Merge pull request #4922 from cpcloud/eval-syntax-docs

DOC: show disallowed eval syntax

Author: cpcloud

doc/source/enhancingperf.rst

@@ -330,6 +330,42 @@ engine in addition to some extensions available only in pandas.
    The larger the frame and the larger the expression the more speedup you will
    see from using :func:`~pandas.eval`.
 
+Supported Syntax
+~~~~~~~~~~~~~~~~
+
+These operations are supported by :func:`~pandas.eval`:
+
+- Arithmetic operations except for the left shift (``<<``) and right shift
+  (``>>``) operators, e.g., ``df + 2 * pi / s ** 4 % 42 - the_golden_ratio``
+- Comparison operations, e.g., ``2 < df < df2``
+- Boolean operations, e.g., ``df < df2 and df3 < df4 or not df_bool``
+- ``list`` and ``tuple`` literals, e.g., ``[1, 2]`` or ``(1, 2)``
+- Attribute access, e.g., ``df.a``
+- Subscript expressions, e.g., ``df[0]``
+- Simple variable evaluation, e.g., ``pd.eval('df')`` (this is not very useful)
+
+This Python syntax is **not** allowed:
+
+* Expressions
+
+  - Function calls
+  - ``is``/``is not`` operations
+  - ``if`` expressions
+  - ``lambda`` expressions
+  - ``list``/``set``/``dict`` comprehensions
+  - Literal ``dict`` and ``set`` expressions
+  - ``yield`` expressions
+  - Generator expressions
+  - Boolean expressions consisting of only scalar values
+
+* Statements
+
+  - Neither `simple <http://docs.python.org/2/reference/simple_stmts.html>`__
+    nor `compound <http://docs.python.org/2/reference/compound_stmts.html>`__
+    statements are allowed. This includes things like ``for``, ``while``, and
+    ``if``.
+
+
 
 :func:`~pandas.eval` Examples
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~