ENH: allow in-line expression assignment with df.eval

doc/source/enhancingperf.rst

@@ -441,18 +441,27 @@ The ``DataFrame.eval`` method (Experimental)
 In addition to the top level :func:`~pandas.eval` function you can also
 evaluate an expression in the "context" of a ``DataFrame``.
 
-
 .. ipython:: python
 
    df = DataFrame(randn(5, 2), columns=['a', 'b'])
    df.eval('a + b')
 
-
 Any expression that is a valid :func:`~pandas.eval` expression is also a valid
 ``DataFrame.eval`` expression, with the added benefit that *you don't have to
 prefix the name of the* ``DataFrame`` *to the column you're interested in
 evaluating*.
 
+In addition, you can perform in-line assignment of columns within an expression.
+This can allow for *formulaic evaluation*. Only a signle assignement is permitted.
+It can be a new column name or an existing column name. It must be a string-like.
+
+.. ipython:: python
+
+   df = DataFrame(dict(a = range(5), b = range(5,10)))
+   df.eval('c=a+b')
+   df.eval('d=a+b+c')
+   df.eval('a=1')
+   df
 
 Local Variables
 ~~~~~~~~~~~~~~~

doc/source/release.rst

@@ -72,7 +72,8 @@ Experimental Features
     ``numexpr`` behind the scenes. This results in large speedups for complicated
     expressions involving large DataFrames/Series.
   - :class:`~pandas.DataFrame` has a new :meth:`~pandas.DataFrame.eval` that
-    evaluates an expression in the context of the ``DataFrame``.
+    evaluates an expression in the context of the ``DataFrame``; allows
+    inline expression assignment
   - A :meth:`~pandas.DataFrame.query` method has been added that allows
     you to select elements of a ``DataFrame`` using a natural query syntax nearly
     identical to Python syntax.

pandas/computation/eval.py

@@ -113,7 +113,8 @@ def _convert_expression(expr):
 
 
 def eval(expr, parser='pandas', engine='numexpr', truediv=True,
-         local_dict=None, global_dict=None, resolvers=None, level=2):
+         local_dict=None, global_dict=None, resolvers=None, level=2,
+         target=None):
     """Evaluate a Python expression as a string using various backends.
 
     The following arithmetic operations are supported: ``+``, ``-``, ``*``,
@@ -169,6 +170,8 @@ def eval(expr, parser='pandas', engine='numexpr', truediv=True,
     level : int, optional
         The number of prior stack frames to traverse and add to the current
         scope. Most users will **not** need to change this parameter.
+    target : a target object for assignment, optional, default is None
+        essentially this is a passed in resolver
 
     Returns
     -------
@@ -194,7 +197,7 @@ def eval(expr, parser='pandas', engine='numexpr', truediv=True,
 
     # get our (possibly passed-in) scope
     env = _ensure_scope(global_dict=global_dict, local_dict=local_dict,
-                        resolvers=resolvers, level=level)
+                        resolvers=resolvers, level=level, target=target)
 
     parsed_expr = Expr(expr, engine=engine, parser=parser, env=env,
                        truediv=truediv)
@@ -203,4 +206,10 @@ def eval(expr, parser='pandas', engine='numexpr', truediv=True,
     eng = _engines[engine]
     eng_inst = eng(parsed_expr)
     ret = eng_inst.evaluate()
+
+    # assign if needed
+    if env.target is not None and parsed_expr.assigner is not None:
+        env.target[parsed_expr.assigner] = ret
+        return None
+
     return ret

pandas/computation/expr.py

@@ -21,13 +21,14 @@
                                     _arith_ops_syms, _unary_ops_syms, is_term)
 from pandas.computation.ops import _reductions, _mathops, _LOCAL_TAG
 from pandas.computation.ops import Op, BinOp, UnaryOp, Term, Constant, Div
+from pandas.computation.ops import UndefinedVariableError
 
 
 def _ensure_scope(level=2, global_dict=None, local_dict=None, resolvers=None,
-                  **kwargs):
+                  target=None, **kwargs):
     """Ensure that we are grabbing the correct scope."""
     return Scope(gbls=global_dict, lcls=local_dict, level=level,
-                 resolvers=resolvers)
+                 resolvers=resolvers, target=target)
 
 
 def _check_disjoint_resolver_names(resolver_keys, local_keys, global_keys):
@@ -88,20 +89,23 @@ class Scope(StringMixin):
     resolver_keys : frozenset
     """
     __slots__ = ('globals', 'locals', 'resolvers', '_global_resolvers',
-                 'resolver_keys', '_resolver', 'level', 'ntemps')
+                 'resolver_keys', '_resolver', 'level', 'ntemps', 'target')
 
-    def __init__(self, gbls=None, lcls=None, level=1, resolvers=None):
+    def __init__(self, gbls=None, lcls=None, level=1, resolvers=None, target=None):
         self.level = level
         self.resolvers = tuple(resolvers or [])
         self.globals = dict()
         self.locals = dict()
+        self.target = target
         self.ntemps = 1  # number of temporary variables in this scope
 
         if isinstance(lcls, Scope):
             ld, lcls = lcls, dict()
             self.locals.update(ld.locals.copy())
             self.globals.update(ld.globals.copy())
             self.resolvers += ld.resolvers
+            if ld.target is not None:
+                self.target = ld.target
             self.update(ld.level)
 
         frame = sys._getframe(level)
@@ -130,9 +134,10 @@ def __init__(self, gbls=None, lcls=None, level=1, resolvers=None):
 
     def __unicode__(self):
         return com.pprint_thing("locals: {0}\nglobals: {0}\nresolvers: "
-                                "{0}".format(list(self.locals.keys()),
-                                             list(self.globals.keys()),
-                                             list(self.resolver_keys)))
+                                "{0}\ntarget: {0}".format(list(self.locals.keys()),
+                                                          list(self.globals.keys()),
+                                                          list(self.resolver_keys),
+                                                          self.target))
 
     def __getitem__(self, key):
         return self.resolve(key, globally=False)
@@ -417,6 +422,7 @@ def __init__(self, env, engine, parser, preparser=_preparse):
         self.engine = engine
         self.parser = parser
         self.preparser = preparser
+        self.assigner = None
 
     def visit(self, node, **kwargs):
         if isinstance(node, string_types):
@@ -575,9 +581,33 @@ def visit_Slice(self, node, **kwargs):
         return slice(lower, upper, step)
 
     def visit_Assign(self, node, **kwargs):
-        cmpr = ast.Compare(ops=[ast.Eq()], left=node.targets[0],
-                           comparators=[node.value])
-        return self.visit(cmpr)
+        """
+        support a single assignment node, like
+
+        c = a + b
+
+        set the assigner at the top level, must be a Name node which
+        might or might not exist in the resolvers
+
+        """
+
+        if len(node.targets) != 1:
+            raise SyntaxError('can only assign a single expression')
+        if not isinstance(node.targets[0], ast.Name):
+            raise SyntaxError('left hand side of an assignment must be a single name')
+        if self.env.target is None:
+            raise ValueError('cannot assign without a target object')
+
+        try:
+            assigner = self.visit(node.targets[0], **kwargs)
+        except (UndefinedVariableError):
+            assigner = node.targets[0].id
+
+        self.assigner = getattr(assigner,'name',assigner)
+        if self.assigner is None:
+            raise SyntaxError('left hand side of an assignment must be a single resolvable name')
+
+        return self.visit(node.value, **kwargs)
 
     def visit_Attribute(self, node, **kwargs):
         attr = node.attr
@@ -669,7 +699,7 @@ def visitor(x, y):
         return reduce(visitor, operands)
 
 
-_python_not_supported = frozenset(['Assign', 'Dict', 'Call', 'BoolOp',
+_python_not_supported = frozenset(['Dict', 'Call', 'BoolOp',
                                    'In', 'NotIn'])
 _numexpr_supported_calls = frozenset(_reductions + _mathops)
 
@@ -712,6 +742,10 @@ def __init__(self, expr, engine='numexpr', parser='pandas', env=None,
         self.terms = self.parse()
         self.truediv = truediv
 
+    @property
+    def assigner(self):
+        return getattr(self._visitor,'assigner',None)
+
     def __call__(self):
         self.env.locals['truediv'] = self.truediv
         return self.terms(self.env)

pandas/computation/pytables.py

@@ -389,6 +389,11 @@ def visit_USub(self, node, **kwargs):
     def visit_Index(self, node, **kwargs):
         return self.visit(node.value).value
 
+    def visit_Assign(self, node, **kwargs):
+        cmpr = ast.Compare(ops=[ast.Eq()], left=node.targets[0],
+                           comparators=[node.value])
+        return self.visit(cmpr)
+
     def visit_Subscript(self, node, **kwargs):
         value = self.visit(node.value)
         slobj = self.visit(node.slice)

pandas/computation/tests/test_eval.py

@@ -24,6 +24,7 @@
 from pandas.computation.ops import (_binary_ops_dict, _unary_ops_dict,
                                     _special_case_arith_ops_syms,
                                     _arith_ops_syms, _bool_ops_syms)
+from pandas.computation.common import NameResolutionError
 import pandas.computation.expr as expr
 import pandas.util.testing as tm
 from pandas.util.testing import (assert_frame_equal, randbool,
@@ -1151,9 +1152,65 @@ def test_assignment_fails(self):
         df = DataFrame(np.random.randn(5, 3), columns=list('abc'))
         df2 = DataFrame(np.random.randn(5, 3))
         expr1 = 'df = df2'
-        self.assertRaises(NotImplementedError, self.eval, expr1,
+        self.assertRaises(ValueError, self.eval, expr1,
                           local_dict={'df': df, 'df2': df2})
 
+    def test_assignment_column(self):
+        skip_if_no_ne('numexpr')
+        df = DataFrame(np.random.randn(5, 2), columns=list('ab'))
+        orig_df = df.copy()
+
+        # multiple assignees
+        self.assertRaises(SyntaxError, df.eval, 'd c = a + b')
+
+        # invalid assignees
+        self.assertRaises(SyntaxError, df.eval, 'd,c = a + b')
+        self.assertRaises(SyntaxError, df.eval, 'Timestamp("20131001") = a + b')
+
+        # single assignment - existing variable
+        expected = orig_df.copy()
+        expected['a'] = expected['a'] + expected['b']
+        df = orig_df.copy()
+        df.eval('a = a + b')
+        assert_frame_equal(df,expected)
+
+        # single assignment - new variable
+        expected = orig_df.copy()
+        expected['c'] = expected['a'] + expected['b']
+        df = orig_df.copy()
+        df.eval('c = a + b')
+        assert_frame_equal(df,expected)
+
+        # with a local name overlap
+        def f():
+            df = orig_df.copy()
+            a = 1
+            df.eval('a = 1 + b')
+            return df
+
+        df = f()
+        expected = orig_df.copy()
+        expected['a'] = 1 + expected['b']
+        assert_frame_equal(df,expected)
+
+        df = orig_df.copy()
+        def f():
+            a = 1
+            df.eval('a=a+b')
+        self.assertRaises(NameResolutionError, f)
+
+        # multiple assignment
+        df = orig_df.copy()
+        df.eval('c = a + b')
+        self.assertRaises(SyntaxError, df.eval, 'c = a = b')
+
+        # explicit targets
+        df = orig_df.copy()
+        self.eval('c = df.a + df.b', local_dict={'df' : df}, target=df)
+        expected = orig_df.copy()
+        expected['c'] = expected['a'] + expected['b']
+        assert_frame_equal(df,expected)
+
     def test_basic_period_index_boolean_expression(self):
         df = mkdf(2, 2, data_gen_f=f, c_idx_type='p', r_idx_type='i')
 

pandas/core/frame.py

@@ -1797,12 +1797,14 @@ def eval(self, expr, **kwargs):
         >>> from pandas import DataFrame
         >>> df = DataFrame(randn(10, 2), columns=list('ab'))
         >>> df.eval('a + b')
+        >>> df.eval('c=a + b')
         """
         resolvers = kwargs.pop('resolvers', None)
         if resolvers is None:
             index_resolvers = self._get_resolvers()
             resolvers = [self, index_resolvers]
         kwargs['local_dict'] = _ensure_scope(resolvers=resolvers, **kwargs)
+        kwargs['target'] = self
         return _eval(expr, **kwargs)
 
     def _slice(self, slobj, axis=0, raise_on_error=False, typ=None):