help with explaining how to split a list of tuples into parts

[Peter Otten]

peter at ifoley.id.au wrote:

> Hi List,
> 
> I am new to Python and wondering if there is a better python way to do
> something.  As a learning exercise I decided to create a python bash
> script to wrap around the Python Crypt library (Version 2.7).
> 
> My attempt is located here - https://gist.github.com/pjfoley/5989653
> 
> I am trying to wrap my head around list comprehensions, I have read the
> docs at
> http://docs.python.org/2/tutorial/datastructures.html#list-comprehensions
> and read various google results.  I think my lack of knowledge is making
> it difficult to know what key word to search on.
> 
> Essentially I have this list of tuples
> 
> # Tuple == (Hash Method, Salt Length, Magic String, Hashed Password
> # Length)
> supported_hashes=[('crypt',2,'',13), ('md5',8,'$1$',22),
> ('sha256',16,'$5$',43), ('sha512',16,'$6$',86)]
> 
> This list contains the valid hash methods that the Crypt Library supports
> plus some lookup values I want to use in the code.
> 
> I have managed to work out how to extract a list of just the first value
> of each tuple (line 16) which I use as part of the validation against the
> --hash argparse option.
> 
> My Question.
> 
> Looking at line 27, This line returns the tuple that mataches the hash
> type the user selects from the command line.  Which I then split the
> seperate parts over lines 29 to 31.
> 
> I am wondering if there is a more efficient way to do this such that I
> could do:
> 
> salt_length, hash_type, expected_password_length = [x for x in
> supported_hashes if x[0] == args.hash]
> 
> From my limited understanding the first x is the return value from the
> function which meets the criteria.  So could I do something like:
> 
> ... = [(x[0][1], x[0][2], x[0][3]) for x in supported_hashes if x[0] ==
> args.hash]
> 
> I am happy to be pointed to some documentation which might help clarify
> what I need to do.
> 
> Also if there is anything else that could be improved on with the code
> happy to be contacted off list.

Every time when you have to look up something you should think 'dict', and I
expect that pretty that will happen automatically.
Also, to split a tuple into its items you can "unpack" it:

triple = (1, 2, 3)
one, two, three = triple
assert one == 1 and two == 2 and three == 3

So:

supported_hashes = {
    "crypt": (2, "", 13),
    "md5": (8, "$1$", 22),
    ...
}
...
parser.add_argument(
    '--hash', default='sha512', 
    choices=supported_hashes, # accept the keys
    help='Which Hash function to use')
...
salt_length, hash_type, expected_password_length = supported_hashes[args.hash]
...