/pandas/core/computation/ops.py
index b49220ae701bc..28b6aef693bfe 100644
--- a/pandas/core/computation/ops.py
+++ b/pandas/core/computation/ops.py
@@ -476,8 +476,8 @@ def isnumeric(dtype):
 
 
 class Div(BinOp):
-
-    """Div operator to special case casting.
+    """
+    Div operator to special case casting.
 
     Parameters
     ----------
@@ -508,8 +508,8 @@ def __init__(self, lhs, rhs, truediv, *args, **kwargs):
 
 
 class UnaryOp(Op):
-
-    """Hold a unary operator and its operands
+    """
+    Hold a unary operator and its operands.
 
     Parameters
     ----------
diff --git a/pandas/core/computation/scope.py b/pandas/core/computation/scope.py
index 8ddd0dd7622e7..b11411eb2dc66 100644
--- a/pandas/core/computation/scope.py
+++ b/pandas/core/computation/scope.py
@@ -15,9 +15,6 @@
 from pandas._libs.tslibs import Timestamp
 from pandas.compat.chainmap import DeepChainMap
 
-import pandas.core.computation as compu
-from pandas.core.computation.common import StringMixin
-
 
 def _ensure_scope(
     level, global_dict=None, local_dict=None, resolvers=(), target=None, **kwargs
@@ -67,7 +64,8 @@ def _raw_hex_id(obj):
 
 
 def _get_pretty_string(obj):
-    """Return a prettier version of obj
+    """
+    Return a prettier version of obj.
 
     Parameters
     ----------
@@ -84,9 +82,9 @@ def _get_pretty_string(obj):
     return sio.getvalue()
 
 
-class Scope(StringMixin):
-
-    """Object to hold scope, with a few bells to deal with some custom syntax
+class Scope:
+    """
+    Object to hold scope, with a few bells to deal with some custom syntax
     and contexts added by pandas.
 
     Parameters
@@ -105,7 +103,7 @@ class Scope(StringMixin):
     temps : dict
     """
 
-    __slots__ = "level", "scope", "target", "temps"
+    __slots__ = ["level", "scope", "target", "resolvers", "temps"]
 
     def __init__(
         self, level, global_dict=None, local_dict=None, resolvers=(), target=None
@@ -163,7 +161,8 @@ def has_resolvers(self):
         return bool(len(self.resolvers))
 
     def resolve(self, key, is_local):
-        """Resolve a variable name in a possibly local context
+        """
+        Resolve a variable name in a possibly local context.
 
         Parameters
         ----------
@@ -198,10 +197,14 @@ def resolve(self, key, is_local):
                 # e.g., df[df > 0]
                 return self.temps[key]
             except KeyError:
-                raise compu.ops.UndefinedVariableError(key, is_local)
+                # runtime import because ops imports from scope
+                from pandas.core.computation.ops import UndefinedVariableError
+
+                raise UndefinedVariableError(key, is_local)
 
     def swapkey(self, old_key, new_key, new_value=None):
-        """Replace a variable name, with a potentially new value.
+        """
+        Replace a variable name, with a potentially new value.
 
         Parameters
         ----------
@@ -225,7 +228,8 @@ def swapkey(self, old_key, new_key, new_value=None):
                 return
 
     def _get_vars(self, stack, scopes):
-        """Get specifically scoped variables from a list of stack frames.
+        """
+        Get specifically scoped variables from a list of stack frames.
 
         Parameters
         ----------
@@ -247,7 +251,8 @@ def _get_vars(self, stack, scopes):
                 del frame
 
     def update(self, level):
-        """Update the current scope by going back `level` levels.
+        """
+        Update the current scope by going back `level` levels.
 
         Parameters
         ----------
@@ -266,7 +271,8 @@ def update(self, level):
             del stack[:], stack
 
     def add_tmp(self, value):
-        """Add a temporary variable to the scope.
+        """
+        Add a temporary variable to the scope.
 
         Parameters
         ----------
@@ -297,7 +303,8 @@ def ntemps(self):
 
     @property
     def full_scope(self):
-        """Return the full scope for use with passing to engines transparently
+        """
+        Return the full scope for use with passing to engines transparently
         as a mapping.
 
         Returns