.. _basic-query-for-python-code:

Basic query for Python code
===========================

Learn to write and run a simple CodeQL query using Visual Studio Code with the CodeQL extension.

.. include:: ../reusables/vs-code-basic-instructions/setup-to-run-queries.rst

About the query
---------------

The query we're going to run performs a basic search of the code for ``if`` statements that are redundant, in the sense that they only include a ``pass`` statement. For example, code such as:

.. code-block:: python

   if error: pass

.. include:: ../reusables/vs-code-basic-instructions/find-database.rst

Running a quick query
---------------------

.. include:: ../reusables/vs-code-basic-instructions/run-quick-query-1.rst

#. In the quick query tab, delete ``select ""`` and paste the following query beneath the import statement ``import python``.

   .. code-block:: ql

      from If ifstmt, Stmt pass
      where pass = ifstmt.getStmt(0) and
        pass instanceof Pass
      select ifstmt, "This 'if' statement is redundant."

.. include:: ../reusables/vs-code-basic-instructions/run-quick-query-2.rst

.. image:: ../images/codeql-for-visual-studio-code/basic-python-query-results-1.png
   :align: center

If any matching code is found, click a link in the ``ifstmt`` column to open the file and highlight the matching ``if`` statement.

.. image:: ../images/codeql-for-visual-studio-code/basic-python-query-results-2.png
   :align: center

.. include:: ../reusables/vs-code-basic-instructions/note-store-quick-query.rst

About the query structure
~~~~~~~~~~~~~~~~~~~~~~~~~

After the initial ``import`` statement, this simple query comprises three parts that serve similar purposes to the FROM, WHERE, and SELECT parts of an SQL query.

+---------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------+
| Query part                                                    | Purpose                                                                                                           | Details                                                                                                                |
+===============================================================+===================================================================================================================+========================================================================================================================+
| ``import python``                                             | Imports the standard CodeQL libraries for Python.                                                                 | Every query begins with one or more ``import`` statements.                                                             |
+---------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------+
| ``from If ifstmt, Stmt pass``                                 | Defines the variables for the query.                                                                              | We use:                                                                                                                |
|                                                               | Declarations are of the form:                                                                                     |                                                                                                                        |
|                                                               | ``<type> <variable name>``                                                                                        | - an ``If`` variable for ``if`` statements                                                                             |
|                                                               |                                                                                                                   | - a ``Stmt`` variable for the statement                                                                                |
+---------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------+
| ``where pass = ifstmt.getStmt(0) and pass instanceof Pass``   | Defines a condition on the variables.                                                                             | ``pass = ifstmt.getStmt(0)``: ``pass`` is the first statement in the ``if`` statement.                                 |
|                                                               |                                                                                                                   |                                                                                                                        |
|                                                               |                                                                                                                   | ``pass instanceof Pass``: ``pass`` must be a pass statement.                                                           |
|                                                               |                                                                                                                   |                                                                                                                        |
|                                                               |                                                                                                                   | In other words, the first statement contained in the ``if`` statement is a ``pass`` statement.                         |
+---------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------+
| ``select ifstmt, "This 'if' statement is redundant."``        | Defines what to report for each match.                                                                            | Reports the resulting ``if`` statement with a string that explains the problem.                                        |
|                                                               |                                                                                                                   |                                                                                                                        |
|                                                               | ``select`` statements for queries that are used to find instances of poor coding practice are always in the form: |                                                                                                                        |
|                                                               | ``select <program element>, "<alert message>"``                                                                   |                                                                                                                        |
+---------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------+

Extend the query
----------------

Query writing is an inherently iterative process. You write a simple query and then, when you run it, you discover examples that you had not previously considered, or opportunities for improvement.

Remove false positive results
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Browsing the results of our basic query shows that it could be improved. Among the results you are likely to find examples of ``if`` statements with an ``else`` branch, where a ``pass`` statement does serve a purpose. For example:

.. code-block:: python

   if cond():
     pass
   else:
     do_something()

In this case, identifying the ``if`` statement with the ``pass`` statement as redundant is a false positive. One solution to this is to modify the query to ignore ``pass`` statements if the ``if`` statement has an ``else`` branch.

To exclude ``if`` statements that have an ``else`` branch:

#. Extend the ``where`` clause to include the following extra condition:

   .. code-block:: ql

      and not exists(ifstmt.getOrelse())

   The ``where`` clause is now:

   .. code-block:: ql

      where pass = ifstmt.getStmt(0) and
        pass instanceof Pass and
        not exists(ifstmt.getOrelse())

#. Re-run the query.

   There are now fewer results because ``if`` statements with an ``else`` branch are no longer included.

Further reading
---------------

.. include:: ../reusables/python-further-reading.rst
.. include:: ../reusables/codeql-ref-tools-further-reading.rst

.. Article-specific substitutions for the reusables used in docs/codeql/reusables/vs-code-basic-instructions

.. |language-text| replace:: Python

.. |language-code| replace:: ``python``

.. |example-url| replace:: https://github.com/saltstack/salt

.. |image-quick-query| image:: ../images/codeql-for-visual-studio-code/quick-query-tab-python.png

.. |result-col-1|  replace:: The first column corresponds to the expression ``ifstmt`` and is linked to the location in the source code of the project where ``ifstmt`` occurs.