Pylint and the Module hashlib has no md5 member Error
#####################################################

:title: Pylint and the Module hashlib has no md5 member Error
:date: 2014-04-26
:category: Software
:tags: python, programming
:slug: pylint-and-the-module-hashlib-has-no-md5-member-error
:author: Chris Ramsay
:status: published
:language: en
:show_source: True

.. contents::

That Pesky Error in PyLint's Results
------------------------------------

.. PELICAN_BEGIN_SUMMARY

How to avoid `PyLint's`_ `Module 'hashlib' has no 'md5' member` error when using
`hashlib's md5()`_ function in your code, and the consequences of doing so.

I do not use `md5` much at all these days, as it is `known to be fairly
unsafe`_; having said that, `md5` does still have its uses when it comes to
comparing file likeness in non-critical applications. It just so happens that
PyLint does not seem to like invoking hashlib with specific constructors and
will therefore mark down code ratings as a result. See the example below:

.. code-block:: python

    >>> import hashlib
    >>> HASH = hashlib.md5()
    >>> HASH.update('Test my hash')
    >>> HASH.hexdigest()
    'f94b71b31c1d09d352db8b59d4f98892'

.. PELICAN_END_SUMMARY

Checking the source code for a particular file using this method of invocating
md5, PyLint invariably gives me an error message similar to the following:

.. code-block:: bash

    raz@foobox:~/Python/soap$ pylint soap_client.py
    No config file found, using default configuration
    ************* Module somemodule
    E:231:SomeClass._some_method: Module 'hashlib' has no 'md5' member

    [...]

    Messages
    --------

    +-----------+------------+
    |message id |occurrences |
    +===========+============+
    |E1101      |1           |
    +-----------+------------+



    Global evaluation
    -----------------
    Your code has been rated at 9.68/10

Hmm, that is not good at all. I want a better PyLint score, something like 10/10
would do just fine, thanks.

What to do Slightly Differently
-------------------------------

Seeing as I would like my code to score as highly as possible in PyLint tests,
what to do? The answer is to use md5 in a subtly different way to get rid of
that error...

Instead of invocating the named constructor, such as `hashlib.md5()`, use the
generic `new()`, constructor `hashlib.new('md5')`:

.. code-block:: python

    >>> import hashlib
    >>> HASH = hashlib.new('md5')
    >>> HASH.update('Test my hash')
    >>> HASH.hexdigest()
    'f94b71b31c1d09d352db8b59d4fq98892'

The interesting bit of test output now shows:

.. code-block:: bash

    Global evaluation
    -----------------
    Your code has been rated at 10.00/10 (previous run: 9.68/10)

Hooray, full marks at last! But, read on...!

A Minor Cautionary Note
-----------------------

This does come with a minor health warning: as briefly mentioned in the hashlib
documentation, the `new()` constructor runs slower than the named constructors.
Let me put this to a very basic test, using `Python's timeit module`_.

Firstly with the generic constructor:

.. code-block:: python

    >>> import timeit
    >>> s = """\
    ... import hashlib
    ... HASH = hashlib.new('md5')
    ... HASH.update('Test my hash')
    ... HASH.hexdigest()
    ... """
    >>> t = timeit.Timer(stmt=s)
    >>> print "%.2f usec/pass" % (1000000 * t.timeit(number=100000)/100000)
    3.02 usec/pass

Now, with the named constructor:

.. code-block:: python

    >>> import timeit
    >>> s = """\
    ... import hashlib
    ... HASH = hashlib.md5()
    ... HASH.update('Test my hash')
    ... HASH.hexdigest()
    ... """
    >>> t = timeit.Timer(stmt=s)
    >>> print "%.2f usec/pass" % (1000000 * t.timeit(number=100000)/100000)
    2.41 usec/pass

So we have knocked off approximately 20% of execution time in this code by using
the named constructor at the expense of a drop in the overall PyLint score.

Finally
-------

As with most things, this is not a black and white issue. If performance is an
quality of important consideration in your code, you may just have to put up
with lower PyLint code ratings!

.. _`Pylint's`: http://www.logilab.org/857
.. _`known to be fairly unsafe`: http://en.wikipedia.org/wiki/MD5#Security
.. _`hashlib's md5()`: http://docs.python.org/library/hashlib.html
.. _`Python's timeit module`: http://docs.python.org/library/timeit.html