New eval keyword to disallow assignment

Author: onesandzeroes

pandas/computation/eval.py

@@ -138,7 +138,7 @@ def _check_for_locals(expr, stack_level, parser):
 
 def eval(expr, parser='pandas', engine='numexpr', truediv=True,
          local_dict=None, global_dict=None, resolvers=(), level=0,
-         target=None):
+         target=None, assignment_allowed=True):
     """Evaluate a Python expression as a string using various backends.
 
     The following arithmetic operations are supported: ``+``, ``-``, ``*``,
@@ -196,6 +196,9 @@ def eval(expr, parser='pandas', engine='numexpr', truediv=True,
         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
+    assignment_allowed : bool
+        Whether the eval should be able to modify the input through
+        assigment.
 
     Returns
     -------
@@ -236,7 +239,11 @@ def eval(expr, parser='pandas', engine='numexpr', truediv=True,
 
     # assign if needed
     if env.target is not None and parsed_expr.assigner is not None:
-        env.target[parsed_expr.assigner] = ret
-        return None
+        if assignment_allowed:
+            env.target[parsed_expr.assigner] = ret
+            return None
+        else:
+            raise ValueError("Expression includes assignment statement: \n"
+                             "\t not allowed from DataFrame.query")
 
     return ret

pandas/core/frame.py

@@ -1935,7 +1935,7 @@ def query(self, expr, **kwargs):
         >>> df[df.a > df.b]  # same result as the previous expression
         """
         kwargs['level'] = kwargs.pop('level', 0) + 1
-        res = self.eval(expr, **kwargs)
+        res = self.eval(expr, assignment_allowed=False, **kwargs)
 
         try:
             return self.loc[res]